Expanding the use of Brassica cover crops in onion production

Final Report for ONE14-208

Project Type: Partnership
Funds awarded in 2014: $14,932.00
Projected End Date: 12/31/2016
Region: Northeast
State: New York
Project Leader:
Christine Hoepting
Cornell Cooperative Extension - Cornell Vegetable Program
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Project Information

Summary:

Use of Brassica cover crops following harvest of early bulb onions has become common practice in Michigan as research from Michigan State University has demonstrated that their use results in improved stand, allowing growers to reduce their seeding rate by 10% or more.    On muck soils potential exists for later planting of brassica cover crops due to their higher nutrient, moisture and temperature-holding capacities.  Through a combined approach of in-depth case-studies and replicated on-farm research trials, we studied the benefits of using brassica cover crops in muck onion production including delayed planting, necessity to incorporate mustard, as well as the potential to reduce seeding rate of direct seeded onions and to increase plant population of transplanted onions following incorporated optimally planted mustard cover crops.

Different seeding rates made it impossible to directly compare effect on yield of incorporated optimally planted mustard cover crop to bare ground. Pre-plant soil test results showed that the incorporated optimally planted (Aug 20) mustard cover crop had significantly 36% higher potentially mineralizable nitrogen and 39% more available potassium than bare ground, suggesting that nitrogen and potassium inputs could be reduced by 20-30 lb/A and 50 lb/A, respectively.  Delaying establishment of incorporated mustard cover crop by 10 days to Aug-30 resulted in no significant differences in yield and bulb size distribution in direct seeded onions, or in nitrogen cycling compared to the optimally planted mustard cover crop, but the nutrient cycling of potassium was to a lesser degree.  This treatment also had the highest net return of $11,113/A.

When establishment of incorporated mustard cover crop was delayed by 27 days to Sep-26 and when a late planted mustard cover crop was left to winter-kill, yield and bulb size was less than in the incorporated optimally planted mustard cover crop, but nitrogen cycling still occurred to the extent that nitrogen rates could be reduced by 20-30 lb/A in following onion crops.  The winter-killed late planted mustard had significantly the highest soil compaction and it appeared that the process of incorporating cover crops in the fall reduced compaction the following spring.  Forage radish that was planted 7 days late on Aug 27 had similar bulb size distribution to incorporated optimally planted mustard cover crop; although the total yield was numerically 11% lower, cost of establishment was 35% less and leaving it to winter-kill provided protection against wind erosion. 

Brassica cover crops did not have any effect on damping off seedling disease, bacterial bulb decay or Botrytis neck rot.  Incorporated optimally planted mustard cover crop did not result in increased yields or net returns when plant population of transplanted onions was increased by 21% and higher.  When direct seeded onions were grown at 17% reduced seeding rate following incorporated optimally planted mustard cover crop, at 645 cwt/A, the yield was twice as much as the average onion yield in New York and with the bulb size skewed towards more lucrative larger sizes, at $18.46/cwt, the price was 12% higher than the state average.

By delaying the planting date of mustard cover crop by 10 days, than theoretically, an additional 15% of acreage could potentially be cropped to mustard cover crops that could potentially result in an increase of 62.6 cwt/A and $1,034/A for a total of 93,750 cwt and $1.5 million while saving $110,100 in seed costs, assuming a conservative 20% increase in yield and a conservative 10% reduction in seeding rate.   

Introduction:

New York onion growers suffer significant losses due to soil-borne pests and impaired soil health. A simple newly developed modification to their rotation can result in major improvements.  Planting Brassica cover crops following harvest of early bulb onions suppresses those pests and provides a deep rooting rotation crop.  Adoption of this practice has been high in Michigan where Michigan State University research (Ngouajio et al.) showed 8 to 20% higher stand, allowing growers to reduce their seeding rate by 10% or more.  In these studies, various Brassica cover crops reduced weed density by 52%, increased beneficial soil microorganisms up to 2.7 times and reduced plant-parasitic nematodes up to 56%.  We believe similar results would occur in New York.  Our preliminary study in Elba, NY showed that a brown/yellow mustard increased onion yield by 28%, jumbo-sized bulbs by 3.5-times and net return by 37%.  On mineral soil, forage radishes have been demonstrated to improve water drainage with their deep tap roots, provided they are planted in mid-August to achieve enough biomass. 

Another concern is inefficient use of nitrogen on muck soils as onion growers apply substantial amounts of nitrogen fertilizer in the spring. Much of that nitrate can be lost to leaching and denitrification with this shallow rooted crop. Brassica cover crops are most likely to provide nitrogen to subsequent crops if they decompose in the spring.  The cold hardiness varies with variety, age and growing conditions, so we will determine the useful combinations.  A preliminary study in Elba gave promising results: soil nitrate-nitrogen in mid-March was almost 3-fold higher following forage radish than bare-ground.  Nutrient cycling and potential to reduce fertilizer inputs in onions grown in muck soil following Brassica cover crops will be addressed in our project.

Adoption of Brassica cover crops remains low in New York, where over 10,000 acres of onions are grown predominantly on muck soils, representing 97% of onion production in the Northeast U.S. Brassica cover crops are currently only being used on 6 of these 75 farms. The main reason for low adoption is that with the recommended establishment date of Aug 10-20, only 10% of the onion acreage is eligible for use of brassica cover crops, because the remainder of the acreage is harvested after this time. Also, growers are not aware of their potential benefits.  We see great potential to increase adoption substantially, leading to higher profitability and improved sustainability.

A second reason is that only a small proportion of the onion acreage is harvested in time to sew Brassica cover crops. Using the date of August 20 recommended for mineral soils, only about 10% of the onion acreage that is cropped to early-maturing varieties is available for establishing Brassica cover crops.  The remaining acreage is harvested in late-August and September.  We will study timing of establishing Brassica cover crops on muck soils; potential exists for later establishment due to their higher nutrient, moisture and temperature-holding capacities. 

Demonstration of the benefits of using Brassica cover crops to suppress pests including soil-borne diseases and bacterial diseases is a high research priority for onions in the NY-IPM program. Whether use of Brassica cover crops suppresses common soil-borne diseases including damping off, onion smut and bacterial diseases will be addressed in our project. Other cited benefits from using Brassica cover crops include increased yield, improved drainage, nutrient cycling and, suppression of soil-borne diseases.  The problem is that in New York, which accounts for 97% of onion production in the Northeast U.S., where over 10,000 acres of onions are grown on muck soils, Brassica cover crops are currently only being used on 6 of these 75 farms. 

Project Objectives:

Original Objectives and Plan of Work:

1. To demonstrate benefits of incorporated optimally planted mustard cover crop on following onion crop grown in muck soil.

To determine whether…

2.  Potential exists for later establishment of brassica cover crop in onions grown in the Northeast on muck soil.

3. Incorporation of mustard cover crops is necessary to achieve benefit in onions grown in the Northeast on muck soil.

4. Mulit-year use of brassica cover crops results in improved benefits over single-year use in onion.

5. There is a difference in benefits to onion between different types of brassica cover crops; specifically, yellow/brown mustard vs. tillage radish vs. tillage radish + oats.

6. The use of brassica cover crops in onion grown in muck soil results in nutrient cycling that would allow for the reduction of fertilizer inputs.

7. The use of brassica cover crops in onion grown in muck soil results in reduced soil compaction.

8. The use of brassica cover crops in onion grown in muck soil results in reduced damping off disease and bacterial disease.

9. The seeding rate can be reduced in direct-seeded onions following a mustard cover crop.

10. The plant population of onion transplants can be increased without reducing bulb size, thus, increasing yield per acre when following mustard cover crop.

To address objectives 1-7, three grower cooperators planted mustard and radish cover crops in Fall 2013 in anticipation of this work (Table 1table-1-case-studies). We conducted in-depth case studies on each farm to study how these factors affected onion stand, yield, bulb size, nutrient cycling and bacterial disease.

To address objective 8, a seedling bioassay was conducted using soil collected from the grower fields used in the case study.

To address objective 9 and 10, a small-plot on-farm research trial was conducted for each objective.

Changes to Original Plan:

Instead of working with three grower cooperators as originally planned, we only worked with two, because CY Farms did not end up planting onions following their mustard cover-cropped fields in spring 2014. Thus, we did not evaluate tillage radish + barley in objective 5.  At Mortellaro, onions were also not planted in the field that had mustard cover crop in 2012, and so we did not complete objective No. 4.

Cooperators

Click linked name(s) to expand
  • Dr. Thomas Bjorkman
  • Matt Mortellaro
  • Michael Riner
  • Guy Smith

Research

Materials and methods:

Objectives 1-3 and 5-8: Demonstration of benefits to onions following incorporated optimally planted mustard cover crop. Effect of later establishment, incorporation, cover crop type on efficacy of cover crop benefits to onion; effect of brassica cover crop on nutrient cycling, soil compaction and bacterial disease.

Case Studies (Table 1, table-1-case-studies; Figure 1-2figure-1-triple-case-study-spring; figure-2-mortellaro-case-study-spring):

Brassica cover crops were established by our grower cooperators in the Elba muck land in 2013 to study onion performance in 2014.  At Triple G Farms, we compared the optimal planting date (Aug-20) of yellow/brown mustard (Caliente 199) to planting 10 days after the optimal window (Aug-30) and to a bare ground control in direct seeded yellow onions (c.v. Hendrix).  At Mortellaro farm, we compared an optimal planting of mustard (cv. Caliente 199) on Aug 7 to a very late planting (Sep-26), both of which were incorporated, a late planting (Aug 27) that was left to winter-kill, and a late planting of forage radish (Aug 30) on the effect of transplanted yellow onions (c.v. Bradley).  Each field was divided into six pseudo replicates along its length.

Effect of mustard cover crops on soil fertility and compaction:

Prior to soil disturbance in the spring, composite soil samples were collected from reps 1, 3 & 5 and sent to Cornell Nutrient Analysis Laboratory (CNAL) for complete nutrient analysis and Potentially Mineralizable Nitrogen (PMN). Available soil nitrate-nitrogen (NO3-N) was later analyzed at the 8-10-leaf stages using the same sampling technique. 

Also prior to soil disturbance, maximum soil compaction at depths 0-6” and 7-12” was measured using a penetrometer at 10 random locations in each of 6 replicates per case study field.

Effect of mustard cover crops on plant growth and yield:

Onion stand was quantified at the loop-flag, 3-5-leaf onion stages in direct seeded onions, and in 8-10-leaf onions in both direct seeded and transplanted onions by counting the number of plants in four side-by-side 3-ft sections of row in two sub-samples per replicate. Onion growth was assessed in all but the loop-flag leaf stages by number of leaves per plant, plant height and neck diameter in two sub-samples of 10 randomly selected plants per replicate.  At harvest, 100-bulb samples were collected from two sub-samples per replicate for yield assessment, which included number of bulbs and bulb weight of the total marketable and culls, and size grade distribution. 

Effect of mustard cover crops on bacterial bulb decay:

Bacterial bulb decay was quantified at harvest and the remaining sound bulbs were put into commercial onion storage for four months after which time they were graded again for decay in February 2015. 

Objective 8: Effect of brassica cover crops on damping of disease of onion

Soil Bioassay:

To evaluate the effect of the brassica cover crops on soil-borne pathogens, a soil bioassay was conducted using soil from each of the fields used in the case studies. In addition to the three fields used in the case study at Triple G, for comparison to grower standard practice, soil was also collected from a nearby field that had a cereal cover crop.  The soil collected prior to soil disturbance from the case studies was used to fill one 200-cell plug tray per replicate for each of the six pseudo replicates per field and one raw onion seed (c.v. Hendrix) per cell was sown.  The seedlings were grown in the outdoors with daily watering as needed.  Emergence, seedling stage and mortality was quantified 2 and 5 weeks after planting.  Samples of unhealthy plants suspect of Damping-off pathogens, Phythium, Rhizoctonia and Fusarium were sent to Chris Smart, Cornell Plant Pathology for confirmation.   

Objectives 9 & 10: Effect of mustard cover crop on plant population and potential to reduce seeding rate and increase transplant population

On-farm, Small-Plot Plant Population Studies:

Two small-plot plant population studies were conducted in order to determine whether incorporated optimally planted mustard cover crops would allow a reduced seeding rate of direct seeded onion at Triple G, and whether it would allow a higher plant population in transplanted onions at Mortellaro.  Mustards primarily increase onion seedling survival in direct-seeded onions, and bulb growth in transplanted onions.  We wanted to determine how much the seeding rate can be reduced in direct-seeded onions, and how much the plant population of transplants can be increased without reducing bulb size, thus, increasing yield.

At Triple G, treatments included the grower’s standard (267,981 plants/A = 7.7 seeds per ft), and 17% (~222,424 plants/A = 6.4 seeds per ft) and 38% (~166,148 plants/A = 4.8 seeds per ft) fewer seeds per acre, in a randomized complete block design with 5 replications. The grower planted the trial using his commercial onion seeder stopping to adjust the seeding density according to the randomized plot plan.  Each plot was 2-beds wide (10-ft wide, 4 rows spaced 15”) by 15-ft long.  At harvest, stand, yield and bacterial bulb decay were collected from the inside 10-feet of all 8 rows of onions.

At Mortellaro, treatments included the grower standard (128,938 transplants/A) and 21%, 38% and 62% higher plant populations. The grower’s standard planting configuration was 15-inch between rows on a 5-ft bed with 3.25-inch off-set plant spacing.  For the higher plant populations, we spaced 5 rows 10-inch apart and used 4-, 3.5- and 3-inch plant spacing.  Plots were 1-bed wide (= 5 ft) by 10-ft long in a randomized complete block design with 5 replications.  Transplants were set by hand using custom fabricated dibbles.  Yield and grade was collected from the entire plot area.  Bacterial bulb decay was also quantified.

Statistical Analysis:

Differences among treatments were analyzed using General Analysis of Variance and means were separated using Fisher’s Protected LSD test with a significance of 5%.

Partial Budgets:

To compare the economics among treatments, average prices for each grade were obtained from the grower cooperators and included $6, $10, $16 and $20 per hundred weight (cwt) for undersize (<1.75”), small (1.75-2.25”), medium (2.25-3”) and jumbo (>3”) sized bulbs, respectively. Input costs included the cost of seed and costs associated with establishing the cover crops, including seeding, chopping and plowing.  A cost of $12/A was used for each pass across the field.  At Triple G, cost of seed was also included because there was a difference in seeding rate between the bare ground field and those cover cropped to mustard.  All input costs were provided by the grower cooperators. 

Research results and discussion:

Objectives 1-3 and 5-8: Demonstration of benefits to onions following incorporated optimally planted mustard cover crop. Effect of later establishment, incorporation, cover crop type on efficacy of cover crop benefits to onion; effect of brassica cover crop on nutrient cycling, soil compaction and bacterial disease.

Effect of brassica cover crops on pre-plant pH, organic matter and soil fertility (Table 2table-2-case-study-pre-plant-fertilility).

Triple G Case Study:

There were no significant differences in pre-plant soil nutrient levels including phosphorous (P), magnesium, boron, copper, zinc, aluminum, iron, molybdenum or sulfur among the bare ground and early and late planted mustard cover cropped fields. Levels of P were low, which would require addition of 150 lb of P in fertilizer, while levels of potassium (K) were very high and would not require any additional fertilizer, according to Cornell recommendations.  Organic matter ranged from 53 to 55%, which is normal for the Elba muck land.  The pH was 5.5. in the bare ground field, which is optimum, while it was slightly significantly lower in the mustard cover cropped fields (early – 5.0; late – 5.1).  In 2008, the bare ground field had a pH of 5.1 and the mustard cover cropped fields had a pH of 5.2.  In 2011, the bare ground field was tiled, which brought some underlying marl that is high in pH to the surface.  Since all fields in this study have a history of having low pH and the higher pH in the bare ground field can be attributed to the tiling, it is unlikely that the mustard cover crops contributed to the lower pH in these fields. 

The optimally planted mustard cover cropped field (1,309 lb/A) had significantly 38.7% higher available K than the bare ground field (944 lb/A). There were no significant differences between the optimally planted mustard cover cropped field and the later planted mustard cover cropped field (1,161 lb/A), which had 23% higher available K than the bare ground field. 

The bare ground field (59,138 lb/A) had significantly higher calcium (Ca) than the mustard cover cropped fields (optimum: 39,675 lb/A; late: 41,519 lb/A) by 43 to 49%, which were not significantly different from each other. The difference in Ca could be a result of the mustard cover cropped fields having significantly lower pH (late – 5.1; early – 5.0) than the bare ground field (pH 5.5); Ca availability decreases as pH falls below 5.0. 

Numerically, the optimally planted mustard cover cropped field had 40% higher manganese (Mn) (44.4 lb/A) than the bare ground (31.6 lb/A) or late planted mustard cover cropped fields (34 lb/A). It is possible that the increase in Mn in the mustard cover cropped fields was a function of the lower pH, because Mn availability decreases as pH exceeds 5.2.  However, there was a much greater difference (10 lb/A) between the optimally planted mustard cover cropped field and the later planted mustard cover crop field, which had similarly low pH than there was between this field and the bare ground field (3 lb/A) where the pH differed by 0.4 units.  Thus, it appears that delaying planting of mustard cover crop by 10 days resulted in less Mn availability.  In a previous NESARE study (ONE10-116), we determined that soil levels of Mn of 20 lb/A or higher was adequate. Thus, these fields would not have required additional inputs anyway. 

In conclusion, use of mustard cover crops increased the amounts of K and Mn compared to bare ground. No significant difference occurred between optimal (Aug 20) and late (Aug 30) plantings of the mustard cover crop for K, but only optimal planting of mustard cover crop increased Mn compared to bare ground. 

Mortellaro Case Study:

Compared to Triple G, Mortellaro sites had lower pH by 0.5, lower organic matter (OM) by 8%, and about half as much potassium (K), calcium (Ca) and boron (B). Alternatively, Mortellaro sites had higher manganese (Mn) by 3-times, iron (Fe) by 15-times, sulfur (S) by 3-times and aluminum by 3.5-times, which was likely a function of the lower pH making these ions more readily available.  These nutrient levels were similar to the levels previously found on this farm during a NESARE project (ONE10-116).

Significant differences among treatments occurred for OM, Ca, magnesium (Mg), B and Fe. Levels of phosphorous (P) and K were low and medium, respectively, and would require addition of 150 lb/A of P and 100 lb/a of K  respectively, according to Cornell recommendations.  The levels of Ca and Mg were normal for the Elba muck land.   The incorporated late planted yellow mustard field had significantly higher %OM (51.5%) than all of the other fields by 4.5 to 9.8 units, while the incorporated optimally planted mustard field had the lowest %OM (41.7%).  It is unlikely that these differences in OM was a treatment effect; rather, they were inherent to the individual fields.  The incorporated late planted mustard cover cropped field had significantly higher Ca than all other fields, which was most likely related to this field also having significantly the highest %OM.  In previous studies, we found a correlation between OM and Ca; as OM increases, Ca increases.  In this study, there was also a significant positive correlation between pH and Ca (Pearson: R= 0.6634; p = 0.0187). The incorporated optimally planted- and winter killed late planted mustard cover cropped fields had significantly higher Mg than the other two fields, which was likely related to these fields also having the lowest pH values as Mg availability decreases as pH decreases.  There was a significant positive correlation between pH and Mg (Pearson; R = 0.9484; p = 0.0000).  The relationship between planting date of incorporated mustard cover crop on available K followed similar trends as at Triple G in that there were no significant differences although the optimal planting had 6.6% (= 30 cwt/A) higher available K than the late planting.  The incorporated late planted mustard cover cropped field had significantly 38% more available B than the other fields, which was most likely related to this field also having the highest OM, because OM holds onto B and as OM increases, the soil’s ability to provide B also increases.  The incorporated optimally planted mustard cover cropped field had significantly 33 to 66% higher Fe than the other fields, which were not different from each other.  There was a significant negative correlation between OM and Fe (Pearson: R = -0.7882; p=0,0023), which may explain these differences.

In conclusion, the pre-plant soil test results did not show any differences between early and late planted mustard cover crops and between forage radish and between winter-killed and incorporated brassica cover crops, because most differences among fields could be explained by the differences in OM and pH.

Effect of brassica cover crops on nitrogen (Table 3table-3-case-study-nitrogen)

Triple G Case Study:

Prior to planting, potentially mineralizable nitrogen (PMN) was 38.9 lb/A in the bare ground field. The optimal (52.8 lb/A) and late planted (70.5 lb/A) mustard cover cropped fields had 1.4-times and significantly 1.8-times as much PMN as the bare ground field, respectively, and there were no differences in PMN between optimal and late planted mustard.  These results suggest that using a mustard cover crop could result in 32 lb/A and 14 lb/A reduced nitrogen inputs when it is planted on time and 10 days late, respectively. 

The grower applied a total of 101 lb/A of nitrogen (N) which was all pre-plant incorporated. In mid-July when the onions were in the 8-10 leaf stage and had already begun to bulb, the optimally planted mustard cover cropped field had twice as much available nitrate-nitrogen (NO3-N) (62.8 lb/A) as the bare ground field, which had 31. 8 lb/A.  The late planted mustard cover cropped field also had significantly 59% more available NO3-N (50.5 lb/A) than the bare ground field, but was significantly less than the optimal planted mustard cover cropped field.  What makes these results challenging to interpret is that the bare ground field was seeded at 8.4 seeds/ft while the mustard cover cropped fields were seeded at 6.4 seeds/ft (24% reduction), which resulted in plant populations that were significantly lower by 16-17% by mid-season.  Therefore, differences in N levels as they relate to plant population were compared.  In the bare ground field, 39 lb/A (already in the soil as PMN pre-plant) plus 101 lb/A added by the grower, minus 32 lb/A remaining in the soil at the 8-10 leaf stage indicates that 108 lb/A of N was used by the crop and/or lost from the system at that time.  Similarly, the late- and optimally planted mustard cover cropped fields resulted in 100 lb/A and 91 lb/A of N that had been used by the crop and/or lost.  When these N values were divided by the plant population, there were no significant differences among treatments.  Numerically, the mustard treatments had 8% and 44.7% higher amount of N in the system for optimal and late planting, respectively, than the bare ground field, which suggests that opportunity exists to reduce N inputs when direct seeded onions follow a mustard cover crop, even when it is planted up to 10 days past the optimal timing.

Mortellaro Case Study:

Compared to Triple G, Mortellaro sites had about half as much PMN prior to planting and about the same NO3-N at the 8-10 leaf stage, even though soil samples were taken about one month apart between the two sites. However, Mortellaro applied 25 lb/A more nitrogen than Triple G, which may explain this difference.  There were no significant differences among treatments in PMN prior to planting or in NO3-N when the onions were in the 8-10 leaf stage.  Although there was a lot of variability in PMN among replicates within fields, contrary to our results at Triple G where optimally planted mustard cover crop had highest PMN prior to planting and highest NO3-N at 8-10 leaf stage, at Mortellaro this treatment had numerically the lowest PMN (16.1 lb/A), and there were no differences among treatments in NO3-N at the 8-10 leaf stage, which ranged from 54 to 62 lb/A.  Mortellaro applied about 125 lb/A nitrogen.  Without having a bare ground field to compare to, we can use the formula where the amount of N in PMN pre-plant is added to the amount applied and then the amount of available NO3-N at the 8-10 leaf stage is subtracted to see how these values compare to the bare ground field at Triple G.  At Mortellaro, the amount of N used/lost ranged from 80 to 107 lb/A, which was similar to Triple G bare ground (108 lb/A).  The potential N used per plant in the mustard treatments (6.03, 6.94, 6.68 lb/10,000 plants) were similar to the mustard treatments at Triple G (lb per 10,000 plants: optimal – 5.10; late – 6.83), while it was higher in the forage radish (7.18 per 10,000 plants).  These results also indicate that N can likely be reduced when a transplanted onion crop follows a mustard cover crop, especially forage radish.  In this study, there were no significant differences between incorporated optimally planted mustard and incorporated late planted (by 37 days) mustard, winter-killed late planted (by 7 days) mustard or winter killed late planted (by 10 days) forage radish cover crops.

Effect of brassica cover crops on soil compaction (Table 4table-4-case-study-soil-compaction)

Triple G Case Study.

Prior to planting at the 1-6 inch depth, the optimally planted mustard cover cropped field had significantly less soil compaction (86.6 psi) than the bare ground (95.8 psi) and late planted mustard cover cropped (100.2 psi) fields, respectively, as measured by a penetrometer. At the 7-12 inch depth, the late planted mustard cover cropped field had significantly the highest soil compaction (141 psi).  It is not known why the late planted field would be significantly more compacted than the other fields; perhaps it was initially more compacted.  Although it is possible that the fibrous root systems of the mustard cover crops reduced soil compaction at least within the top 6 inches, without side-by-side comparisons within the same field, our results do not definitively confirm this.

Mortellaro Case Study.

Prior to spring planting, the winter killed late planted mustard cover cropped field (125 psi) had significantly the highest soil compaction at 1-6” depth as measured by a penetrometer, which was about 5-times higher than the two incorporated treatments (optimal mustard: 27 psi; late mustard: 22 psi) and significantly double the winter killed late planted forage radish field (73 psi). At 7-12”, there was no difference between the winter killed late planted mustard (149 psi) and winter killed forage radish (144 psi), both of which had significantly almost double the compaction of the two incorporated treatments (optimal mustard: 85 psi; late mustard: 72 psi).  At this farm, it appeared that the process of cover crop incorporation via tillage in the late fall resulted in an impressive reduction in soil compaction throughout the soil profile compared to leaving the cover crop undisturbed.  Of the two winter killed cover crops, the soil cropped to forage radish had half as much soil compaction as mustard.  Had the forage radish been planted earlier, the roots may have penetrated deeper and also reduced compaction at the 7-12” depth.

Effect of brassica cover crops on onion stand (Table 5table-5-case-study-stand)

Triple G Case Study:

Unfortunately, not all of the fields were planted at the same seeding density; the bare ground field was seeded at 8.4 seeds/ft and the mustard cover cropped fields were seeded at 6.4 seeds/ft (= 24% reduction), because the grower decided to follow the recommendations at Michigan State University to reduce seeding rate when using mustard cover crops. Consequently, the mustard cover cropped fields had significantly lower stands than the bare ground field by about 26% throughout the growing season.  There were no significant differences in stand as a proportion of seed planted among treatments at loop-flag stages, 3-5 leaf stage or at harvest.  At the 8-10 leaf stage, the bare ground field had significantly 9% less % stand of seed planted than the mustard cover cropped fields, which were not significantly different from each other.  The stand held constant from loop-flag (90-92% of seed planted) to 3-5 leaf (87-90% of seed planted), but then dropped slightly at 8-10 leaf (range: 79 to 82% of seed planted) and again at harvest (range: 73 to 77% of seed planted).  The decrease in stand from the loop stage to harvest was also not different among treatments (range: 13 to 17%, data not shown).  Possible reasons for plant loss during the growing season include, heat stress, wind injury and damping off disease in the flag leaf stage, mortality due to water-saturated soil and ponding, herbicide injury, onion smut, onion maggot and accidental up-rooting during hand weeding operations. 

In general, the 2014 growing season was one of the best on record for stand and yield in onion production in Elba, NY. In our bioassay (Table 9table-9-triple-g-bioassay), we had increased emergence and healthier plants in the mustard cover cropped fields compared to the bare ground.  It is likely that we would have seen differences is stand between the bare ground and mustard cover cropped fields in a more challenging growing season, as they typically observe in Michigan.

Mortellaro Case Study:

Cover crop is not expected to have an effect on emergence in transplanted onions and differences in stand would likely be attributed to other causes such as heat stress during establishment, onion maggot or saturated soil. At the 8-10 leaf stage, incorporated late planted mustard and winter killed late planted mustard had significantly 5% higher stand than the incorporated optimally planted mustard and winter-killed late planted forage radish cover cropped fields.  The stand in each treatment exceeded the targeted plant spacing of 3.7 plants per foot, perhaps due to the accidental placement of more than one plant per hole.  At harvest, there were no significant differences among treatments in stand.  The incorporated optimally planted mustard and winter killed late plant forage radish cover cropped fields had the same stand as they had at the 8-10 leaf stage, while the other fields had lower stands. 

Effect of brassica cover crops on onion plant size (Table 6table-6-case-study-plant-size)

Triple G Case Study:

In June when the onions were at the 3-5 leaf stage, the onions in the mustard cover cropped fields had significantly 2 more leaves per plant, were taller by 16 cm (=6.4 inches) and had a larger neck diameter by 2 mm compared to the bare ground field. There were no differences between the optimal and late plantings of mustard cover crops.  In July, at the 8-10 leaf stage, the late planted mustard cover cropped field had significantly 0.6 more leaves per plant and plants that were 3 cm taller than the optimally planted mustard cover cropped and bare ground fields.  At this time, neck diameter was significantly 3 mm larger in the mustard cover cropped fields than the bare ground field.  Generally, as seeding rate decreases, onion size increases, therefore, we cannot entirely contribute the increased plant size to the mustard cover crops, because the mustard cover crops were planted at a seeding density that was 25% less than the bare ground field.  In our bioassay, we observed more vigorous plant growth in the onions grown in the soil that was cropped to the mustard cover crops compared to the bare ground (Table 9table-9-triple-g-bioassay), so it is possible that some of the increased plant size was due to the effect of the mustard cover crops. 

Another important contributing factor to the large difference in plant size between the bare ground and mustard cover cropped fields is that we saw a general surge in onion growth during the week of June 16th in the Elba muck land.  For example in another one of our trials, onion leaf stage jumped from 3.5 leaves per plant to 5.5 leaves per plant between Jun-13 and Jun-20; normally, onions grow at a rate of about 1 leaf per week.  Even though all fields were evaluated within 3 days of the same number of days after plating, the mustard cover cropped fields were evaluated 9 days after the bare ground field on Jun-25, which was after the general surge in plant growth.  There were no significant differences in plant size between the optimal and the late planted mustard cover cropped field in this case study, unlike in the bioassay where we saw a slight skew towards more advanced growth stages in the optimally planted mustard cover cropped field. 

Mortellaro Case Study.

At the 8-10 leaf stage, there were no differences among fields for number of leaves per plant. The incorporated optimally planted mustard planting and winter-killed late planted forage radish cover cropped fields had significantly the tallest plants by 0.7 to 2.3 inches and widest neck diameters by 0.5 to 2.1 mm than the other treatments.  This is likely a plant spacing effect, because these fields also had significantly reduced stands by 5%. Generally, as plant spacing increases, plant size also increases.  There were no differences in onion plant size between the winter-killed late planted mustard cover crop field and the incorporated late planted mustard cover cropped field.  The incorporated optimally planted mustard cover cropped field had plants that were significantly 0.8 inches taller with necks that were significantly 2.1 mm wider than the incorporated late planted mustard cover cropped field.  The winter-killed forage radish cover cropped field had significantly 3.9 cm taller plants with significantly thicker necks by 1.5 mm thicker necks than the incorporated late planted mustard cover cropped field.  Although not significantly different, winter killed late planted forage radish cover cropped fields had numerically larger plants than the winter killed late planted mustard cover cropped fields. These results suggest that very late planting of mustard cover crop (Sep 26) does not improve plant size compared to optimal mustard cover crop establishment.  The late planting of forage radish resulted in similar sized plants as the incorporated optimally planted mustard crop.

Effect of brassica cover crops on onion yield and bulb size distribution (Table 7table-7-case-study-yield-bulb-size)

Triple G Case Study.

The bare ground field had significantly higher marketable yield by 8-11%, twice as many boilers, 2-4 times as many small bulbs, and twice as many medium and jumbo bulbs than the mustard cover cropped fields. There were no significant differences among the optimal timing and the late planted mustard cropped fields.  Generally, as seeding rate increases, total yield increases with the bulb size distribution skewing towards smaller sized bulbs.  The increased yield and smaller bulb sizes that occurred in the bare ground field was in part a function of the higher seeding rate; a 31% increase in seeding rate resulted in a 35% increase in final stand and a 12% increase in total yield. 

Mortellaro Case Study.

Although there were no significant differences in total yield, the incorporated optimally planted mustard cover cropped field had numerically the highest yield, which was about 13% or 76 cwt/A higher than the other treatments. The incorporated late planted mustard cover crop field was skewed towards a smaller bulb size distribution and had significantly the highest boiler and medium bulb weights by 3-times and 50%, respectively, and significantly 40% fewer jumbo sized bulbs and numerically 11.4% reduced yield than the optimal planted mustard.  These results suggest that the potential yield improvements of an incorporated mustard cover crop are lost when planting the cover crop is delayed 37 days past the optimal planting timing (August 10 to 20), or when it is planted slightly late (7 days) and not incorporated.  Similarly, planting a forage radish crop 10 days late did not result in as good yield as incorporated optimally planted mustard cover crop. 

Effect of brassica cover crops on bacterial bulb decay of onion (Table 8table-8-case-study-disease)

Triple G Case Study.

In general, incidence of bacterial bulb decay was very low at less than 5%. There were no significant differences in total bacterial bulb decay (out of field and storage), although numerically the mustard cover cropped fields had higher levels of decay (bare ground: 2.4%; late planting: 3.5%; optimal planting: 4.9%). Incidence of Botrytis neck rot was less than 2% and there were no differences among fields.

Mortellaro Case Study.

Highest incidence of total bacterial bulb decay (out of field and storage) occurred in the winter killed late planted forage radish cover cropped field (5.3%), which was only significantly higher than the incorporated late planted mustard cover cropped field (1.9%). There were no significant differences among the other fields.  There were no significant differences among fields for incidence of Botrytis neck rot, which ranged from 6.5 – 12.4%.

Our results indicated that brassica cover crops had no effect on bacterial bulb decay or Botrytis neck rot of onion.

 

Objective 8: Effect of brassica cover crops on damping off disease of onion

Triple G soil bioassay (Table 9table-9-triple-g-bioassay; Figure 3figure-3-triple-g-bioassay-vigor).

Seeds planted in soil from the bare ground field had significantly 15-18% and 13-17.5% lower emergence than the mustard cover cropped fields and a field cover cropped to cereal, 2 weeks and 4 weeks post-sowing, respectively.   Seeds planted in soil from the bare ground field had significantly 16-19% and 14-16% fewer healthy plants than the other treatments at 2 weeks and 4 weeks post-sowing, respectively.  There were no significant differences in emergence or incidence of healthy plants among the cereal and mustard cover cropped fields at any evaluation.  There were also no significant differences in incidence of damping off at 2 and 4 weeks post sowing, which totaled 13-18%. 

Two weeks post sowing, there were significant differences in the growth of the healthy plants with a skew towards more advanced stages in the plants grown from the field cropped to optimally planted mustard cover crop.  This treatment had significantly about half as many plants in the poke stage compared to the other treatments, significantly 10% and 17.5% more plants in the flag leaf stage than late planted mustard cover crop and cereal cover crop, and significantly the highest proportion of plants starting the first leaf (3.7% compared to 0.4%, 0.3% 0.8%).  Onion seedlings grown in soil from the optimally planted mustard cover cropped field had 51.8% of its healthy plants in the flag leaf stage, which was at least 10% more than any of the other treatments.

These results suggest that when onions are sown following an optimally planted mustard cover crop that stand and vigor would be improved over a bare ground field. Although emergence and proportion of healthy plants grown in soil from the late planted mustard cover crop and cereal cover crop were similar to the optimally planted mustard cover crop, the healthy plants were not as vigorous (less advanced growth stages).  The seedlings grown in soil from the late planted mustard cover crop had 10% more plants in the flag leaf stages than those grown in soil that was cereal cover cropped, suggesting a slight advantage of late planted mustard cover crop over cereal cover crop.  In this bioassay, improved seedling vigor was not related to differences in damping off, as levels of this disease were insignificant among treatments.  Improved vigor was significantly less when the mustard cover crop was planted 10 days past the optimum.

Mortellaro Soil Bioassay (Table 10table-10-mortellaro-bioassay).

Overall emergence of the onions planted in Mortellaro soils were lower than the Triple G fields, which was most likely due to the soil being wetter at the time of sowing. The seeds planted in soil collected from the incorporated late planted mustard field had the highest seedling emergence 2.5 (55%) and 4 weeks (61%) post planting, which was significantly 20% (2.5 week) and 25% (4 week) higher than the incorporated optimally planted mustard cover cropped field.  It was also significantly 29% (2.5 week) and 35% (4 week) higher than the winter-killed late planted mustard cover cropped field.  Winter-killed late planted forage radish had numerically the second highest emergence, but was not significantly different than any of the other treatments. Two and half weeks post seeding, incorporated late planted mustard also had the highest proportion of healthy plants (51%), which was significantly 20% higher than incorporated optimally planted mustard field and 29% higher than winter-killed late planted mustard.  Winter-killed late planted forage radish had the second highest number of healthy plants, but was not significantly different than any of the other treatments.  At the 4 week evaluation, differences among treatments were not significant, but they followed the same trends.  There were no significant differences in growth stage of the healthy plants among the treatments; numerically the treatment with the highest emergence rate (incorporated late planted mustard) was also skewed towards the more advanced crop stages.

At 2.5 weeks post seeding, there were no differences among treatments for incidence of damping off or stress related seedling mortality, which ranged from 1.6 to 3.0% and 0.7 to 0.9%, respectively. At the 4 week evaluation, total incidence of damping off ranged from 7 to 16%; although there were no significant differences, the winter-killed late planted mustard had numerically the highest level of damping off.  

These results indicated that when incorporated, planting a mustard cover crop 26 days past the optimal timing did not result in significantly reduced seedling vigor as measured by emergence and growth stage. When mustard cover crop was planted late by 7 days and then left to winter-kill, there was no improved emergence or plant vigor.  Generally, the late planted winter-killed forage radish resulted in similar emergence and seedling vigor as the incorporated mustard treatments.

 

Objective 9: Effect of mustard cover crop on plant population and potential to reduce seeding rate in direct seeded onions

Planting density study at Triple G (Table 11table-11-small-plot-seeding-rate-triple-g).

Significant differences among treatments occurred in medium bulbs and final stand. Despite having the highest seeding rate, at final harvest the standard treatment had the same number of seeds per foot (4.8) as the 17% reduced seeding rate (4.6), and there were no significant differences between these two treatments, although numerically the 17% reduced seeding rate (645 cwt) yielded 35 cwt (=5.7%) higher than the standard (610 cwt).  In the case study, this field (6.4 seeds/ft) yielded 627 cwt with very similar bulb size distribution as the 17% reduced seedling rate (6.4 seeds/ft).  The 38% reduced seeding rate had significantly 26% less final stand (3.4 seeds/ft), significantly 50% fewer medium bulbs and numerically 16% lower total yield than the standard/17% reduced treatments, which suggests that lowering the seeding rate by 25% to 4.8 seeds/ft from 6.4 seeds/ft was too much.  Even though this reduced seeding rate (4.8 seeds/ft) did well with 66% of its bulbs being jumbos compared to only 50% being jumbos in the 6.4 seeds/ft treatment, there simply were not enough bulbs to make as high a yield.  The 6.4 seeds/ft treatment grossed 16% (=$1,615/A) more than the 4.8 seeds/ft treatment; even after the cost of seed was taken into consideration, the 6.4 seed/ft treatment net 15% (=$1,460/A) more than the 4.8 seeds/ft treatment (Table 15). 

 

Objective 10: Effect of mustard cover crop on potential to increase population of transplanted onions

Plant Population Study at Mortellaro (Table 12table-12-small-plot-poulation-study-mortellaro).

There were no differences among treatments at harvest for total yield and bulb size distribution, except in the medium size bulb class, where the standard plant population (128,937 plants/acre) and the 21% higher plant population were not significantly different than each other, but had significantly 18% to 46% fewer medium bulbs than the 38% higher and 62% higher plant populations. Not surprisingly, the higher planting populations skewed towards smaller bulb size distribution.  The 62% higher plant population had numerically the highest yield (515 cwt/A); compared to the standard (493 cwt), it had 40% more medium bulbs and half as many jumbos, which resulted in only a 4.5% increase in total yield.  Compared to the standard, the 21% higher plant population (483 cwt) resulted in 23% more medium bulbs and 21% fewer jumbo bulbs, which resulted in only a 3.4% decrease in yield.  With 2.5 times more small bulbs, 1.5 times more medium bulbs and half as many jumbo bulbs, the 38% higher plant population (453 cwt) resulted in an 8% total yield reduction.  In the case study, this field (incorporated optimally planted mustard) yielded 647 cwt/A; perhaps the difference being that the plant population study was located towards the edge of the field where the musk was shallower.  These results indicate that plant population could be increased by 21% to 157,000 per acre without any significant reduction in yield or bulb size distribution.

 

Research conclusions:

Benefits of incorporated optimally planted mustard cover crop: improved plant vigor and yield, and nutrient cycling resulting in reduced fertilizer inputs.

In our Triple G case study, we compared direct seeded onion crops following incorporated and optimally planted mustard cover crop. The grower cooperator seeded the mustard cover cropped fields at a 25% reduced seeding rate (6.4 seeds/ft) compared to the bare ground field (8.4 seeds/ft), because research results from Michigan State University indicated that seeding rate following mustard cover crop could be reduced by at least 10%.  The difference in seeding rate made it impossible to directly compare bare ground to mustard cover cropped fields.

Prior to planting onions, there were significant differences in soil parameters among the fields.  The soil bioassay showed that soil collected from the optimally planted mustard cover cropped field had significantly higher emergence by 15% and higher proportion of healthy plants by 14%.  Of the healthy plants, the ones grown in the soil from the optimally planted mustard cover cropped field were more vigorous than those grown in the bare ground soil, with significantly 12times- and 2.3-times more plants in the flag and loop stages, respectively, 2 weeks after seeding.  These results suggest that an optimally planted and incorporated mustard cover crop could improve yields of direct seeded onion, similar to our preliminary study in 2013 where the mustard cover crop resulted in a 28% increase in yield and 37% increase in net return, valued at $1,477/A due to 3.5 times more jumbo yield (=130 cwt/A).

Pre-plant soil tests results showed that compared to bare ground, the optimally planted mustard cover cropped field had significantly 36% (=14 cwt/A) more potentiall mineralizable nitrogen (PMN), 39% (=365 cwt/A) more available potassium (K) and numerically 40% (=13 cwt/A) more manganese (Mn).  Cornell recommends 100 to 120 lb/A of nitrogen for onions.  Based on this case study, theoretically the rate of nitrogen fertilizer could conservatively be reduced by 25% (= 20 – 30 lb/A) when onions follow an incorporated optimally planted mustard cover crop, representing a savings in fertilizer cost of $5/A ($400/ton urea).  At Triple G, levels of K were very high and no additional K would have been required.  In a field where K is low (220 – 370 cwt/A), theoretically a 39% increase in K (= 89 – 144 lb/A) could bring soil levels to medium, which would require an input of 100 lb/A of K instead of 150 lb/A for a savings in fertilizer costs of 50 lb and $17 per acre ($685/ton of potash).  In a previous NESARE Partnership project (ONE10-116), we learned that there was no yield response to Mn when soil levels exceeded 20 lb/A.  At Triple G, levels of Mn were sufficient.  In a field where the soil level of Mn is less than 20 lb/A, theoretically a 40% increase from incorporated optimally planted mustard cover crop could eliminate need for additional inputs at up to $58/A (dry fertilizer pre-plant at 4 lb Mn/A @ $3.58/lb and 2 applications of 2 lb foliar @ $24.66 per app).

Planting of incorporated mustard cover crop can be delayed 10 days to Aug-30 and still gain some benefit over bare ground in following direct seeded onion crop:

At Triple G, we compared delaying establishment of a mustard cover crop by 10 days (Aug 30) to an optimally planted mustard cover crop (Aug 20) on direct seeded onions. In the field, there was no difference between the onions in these two fields in stand, plant size, total yield or bulb size distribution, except at the 8-10 leaf stage, where the onions in the late planted mustard had significantly 0.5 more leaves per plant and 3.1 cm taller plants than those in the optimally planted mustard field.  In the pre-plant soil bioassay, seedlings grown in soil from the late planted mustard cover cropped field had similar emergence and proportion of healthy plants as the optimally planted mustard field, except that the healthy plants were not as vigorous and had significantly 1.7 times more plants in the poke stage and only 21% as many plants in the flag leaf stage as the optimally planted mustard soil.  The late planted mustard had statistically 15% higher emergence and 24% more healthy plants than the bare ground, but its plant vigor was not significantly different than the bare ground.  Prior to planting the onions, there was no difference in available K between the late and optimally planted mustard cover crop fields and the late planted mustard cover crop had significantly 23% higher K (= 217 lb/A) than the bare ground field.  If soil levels were low (220 – 370 lb/A), a 23% increase in K following a late planted mustard cover crop would only result in a reduction of K inputs if the available K was initially 300 lb/A or higher.  The late planted mustard cover crop did not result in higher levels of Mn as the optimally planted cover crop did.  However, it did have significantly 81% higher amounts of potentially mineralizable nitrogen (PMN) (= 32 lb/A) compared to the bare ground field, which was not significantly different than the optimally planted mustard cover cropped field.  By mid-season, the late planted mustard field had significantly 19.5% less available NO3-N (=12.2 lb/A) than the optimally planted mustard cover cropped field, but was still 58% higher (= 19 lb/A) than the bare ground field.  By mid-season, there was no difference in potential nitrogen use per plant between the late and optimally planted mustard cover cropped fields. Thus, even a mustard cover crop that is planted 10 days later than the optimal timing (Aug 30) would result in increased available nitrogen compared to bare ground, and theoretically nitrogen inputs could be reduced by a conservative 25% (= 20-30 lb/A) at a savings of $5/A.  These results suggest some benefit to a direct seeded onion crop following a later planted mustard cover crop, but not to the same extent as one that is planted by Aug-20.

At Mortellaro, we compared delayed planting of mustard cover crop by 37 days (Sep-26) compared to optimally planted mustard cover crop (Aug-7) in transplanted onions. Total onion yield of the late planted mustard cover cropped field (571 cwt/A) was numerically 11.7% less (= 76 cwt/A) than the optimally planted field (647 cwt/A) and it was skewed towards a smaller bulb size distribution and had significantly 2.8 times higher boiler yield, 1.5 times higher medium bulb yield and only 60% of the jumbo yield. In the pre-plant soil bioassay, the seeds planted in soil collected from the late planted mustard field had significantly 68% higher emergence and numerically 67% more healthy plants 4 weeks after seeding compared to the optimally planted mustard cover cropped field.  However, there were no differences in seedling vigor as measured by development stage of healthy plants 2.5 weeks post seeding.  In the field at the 8-10 leaf stage, the late planted mustard cover crop had numerically 0.2 fewer leaves, significantly 2.2 cm shorter plants and a significantly 2.1 mm smaller neck diameter than the optimally planted mustard cover crop. These results suggest that the potential yield improvements of an incorporated mustard cover crop are lost when planting the cover crop is delayed 37 days past the optimal planting timing (August 10 to 20). By not planting a mustard cover crops at this time, growers could save $111 per acre in cost of cover crop seed and establishment.

Differences in inherent pH and organic matter among the fields at Mortellaro made it challenging to determine treatment effects beyond differences in nutrient availability related to pH and %OM. Particularly, there was a strong correlation between pH and available magnesium (Mg) (Pearson: R = 0.9484; p = 0.0000), but also with Calcium (Ca) (Pearson; R = 0.6634; p = 0.0187).  And there were also correlations between %OM and Mg (Pearson: R = -0,6380; p = 0.0256), Mn (Pearson: R = 0.6241; p = 0-0301) and Fe (Pearson: R = -0.7882; p=0.0023).  Significant differences between optimally and late planted mustard cover crops were attributed to differences in pH and OM and not the planting timing of the cover crops.  There were no differences in pre-plant PMN, available NO3-N mid-season or potential nitrogen use per plant between late and optimally planted mustard cover cropped fields, although numerically the late planted field had double (=16 lb/A) pre-plant PMN and 15% (= 0.9 lb/10,000 plants) more potential nitrogen used per plant than the optimally planted mustard cover cropped field. Theoretically, rate of applied nitrogen in onion crop following late planted mustard cover crop could be reduced by 25% (=25-30 lb/A) for a savings of $5/A.

Incorporation of mustard cover crop essential to improve onion yield

At Mortellaro, we compared allowing late planted (by 7 days) mustard cover crop to winter-kill to incorporated late planted (by 37 days) and optimally planted (on Aug 7) mustard cover crops. In the field, the winter-killed mustard cover crop had statistically the same yield (573 cwt/A) and bulb size distribution as the incorporated late planted mustard cover crop (571 cwt/A), which were numerically 11.7% less (= 76 cwt/A) than the optimally planted field (647 cwt/A) and skewed towards smaller bulb size distribution.  The winter-killed late planted mustard cover crop had 67% of the medium bulb yield and 39% higher jumbo bulb yield than the incorporated late planted mustard cover crop.  In the pre-plant soil bioassay, the winter-killed late planted mustard cover crop had significantly 29% lower emergence and 40% fewer healthy plants that were less vigorous than the incorporated late planted mustard cover cropped field, but was similar to the incorporated optimally planted mustard cover crop.  In the field in mid-season, the winter killed late planted mustard cover crop was similar in size the incorporated late planted mustard cover crop, which were slightly smaller than the incorporated optimally planted mustard cover cropped field.  In the pre-plant bioassay, the incorporated late planted mustard cover cropped soil had 36% higher emergence and 28% more healthy plants that were more vigorous than the winter killed late planted mustard cover crop.  There were no differences in pre-plant PMN, mid-season available NO3-N or potential nitrogen used per plant between the winter killed late planted and incorporated late planted mustard cover cropped fields, which were slightly higher than the incorporated optimally planted mustard cover cropped field.  These results suggest that there would not be an increase in yield if a mustard cover crop was not incorporated and left to winter kill. Theoretically, the rate of nitrogen used in the following onion crop could be reduced by 25% (=20-30 lb/A) for a savings of $5/A. However, the cost of cover crop establishment ($75/A) exceeds the savings in nitrogen fertilizer.  

Forage radish planted 7 days late (Aug 27) as good as incorporated optimally planted mustard:

At Mortellaro, we compared winter killed late planted (by 10 days) forage radish to incorporated optimally- and late- (by 37 days) planted mustard cover crops. There were no significant differences in total yield between late planted forage radish (576 cwt/A) and incorporated late planted mustard (571 cwt/A), which was numerically 11% (=71 cwt/A) less than the incorporated optimally planted mustard cover cropped field.  Bulb size distribution was the same as the incorporated optimally planted mustard cover cropped field, which was skewed towards larger bulb size distribution than the incorporated late planted mustard cover cropped field.  Similarly, there were no differences in plant size in mid-season between winter killed late planted forage radish and incorporated optimally planted mustard.  In the pre-plant soil bioassay, forage radish had the second highest emergence and healthy plants and the most vigorous plants, which were not significantly different than the optimally planted mustard cover crop.  There were no differences among treatments for pre-plant PMN, mid-season available NO3-N and potential nitrogen use per plant (although winter killed late planted forage radish numerically had the highest).  These results suggest that winter killed late planted (by 7 days) forage radish would offer yield benefits to following onion crop similar to incorporated optimally planted mustard cover crop. Forage radish can be established for 35% less (=$72.60/A) than mustard cover crop, because it does not need to be incorporated.  Leaving the foliage to winter kill also provides protection from wind erosion.  

Process of incorporating cover crops in late fall resulted in less soil compaction:

At Triple G, it appeared that differences in pre-plant soil compaction may have masked any treatment effects. At Mortellaro, it appeared that the process of incorporating the cover crops using tillage in late fall resulted in an impressive reduction in soil compaction throughout the soil profile compared to leaving the cover crop undisturbed.  At the 1-6” depth, the winter killed late planted mustard cover cropped field (125 psi) had significantly the highest soil compaction at 1-6” depth as measured by a penetrometer, which was about 5-times higher than the two incorporated treatments (optimal mustard: 27 psi; late mustard: 22 psi) and significantly double the winter killed late planted forage radish field (73 psi).  At the 7-12” depth, the two fields where the cover crops were incorporated had significantly 41 to 52% (= 59 – 77 psi) less soil compaction compared to the fields where the cover crops were winter killed.  Had the forage radish been planted earlier, the roots may have penetrated deeper and also reduced compaction at the 7-12” depth.

Brassica cover crops had no effect on damping off, bacterial bulb decay or Botrytis neck rot:

In the pre-plant soil bioassays, there were no differences among treatments for incidence of damping off, which ranged from 13 to 18% at Triple G and from 7 to 16% at Mortellaro. At Triple G, there was also no difference in % stand (of seeding rate) among treatments that would suggest any differences in damping off seedling diseases.  Incidence of bacterial bulb decay was very low at 5% or less.  There were no significant differences among treatments at Triple G, although numerically the onions following the mustard cover crops had higher levels of bacterial bulb decay than the bare ground.  At Mortellaro, the onions grown in the field following the incorporated late planted mustard cover crop had the lowest incidence of bacterial bulb decay (2% rot), which was significantly only lower than the winter-killed late planted forage radish cover cropped field (5% rot).  At Mortellaro, there were also no differences in incidence of Botrytis neck rot, which ranged from 6.5 to 12%. These results indicate that use of brassica cover crops had no effect on incidence of damping off seedling diseases, bacterial bulb decay or Botrytis neck rot.

The seeding rate can be reduced in direct-seeded onions following a mustard cover crop:

In the small plot field trial located at Triple G in the field where mustard cover crop was planted on time (Aug-20) and incorporated in late fall, a 17% reduction in seeding rate from the standard from 7.7 to 6.4 seeds/ft resulted in a total yield of 645 cwt/A. With 50% of the bulbs being jumbos, this grossed $11,909/A and after the cost of seed was taken into consideration, it netted $11,297/A.  Unfortunately, the standard treatment had the same number of bulbs as the 17% reduced rate treatment at harvest, so a proper comparison could not be made. However, 645 cwt/A and $11,909/A (=$18.46/cwt) is twice as much as the average onion yield in New York (313 cwt/A) and 12% higher than the average price ($16.52/cwt). Therefore, theoretically, growers should be able to reduce their seeding rate by 17% or more, resulting in savings of seed costs of at least $122/A (7.7 seeds/ft to 6.4 seeds/ft).

Plant population cannot be increased in transplanted onions following a mustard cover crop:

In a small-plot field trial located at Mortellaro in the field where mustard cover crop was planted on time (Aug 7) and incorporated in late fall, 21% increase in plant population from 128,937 plants/A to 156,817 plants/A resulted in numerically 23% more medium bulbs and 21% fewer jumbo bulbs, which resulted in only a 3.4% (=10 cwt/A) decrease in yield from 493 cwt/A to 483 cwt/A. The 21% higher plant population ($8,492/A) grossed only 4% less (=$368/A) than the standard ($8,860/A).  After the cost of transplants was taken into consideration, it ($6,860/A) netted 11% less (=$799/A) than the standard ($6,860/A).  Whether a mustard cover crop could support a plant population that is higher than the standard to a lesser extent than 21% warrants further study.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

Outreach:

Hoepting, C.A. 2014. Benefits of using mustard cover crops in muck onion production.  Annual Elba Muck Onion Twilight Meeting.  Elba, NY, USA: August 7, 2014 (29 participants).

Due to the delay in analysis of the results of this project, new information on use of brassica cover crops has not been available. Now this this project has been completed, we will disseminate the results of this study with onion growers via newsletter article(s) and presentation at grower meetings, as well as via direct consultation. 

Project Outcomes

Project outcomes:

Triple G Case Study (Table 13table-13-eco-analysis-triple-g-case-study):

In the Triple G case study, the grower planted the mustard cover cropped fields at a 25% reduced seeding rate in response to recommendations from Michigan State University. The higher seeding rate in the bare ground field had input costs that were $192/A more than the mustard cover cropped fields, even without the added cost of a mustard cover crops ($106/A).  The bare ground field had the highest yield by 9% and 12% compared to the mustard treatments.  However, the optimally planted mustard cover cropped field had almost double the number of jumbo bulbs, which draw $20/cwt instead of $16/cwt or $10/cwt for medium and small bulbs, respectively.  Consequently, the optimally planted cover cropped field grossed almost the same as the bare ground field, and with the slightly lower input costs it netted $76/A more than the bare ground.  Additionally, the results of our study suggest that the rate of nitrogen could be reduced by 25% (20-30 lb/A) when onions followed an optimally planted mustard cover crop, thus saving an additional $5/A in input costs over the bare ground.

In the field where establishment of the mustard cover crop was delayed by 10 days (Aug 30), there was no difference in yield or bulb size distribution compared to the optimally planted mustard cover crop. Consequently, the late planted mustard cover crop netted an insignificant 4.3% (=$479/A) less per acre than the optimally planted mustard cover crop.  If the planting date of mustard cover crop could be delayed by 10 days to Aug-30, than theoretically, an additional 15% of acreage could be cropped to mustard cover crops.  A conservative 20% increase in yield (based on a 28% increase in yield with incorporated optimally planted mustard cover crop compared to bare ground at Mortellaro in 2013) with a conservative 10% reduction in seeding rate (our studies show seeding rate can be reduced 17% to 6.4 seeds/ft) on 1500 acres could result in an increase of 62.6 cwt/A and $1,034/A (based on average NYS yield, 313 cwt/A and price, $16.52/cwt) for a total of 93,750 cwt and $1.5 million while saving $110,100 ($73.40/A = 10% of 7.7 seeds/ft) in seed costs.   

Mortellaro Case Study (Table 14table-14-eco-analysis-mortellaro-case-study):

At Mortellaro, we compared delayed planting of incorporated mustard cover crop by 37 days (Sep-26) compared to optimally planted mustard cover crop (Aug-7) in transplanted onions. Total onion yield of the late planted mustard cover cropped field (571 cwt/A) was numerically 11.7% less (= 76 cwt/A) than the optimally planted field (647 cwt/A) and it was skewed towards a smaller bulb size distribution with only 60% of the jumbo yield. With such a difference in jumbo yield, the late planted mustard grossed and net 17% less or $2,045/A than the optimally planted mustard cover cropped field.

When we compared winter-killed late planted (by 7 days) mustard cover crop to incorporated late planted (by 37 days) and optimally planted (on Aug 7) mustard cover crops, the winter-killed mustard cover crop had statistically the same yield (573 cwt/A) and bulb size distribution as the incorporated late planted mustard cover crop (571 cwt/A), which were numerically 11.7% less (= 76 cwt/A) than the optimally planted field (647 cwt/A) and skewed towards smaller bulb size distribution.  Without the cost of incorporating the cover crop, the input cost of the winter-killed mustard was 32% less than the incorporated mustard cover crops, and so it grossed and net 5% or $531/A more than the incorporated late planted mustard cover crop.  Clearly, delayed planting of mustard cover crop by 37 days and not incorporating it did not improve economic return compared to incorporated optimally planted mustard cover crop.  The winter killed late planted forage radish had the lowest input cost ($38/A), because the cost of seed was the cheapest and it did not require incorporation, but because it did not yield as high as the incorporated optimally planted mustard cover crop or have as many jumbo bulbs, it also net and grossed 12% less or $1,509/A (gross)/$1,436/A (net).  

Potential to reduce seeding rate of direct seeded onions following incorporated optimally planted mustard cover crop, small-plot trial at Triple G (Table 15table-15-eco-analysis-small-plot):

In the small plot field trial located at Triple G in the field where mustard cover crop was planted on time (Aug-20) and incorporated in late fall, a 17% reduction in seeding rate from the standard at 7.7 to 6.4 seeds/ft resulted in a total yield of 645 cwt/A. With 50% of the bulbs being jumbos, this grossed $11,909/A and after the cost of seed was taken into consideration, it netted $11,297/A.  Unfortunately, the standard treatment had the same number of bulbs as the 17% reduced rate treatment at harvest, so a proper comparison could not be made. However, since the standard treatment was skewed towards smaller bulb sizes and had 14% fewer jumbos than the 17% reduced rate, it had a lower total yield and gross by 5% (=35 cwt/A) and 7% (=$817/A), respectively.  After the higher cost of seed was taken into consideration the standard net 8% (=$938/A) less than the 17% reduced rate.  The 38% reduced seeding rate had fewer bulbs and 17% (=105 cwt/A) lower yield and 13.6% (=$1,615/A) lower gross than the 17% reduced seeding rate.  Even after the lower cost of seed was considered, it still net 135 (=$1,460/A) less than the 17% reduced rate.   

Potential to increase plant population of transplanted onions following incorporated optimally planted mustard cover crop, small-plot trial at Mortellaro (Table 15table-15-eco-analysis-small-plot):

In a small-plot field trial located at Mortellaro in the field where mustard cover crop was planted on time (Aug 7) and incorporated in late fall, 21% increase in plant population from 128,937 plants/A to 156,817 plants/A resulted in numerically 23% more medium bulbs and 21% fewer jumbo bulbs, which resulted in only a 3.4% (=10 cwt/A) decrease in yield from 493 cwt/A to 483 cwt/A. The 21% higher plant population ($8,492/A) grossed only 4% less (=$368/A) than the standard ($8,860/A).  After the cost of transplants was taken into consideration, it ($6,860/A) netted 11% less (=$799/A) than the standard ($6,860/A).  The 38% and 62% higher plant populations were skewed towards smaller bulb size distributions.  The 62% higher plant population had a total yield that was 6% higher than the 21% plant population, but with 42% less jumbos, it net the same as the 21% higher plant population; after the higher cost of transplants was taken into consideration, it net 20% or $1,200/A less than the 21% higher plant population.

Farmer Adoption

In general, interest in use of brassica cover crops in muck-grown onions has dissipated as grower attention has shifted to herbicide use in the fall to manage problem perennial weeds. Examples include use of Dual Magnum, which is applied in late fall to control yellow nutsedge, and use of burn down herbicides such as Roundup, 2,4-D and dicamba that are also applied in the fall to control perennial sow thistle and creeping yellowcress.  All of these management programs were derived from research conducted by Hoepting in response to grower needs.  Unfortunately, these management strategies that involve herbicide use in the fall are in direct conflict with establishment of brassica cover crops that also need to be established and incorporated in the fall.  Ideally, once these problem weeds are under control, muck onion growers will be able to revisit and eventually expand use of brassica cover crops as our research has clearly demonstrated their benefits.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

In this project, we were not able to accurately assess the impact of the mustard cover crops compared to bare ground soil due to the confounding variable of the different seeding rates, as our grower cooperator followed Michigan State University recommendations to use a lower seeding rate following mustard cover crops in onion. It would also be beneficial to compare effect of mustard cover crop to a cereal cover crop, which is also a very common practice in muck onion production.

In the direct seeded onion case study, our results indicated that benefit could still be derived when incorporated mustard cover crop was planted 10 days later than optimal timing. In the transplanted onion case study, delaying planting of mustard cover crop by 37 days did not result in any benefit. Thus, further research is required to determine the extent to which planting of mustard cover crop can be delayed to obtain worthwhile benefit; for example, 10, 15, 20 or 25 days.

Our seeding rate study indicated that a 17% reduction in seeding rate of direct seeded onions following an incorporated optimally planted mustard cover crop could be feasible. Further research is warranted to fine-tune the degree to which the seeding rate can be reduced; for example, 10%; 15% and 20%.

Our plant population study indicated that incorporated optimally planted mustard cover crop could not support an increase in plant population of 21% or more. Further research is warranted to determine whether it could support an increase in plant population to a lesser degree; for example, 10% or 15%.

When delayed planting of mustard cover crop is determined to yield same worthwhile benefit as optimal planting does, further research is required to determine whether such treatments could support reduced seeding rates in direct seeded onion or increased plant population in transplanted onion.  

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.