Expanding the use of Brassica cover crops in onion production

Expanding the use of Brassica cover crops in onion production

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. Other cited benefits from using Brassica cover crops include increased yield, improved drainage and nutrient cycling.  The problem is that in New York, 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.  The main reason for low adoption is that growers are not aware of their potential benefits.  The goal of this project is to expand use of Brassica cover crops to most of the muck onion farms in NY so that they too can reap their benefits and enjoy sustainable production. 

In 2014, we conducted 7 in-depth on-farm case studies to determine the effects of planting date and incorporation of mustard on onion stand, yield, bulb size, nutrient cycling, compaction, and suppression of bacterial diseases. Small-plot trials were conducted at two locations to optimize onion plant population following Brassica cover crops.  A plug tray bioassay was conducted to determine if Brassica cover crops have an effect on diseases caused by soil-borne pathogens, emergence and seedling vigor.  Preliminary bioassay results indicated that mustard cover crops increased onion emergence by up to 18% and significantly improved seedling vigor, which did not appear to be related to damping off disease.  In the field studies, confounding variables made it challenging to interpret the effect of the mustard cover crops, although it appeared that less nitrogen could be applied following a mustard cover crop.  In 2016, data interpretation will continue and cost-benefit analysis conducted, all of which will be reported to onion growers. 

Objectives/Performance Targets

This project has received a no-cost extension to April 15, 2016 to complete data analysis and final report writing. The majority of the research was completed as planned by the end of 2014 and included in the 2014 annual report.  For this annual report, we will only comment on progress made in 2015.

Triple G case study:

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.

2015 progress: Bulbs were assessed for bacterial bulb decay on March 17, 2015 after 4 months in commercial storage. All data from this trial has been entered, analyzed and summarized, and the results section of the final report written.  Discussion needs to be completed.  One issue with this trial was that the grower planted the mustard cover cropped fields to a lower seeding density than the bare ground field, which made the true effect of the cover crops on yield challenging to determine.  The bare ground field also inherently had a higher pH than the two fields that were cropped to the mustard crops, which again make differences in soil nutrients challenging to interpret.

Mortellaro case study:

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) in transplanted yellow onions.

2015 progress: Bulbs were assessed for bacterial bulb decay on February 19, 2015 after 3 months in commercial storage. All data has been entered, analyzed and summarized for this study, but it has not yet been interpreted or written up.

Triple G soil bioassay:

This experiment was designed to evaluate the effect of the mustard cover crops on soil-borne pathogens, germination/emergence and seedling vigor. Soil was collected from the case study fields prior to soil disturbance and was used to fill one 200-cell plug tray per field rep and one raw onion seed per cell was sown.  The seedlings were grown in the outdoors with daily watering as needed.  

2015 progress: All of the data from this trial has been entered, analyzed, summarized and interpreted.

Mortellaro soil bioassay:

Same as for Triple G bioassay.

2015 progress: All of the data from this trial has been entered, analyzed and summarized. It has not yet been interpreted or written up.

Triple G plant population study:

Following optimum treatment of mustard cover crop, small-plot replicated trial was conducted to determine whether the mustards will allow a reduced seeding rate of direct seeded onions. 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

2015 progress: All of the data from this trial has been entered, analyzed, summarized and interpreted.

Mortellaro plant population study:

Following optimum treatment of mustard cover crop, small-plot replicated trial was conducted to determine whether mustards would allow a higher plant population in transplanted onions without reducing bulb size, thus increasing yield. Treatments included the grower standard (128,938 transplants/A) and 21%, 38% and 62% higher plant population.

2015 progress: All of the data from this trial has been entered and analyzed. It needs to be summarized, interpreted and written up.

Cost-benefit analysis has not been started for any of the trials or case studies.

Accomplishments/Milestones

Triple G soil bioassay, plant population study and case studies have been completed with respect to data entry, analysis, summary, interpretation and writing up. Only a little more work needs to be done on the discussion section of the case study.

Data has been entered and analyzed for the three Mortellaro studies. It has been summarized for the case study and bioassay, but none of the projects have been interpreted or written up.  This is scheduled for winter 2016. 

Evaluation of bacterial bulb decay out of storage. Hoepting and technician.  February 2015. Completed.

Complete data analysis; cost-benefit analysis; write and disseminate newsletter article highlighting what was learned in first year of study; add provisional recommendation to the Cornell cover crop website; project completed. Hoepting and Bjorkman.  March 2015. Re-scheduled for Winter 2016.

Submit Final report to NESARE. Hoepting.  June 15, 2016. On-track.

Impacts and Contributions/Outcomes

Triple G soil bioassay

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. 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 and managed mustard cover crop that stand and vigor will 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 and managed 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.

Triple G case study

Effect of mustard cover crop on pre-plant soil fertility.

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). However, the higher pH in the bare ground field was likely a result of tiling and bringing lower lying soil of higher pH to the surface. The bare ground field had significantly lower potassium than the mustard cover crop fields by 19 to 28%; and the optimally planted mustard cover crop field had numerically 10% higher potassium than the later planted mustard cover cropped field. Alternatively, the bare ground field had significantly higher calcium than the mustard cover cropped fields by 43 to 49%, and the late planted mustard had numerically 5% higher calcium than the optimally planted mustard. The optimally planted mustard cover cropped field had significantly 2.2 and 2-times higher chlorine than the late planted mustard cropped and bare ground fields, respectively, which were not significantly different from each other. Numerically, the optimally planted mustard cover cropped field had 40% higher manganese than the bare ground or late planted mustard cover cropped fields. The difference in calcium 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); calcium availability decreases as pH falls below 5.0. Whether differences in potassium, manganese, copper and chlorine are a treatment effect is unknown and needs to be investigated further.

Nitrogen.

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 more PMN than the bare ground, respectively. 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 (63 lb/A) than the bare ground field, which had 31. 8 lb/A. The late planted mustard cover cropped field also had significantly 59% more available nitrate-nitrogen (50.5 lb/A) than the bare ground field, but was significantly less than that in the optimal planted mustard cover cropped field. These results suggest that using a mustard cover crop would allow reduced nitrogen inputs, but needs to be investigated further.

Soil compaction.

Although it is possible that the fibrous root systems of the mustard cover crops reduced soil compaction, without side-by-side comparisons within the same field, our results do not definitively confirm this.

Stand.

Unfortunately, not all of the fields were planted to 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, 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. At the loop stage, emergence (% stand as a proportion of seeds planted) ranged from 90 to 92% and was not significantly different among the three fields. There were no significant differences in stand as a proportion of seed planted among treatments at the 3-5 leaf and 8-10 leaf stages or at harvest. The stand held constant from loop to 3-5 leaf (range: 87 to 90% of seed planted), but then dropped slightly at 8-10 leaf (range: 79 to 82% of seed planted) and 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%). Possible reasons for plant loss during the growing season include, heat stress, wind injury and damping off disease in the flag leaf stage, 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, 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.

Plant Size.

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 two timings 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 early 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. 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, so it is possible that some of the increased plant size is due to the mustard cover crops. There were no significant differences in plant size between the optimal and the late planted mustard cover crops 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.

Yield.

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 occurred in the bare ground field was in part a function of the higher seeding rate; a 20% increase in seeding rate resulted in a 28% increase in final stand and a 9% increase in total yield. Numerically, the optimal timing of the mustard cover cropped field had less total yield and jumbo bulbs, and more boilers, small and medium bulbs than the later planted mustard.

Bulb decay. There were no significant differences in total bacterial bulb decay (out of field and out of storage) or Botrytis neck rot (out of storage).

Triple G Case Study Conclusions:

There did not appear to be any differences between the optimal planting of mustard cover crop compared to delaying planting by 10 days. However, it was not possible to determine whether there was any benefit to the mustard cover crops over bare ground because the bare ground field was planted at a 20% higher seeding rate, which masked any differences that may have occurred with respect to stand, plant size and yield. The higher amounts of nitrogen in the soil in the mustard cover cropped fields indicate that there is potential for reduced nitrogen inputs and environmental contamination when onions are planted followed by a fall mustard cover crop.

Planting density study at Triple G

The 38% reduced planting rate resulted in about half as many medium bulbs as the standard and the 17% reduced seeding rate, and there were no significant differences between the standard and 17% reduced seeding rate. Medium bulb size was the only size class that was correlated with total yield (R = 0.5626; p = 0.0290). Numerically, the 17% reduced seeding rate resulted in 5.7% higher total marketable yield than the standard and 38% reduced seeding rates. There were no significant differences in the final stand between the standard and the 17% reduced seeding rate, which were both significantly 37-41% higher than the 38% reduced seeding rate. At harvest, the standard, 17% reduced seeding rate and 38% reduced seeding rate had 62.3%, 71.9% and 70.8% remaining stand of the original seeded rate.

In this study, when onions were grown following an optimally planted and managed mustard cover crop, a 20% increase in seeding rate from 6.4 to 7.7 seeds per foot resulted in a 4% increase in final stand and a 5% decrease in total yield. In the case study, when a seeding rate of 6.4 seeds per foot in mustard cover cropped fields increased to 8.4 seeds per foot in the bare ground field, this 31% increase in seeding density resulted in a 28% increase in final stand and a 9% increase in total yield (Table ??). The major difference between these scenarios is that there was a greater drop between the seeding rate and the final stand in the small plot trial (62.3% remaining) than in the case study (73% remaining), thus when yield was compared for initial seeding rates, the difference in final stand was 2.1 and 0.2 seeds/ft for the case study and field trial, respectively. Therefore, the similarities in total yield and bulb size distribution between the 7.7 seeds per foot and 6.4 seeds per foot treatments in the planting density study are most likely due to them having similar final stands. Had the final stands among treatments been proportionately reduced, we could fairly assess whether seeding rate should or could be effectively reduced following a fall mustard cover crop. It is unknown why a non-uniform drop in stand occurred among treatments in the trial; in the case study, the stand of two planting rates dropped uniformly.

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