Managing Cover Crop and Conservation Tillage Systems To Enhance Vegetable Crop Yields, Economic Returns and Environmental Quality

Managing Cover Crop and Conservation Tillage Systems To Enhance Vegetable Crop Yields, Economic Returns and Environmental Quality

Summary

In this on-farm research and education project, we continued the development of conservation tillage production systems for vegetable crops. Our focus was on evaluating cover crop mixtures for increasing sweet corn yield and for improving efficiency of strip-tillage operations. We conducted eight on-farm trials on five farms in the Willamette Valley. In strip-tillage trials on four farms where good cover crop establishment and growth occurred, the oat-legume mixture increased average corn yields by an average of 1.6 tons per acre (18%) compared to the non-cover cropped plots. After factoring costs of cover crop establishment, the oat-legume cover crop increase net profit by $86/acre compared to the non-cover cropped fallow treatment. The oat-legume cover crop also increased corn yield over the straight oat cover crop by 1.7 tons/acre, a 19% increase. The phacelia-legume cover crop also increased yield compared to fallow and straight oats, but to a lesser degree (7% and 8% increase respectively).
In the two fields where reduced tillage was used instead of strip-tillage (same farm, two years), the oat cover crop produced the highest graded yields, producing 1.1 tons (12%) more than the no-cover crop, fallow plots. The oat-legume cover crop also produced more corn than the fallow plots, but only by 6%.
Mechanical methods of cover crop suppression were investigated for organic production, with a focus on a phacelia-vetch cover crop mixture. Flail mowing gave excellent kill of the cover crop, however rolling with a ring-roller killed the phacelia and only partially suppressed the vetch in the mixture.

Objectives/Performance Targets

1. To enhance farmers’ ability to select and manage cover crops in conservation tillage vegetable crop production systems.

   To develop and evaluate conservation tillage practices for sustainable and organic farming systems.

Accomplishments/Milestones

Objective 1. To enhance farmers’ ability to select and manage cover crops in conservation tillage vegetable crop production systems
Alternative cover crop species and mixtures were evaluated on several farms in the Willamette Valley. Cover crop treatments were identical on all farms and included (1) oats (2) an oat-legume mixture (oats, common vetch, and crimson clover), (3) a phacelia (Phacelia tanacetifolia)-legume mixture (phacelia, common vetch and crimson clover), and (4) no cover crop (naturally occurring weeds). In the first year of the study, cover crops were planted in seven on-farm trials from Sept. 5 to Oct. 28, 2002. In the second year of this experiment, seeding rates were adjusted slightly (based on results from the first year). Cover crops were planted on four farms from Sept. 19 to Oct. 25, 2003. Individual treatment blocks varied from 2-4 acres to permit the use of commercial harvesting equipment to obtain realistic assessment of crop yield. Each field represented a single replication in a randomized block experimental design.
Cover crop biomass was estimated (see below) in the spring of 2003 and 2004 within a week) of the cooperating grower’s decision to kill the cover crop with glyphosate. Cover crop sampling occurred from April 3 to April 22, 2003 and May 23 to June 13, 2003. In all fields except one, a strip tillage system was used to prepare a seedbed. Sweet corn was planted in May and June each year using the growers’ planting equipment (see Table 1 for specific varieties, planting and harvest dates).

Data Collection:
Cover crop biomass was estimated by randomly selecting 4-6 locations within each cover crop block by tossing a 0.25m2 aluminum quadrat. The quadrat was worked through the cover crop foliage to the soil surface and the foliage clipped within the quadrat. Individual cover crop species within the mixtures were separated into paper bags and returned to laboratory for drying and weighing. Samples were taken to the OSU Central Analytical Laboratory for analysis of percent carbon and nitrogen.
Corn yield was determined using the participating growers’ commercial harvesting equipment. Corn was hauled to the Norpac processing facility where harvest weights and quality grades were determined. Harvested plot areas in the field were measured to calculate crop yields. Cover crop seed costs were obtained from the seed suppliers for the economic analysis.

Results
Of the original seven fields where cover crops were planted in the fall of 2002 (year 1), cover crop samples were taken in only four fields in the spring of 2003. Cover crop establishment was very uneven in two of the fields and were dropped from the trial. In another field the cover crop was killed with herbicide very early in the season before biomass estimates were obtained. In 2004, one of the trial corn fields was inadvertently harvested before yield data could be taken.
Corn yield response to cover crop treatments varied across the farms, however several trends are suggested by the data. In the four strip-till fields where good cover crop establishment and growth occurred (Table 1), corn yields were increased by 1.6 tons per acre (18%) following the oat-legume mixture average compared to the non-cover cropped, fallow plots (Fig. 1). Corn yields were also higher in the oat-legume cover crop plots compared to the oat only cover crop by 1.7 tons/acre, a 19% increase. The phacelia-legume cover crop also increased yield compared to fallow and straight oats, but to a lesser degree (7% and 8% increase respectively). Corn yield was highly correlated to the percent usable corn based on the Norpac cannery grading standards (Fig. 2). Average cob weight was a primary factor in determining final grade (Fig. 3).
In the two fields (same farm, two years) where reduced tillage was practiced rather than strip-tillage, the straight oat cover crop produced the highest graded yields, producing 1.1 tons (12%) more than the no-cover crop, fallow plots (Table 2, Fig. 4). The oat-legume cover crop and the phacelia-legume treatments produced identical graded yields (9.4 tons/acre), only 3% higher than the fallow (8.9 tons/acre). In the first year of the study, however, the seeding rate of phacelia in the phacelia-legume mixture was too high and prevented any growth of the legume. Since the potential “legume benefit” was missing from that treatment, few firm conclusions can be made about the effect of cover crops on corn yield in the minimum tillage situation.
One of the strip-till fields in the first year of the trial was heavily infested with symphylans, a soil-dwelling arthropod that feeds on crop roots. Considerable damage occurred to the corn plants (Fig. 6). In this field the oats in the oat-legume treatment crowded out the legume during establishment, thus there was no true “oat-legume” treatment. Slightly higher corn yields were produced in the no-cover crop, fallow plot compared to any of the cover crop treatments (Table 3, Fig. 5).
Cover crop biomass and nitrogen accumulation
The oat cover crop produced the highest quantity of above-ground biomass (2.4 tons/a) compared to oat-legume mixture (2.0 tons/a), with the phacelia-legume mixture intermediate at 2.2 tons per acre (Table 4, Fig. 6). Only 0.2 tons of weed biomass was growing in the no-cover crop, fallow plots. The phacelia-legume mixture produced the highest quantity of nitrogen (170 lbs/a), with the oat-legume mixture producing 120 lbs N/a and the oat only cover crop producing 50 lbs N/a) (Table 4, Fig. 7). The high nitrogen contribution by the phacelia-legume mix is questionable, however, since the laboratory analysis of percent N of the phacelia component in one of the fields (Sweeney 03) was suspiciously high (4.3%). One other field (Dickman 04) had a relatively high N concentration of 3.6%. The average %N of phacelia in the other five fields was only 1.6%. However, with the very leafy plant canopy of phacelia, there is a reasonable possibility of the N content to be high just before flowering. Clearly more work is needed to understand the variability of N content of phacelia related to crop phenology.
Legume proportion of cover crop mix and C:N ratios.
The vetch was the primary component of the cover crop mixtures (Tables 4 & 5.) The crimson clover contribution to the cover crop mixtures was quite small, (average less than 5% of the total biomass), with the exception of one field (Hendricks 04), in which the crimson clover was 26% percent of the total (Tables 4 & 5).
Carbon-to-nitrogen ratios have been used to indicate the relative percent nitrogen in cover crops, since the percent carbon of most plant tissues remains relatively constant. Percent nitrogen content of plant tissue varies considerably, not only between species, but also within various growth stages of a given species. For example, an immature cereal crop has a higher nitrogen content (and hence lower C: N ratio) than a mature cereal crop. The C: N ratio is commonly used as a relative indicator to predict how rapidly the plant tissue will degrade after it is killed, either in the soil or on the soil surface as a mulch. Plant tissue with high C: N ratios (>30) will be degraded more slowly than tissue with low C: N ratios such as <15.
In these trials, the C:N ratios (and percent nitrogen) of the cover crops, as well as the increased quantity of cover crop nitrogen, may be key factors in the increase of corn yields in the legume-based cover crop mixtures. In the two fields where corn yield was increased the most by adding legume to the mixture (Sweeney 04 and Hendricks 04), legumes comprised from 67 to 75% of the oat-legume mixture, contributing 90 and 110 lbs nitrogen per acre respectively. In the oat-legume mixtures, the C: N ratio of the oat tissue was more than 1/3 less (28) compared to the oat tissue from oats grown alone (46) (Table 6). Apparently, the legumes in the mixtures are releasing nitrogen in the soil, which is taken up by the oats, reducing the C: N ratio of the oat tissue. In this situation, the oats with the lower C: N ratio in the mixture would theoretically be decomposed more rapidly by soil microorganisms, thereby releasing nitrogen more quickly. However, legumes are also known to improve a variety of soil quality parameters, including enzymes.
The lack of increase in corn yields in the strip-till fields by oat cover crops alone compared to the no-cover crop plots, may be due to several factors, including immobilization of soil nitrogen by soil bacteria engaged in degrading high C: N ratio plant material. Oats may contain allelopathic compounds that could retard corn growth as well.
In the reduced-tillage fields, however, oat cover crops produced the highest yields. The physical incorporation of the cover crop residue throughout the surface of the field would likely increase the rate of microbial decomposition compared to the strip-till fields that only had 12- to 14-inch-wide strips tilled on 30-inch centers.
Economic analysis of cover crop treatments
It is important here to note that these analyses are conducted on data from only four fields from two farms. More on-farm replications of these treatments will be required to have adequate confidence in the interpretations.
Cover crop establishment costs depend on seed costs, seeding rates, planting equipment, and labor costs. Seed costs per acre for the cover crop treatments in 2004 were oats, $9.00; oat-legume, $17.40; and phacelia-legume, $23.40 (Table 7). Note that this analysis does not include crimson clover, since clover was a very poor competitor in the cover crop mixtures and did not form a significant component of the cover crop biomass. (Future cover crop recommendations would not include crimson clover.) Planting equipment and labor costs are based on OSU Extension cost estimates for two disk passes to prepare a seedbed, followed seeding with a grain drill ($33.00/a). Estimated benefits from the cover crop treatments can be calculated by subtracting cover crop costs from the net return (graded yield x corn price).
The oat-legume treatment produced an 11% increase ($86/acre) in net profit compared to the fallow (Table 8). Obviously, the high corn yield from this treatment produced the high economic returns. Oat cover crops and the phacelia-legume mixture produced negative -$8 and –$5, respectively, compared to the fallow (Table 8). Although the phacelia-legume mixture produced higher yields than the fallow, the relatively high establishment costs reduced the net economic value.
Educational Goals: Develop an educational outreach program to enhance availability of information and adoption of successful practices.
Presentations in 2004 were made at the OSU Soil Biology Workshop, Aurora, OR (47 attendees), the OSU Soil Management Workshop, Aurora (60 attendees), a Vineyard Cover Crop and Environmental Land Use Workshop, Aurora (55 attendees), the Far West Agribusiness Assn, Salem (40 attendees), and the Western Region Conservation Tillage Conference in Five Points, CA in 2004 (350 attendees).
Objective 2. To develop and evaluate conservation tillage practices for sustainable and organic farming systems.
An experiment was established at the OSU Vegetable Research Farm in the fall of 2004. A 2-acre block of land (previously alfalfa) was seeded to a cover crop mixture of phacelia and vetch, as described above for Objective 1. Objectives of the experiment were to evaluate alternative methods of cover crop and suppression in an organically managed system for broccoli production. Cover crop biomass was estimated at four locations in the field on two separate dates in the spring using a sickle-bar mower to mow 10’ x 10’ quadrats. Because of the sprawling growth habit of mature phacelia, along with the intertwining growth of the vetch, we did not feel the 0.25 m quadrat method of cover crop estimation was valid. The larger sample size gave us a much better estimate of biomass. The 100 sq. ft. samples were weighed wet in the field and four subsamples (each filling a large grocery sack) were returned to the laboratory. Samples were immediately sorted into the component cover crop species and then weighed again to determine wet weight. Separated subsamples were then dried for 96 hours at 100 deg. F. Analysis of carbon and nitrogen content was performed by the OSU Central Analytical Laboratory.
Experimental treatments were laid out and the pre-tillage cover crop suppression methods were imposed, which included flail mowing and rolling with a Schmeiser ring roller. Unfortunately for this experiment, however, western Oregon experienced one of most continuously rainy springs in the last 25 years. Soils remained wet, especially under the standing cover crops, delaying the strip tillage. There were almost no drying periods longer than a few days at a time. Stahlbush Island Farms, our cooperator who was to supply the strip-tillage equipment, also experienced the crisis of not being able to prepare soil in a timely fashion. We were unable to obtain equipment during the few periods when the soil was dry enough to till. Due to a major land expansion of the farm into southern Oregon (with the acquisition of a new 500-acre organic farm), tractors and equipment were not available at the key points in time. By early July, we had gone past the time of the season suitable for broccoli production, and the experiment was terminated without going farther. One of the lessons learned in this process is the high risk of depending on farmer-supplied equipment. When “push comes to shove,” the farmers’ economic interests outweigh the desire to conduct research trials.
Current Experiments
In the fall of 2004, two new on-farm trials were established with the same cover crop treatments described above (except crimson clover was dropped as a component of the legume mixtures). In the spring of 2005, cover crop biomass was estimated in one field using the 0.25 sq. m quadrat as described above. However, the sickle bar mowing method (described above in the organic trial) was used on the other trial. We feel this method gives a far superior estimate of cover crop biomass, especially in the phacelia/vetch mixtures. Plots were strip-tilled in June 2005, the corn was planted, and yield will be determined in late September.

Impacts and Contributions/Outcomes

This on-farm research demonstrates the potential “win-win” possibilities for using legume-based cover crops in strip-till sweet corn production. The increased corn yields following an oat-legume or phacelia-legume cover crop can clearly offset the additional costs associated with growing the cover crop and potentially produce increased profits within a single crop year. The long-term, multiple benefits of cover cropping are well known and include erosion prevention and improvement of soil and water quality. Several of the Willamette Valley farmers who have been using both cover crops and strip tillage for several years have seen the combined value of these practices to increase soil quality, reduce input costs, and increase crop yields.
One of the primary reasons for evaluating phacelia as a cover crop was the possibility for substitution of phacelia for cereals in the cover crop mixtures. Phacelia has a very different root structure than cereals, and the above-ground plant is much more succulent than the cereals as it approaches maturity. These differences in physical structure of the cover crop may allow growers to delay killing the cover crop with glyphosate and accumulate more biologically fixed nitrogen in the legume component of the cover crop. In addition, more biomass carbon is accumulated, adding to soil organic matter.
The difference in plant structural characteristics between the phacelia and the cereal may also facilitate the adoption of strip tillage. The killing of cover crops has been timed in the early spring to prevent excessive growth of the cereal cover crops since these crops are difficult to work with the strip tillage equipment. In this project, we used a “shank-coulter” type strip-tillage machine, which uses two fluted coulters running behind and alongside a subsoiling shank to mix and incorporate the cover crop. One of the participating growers made the following comments about the phacelia-legume cover crop mixture after preparing seed beds with the strip-tillage equipment,
“The phacelia plot was remarkably easy to strip till. In fact one pass may have been enough rather than the two or three required in the oat test plots.”
This grower also points out one of the potential disadvantages of large quantities of cover crop biomass remaining on the soil surface in the strip tillage systems,
“Due to the large amount of crop residue, it was impossible to cultivate with our cultivator, but that was true with all the strip-till plots. An extra spray application of Atrazine and oil was required to control the weeds. The conventional (tillage) plots did not require this spray.”
In the results reported in this research, the oat-legume mixture increased corn yield over the phacelia-legume mixture on two of the four fields in the strip-till analysis (Hendricks 03 and 04). In the other two fields (Sweeney 03 and 04), the two cover crop mixtures yielded comparably.
Although the organic strip-tillage trial was aborted midway through the season because of the unavailability of equipment (see above), we obtained excellent results killing the phacelia-vetch mixture by flail mowing. Killing the cover crops without herbicides has been a significant problem in organic strip-till systems. Both the phacelia and vetch were in full flower at the time of mowing, so the impact of mowing at various vegetative stages of the plants still needs more study. We are very encouraged by the potential to kill phacelia by merely rolling the crop down and we plan to explore development of a cover crop roller similar to one being developed by the Brubaker farm near the Rodale Institute.

Future Work
Six on-farm trials will be established in the fall of 2005 to continue the evaluation of cover crop mixtures in strip-till production systems. For the organic research trial, the need to be able to prepare the soil during wet spring conditions is moving us toward a ridge-till system. We have evaluated ridge tillage in a low-input vegetable production system at Stahlbush in 2004 and 2005 and obtained excellent ability to work the soil earlier than flat-ground culture. The ridges will be formed in the fall after harvest and cover crop planted over the entire area. In the spring, a flail mower will be used to mow the cover crops down to the top of the ridge. Either the strip-tillage machine or a Buffalo cultivator equipped with ridge clearing devices will be used to prepare a seed bed. Various experimental treatments targeted at weed control will be evaluated, including precision cultivation, mowing, and thermal approaches.
Plans are being developed for on-farm field days in the spring of 2006. A planning committee has been formed to develop a NW region Conservation Tillage and Stewardship Conference in Salem, OR in February 2006.

Collaborators: