The purpose of this project was to expand the knowledge base informing sustainable agricultural practices. This project addressed three key challenges for organic, reduced-tillage vegetable production. First, selecting summer cover crops that winter kill eliminates the challenge of mechanically killing the cover crop. Second, we evaluated quantity and timing of nitrogen release from both the cover crops and organic amendment nitrogen during over-wintered vegetable production. Third, we evaluated the weed control potential of the different cover crop residues.
Bulb onions can be fall planted for over-wintered production in the southeastern U.S. In eastern North Carolina transplanting dates are from October through mid-November. The average fall frost date for eastern North Carolina is mid or late October, which would allow for frost kill of a susceptible cover crop to correspond with onion transplanting. Warm season annual cover crops include C4 grasses such as foxtail millet [Setaria italica (L.) Beauv.] and heat adapted legumes such as cowpea ‘Iron & Clay’ [Vigna unguiculata (L.) Walp.], both of which are killed by frost and cold weather.
Onions pose a particular challenge for weed control due to their sparse vegetative structure which incurs greater weed competition than crops that produce a closed canopy. Organically grown onions in Georgia are most often produced by transplanting into plastic mulch for weed control. With in-situ cover crop mulch, no-till transplanting equipment can be used to cut through surface residue, create a narrow furrow for the transplant and close the furrow with weighted press wheels.
Winter cropping with N and weed suppression from summer cover crop residue and surface applied organic amendment N have not been thoroughly studied. Cool winter temperatures slow residue decomposition, which could enhance mulch-based weed control during a season when weed pressure is generally less, and could contribute to a prolonged period of N mineralization for cash crop uptake. This study assessed the contributions of summer annual grass and legume cover crops in different mixture ratios or monocultures and rates of soybean meal as an N amendment on over-wintered, no-till, organic onion production.
1. Summer annual cover crops in monocultures or different ratio bi-cultures for organic fall planted vegetable production.
2. Soybean meal as a N fertility source in combination with cover crops for over-winter production.
3. Cover crop and soybean meal management affects on onion yield.
Field experiments were conducted on first year transitional (non-organic) land in 2006-07 and 2007-08. Cover crops of foxtail millet ‘German Strain R’ [Setaria italica (L.) Beauv.] and cowpea ‘Iron & Clay’ [Vigna unguiculata (L.) Walp.] were grown as monocrops (MIL, COW) and biculture mixtures and compared to a bare ground control (BG). Mixtures of cowpea and millet consisted of seeding rates aimed at producing 70%, 50%, and 30% millet per total biculture biomass (MIX-70, MIX-50, MIX-30). Cover crop residue treatments were evaluated for weed suppression and N contribution to no-till organic onion production. Supplemental N in the form of surface applied soybean meal [Glycine max (L.) Merrill] was applied to cover crop treatment subplots at three rates: 0, 105, and 210 kg N∙ha-1.
Cowpea shows promise as a summer cover crop used as a residue mulch for fall planted crops such as over-wintered onion. COW produced comparable onion yields to BG in both years of this experiment and at commercially reasonable levels. Utilizing cover crops for no-till vegetable production does require some additional management considerations compared to clean till or bare ground cultivation but there are tradeoffs. Many other studies have shown that incorporating cover crops into production rotations provide a variety of benefits, especially over an extended period of time greater than one or two years. This experiment was conducted on two different sites over the two years and therefore did not address the long term affects of these cover crop management options.
Weed control for large scale organic production using in-situ cover crop residues would be a major challenge and expense. Management to reduce weed seed banks and careful selection of low weed pressure fields would be essential to making it commercially feasible. That type of management is not impossible and has been achieved on certain exemplary farms. Even though COW had high weed interference, just two thorough hand weeding events were sufficient to maintain onion yields. Weed control is logistically difficult with surface residue since mechanic cultivation options would disturb residue and tend to catch and drag on equipment. Depending on the field specific weed pressure and types of weed species present, hand weeding through residue is effective but time consuming.
This experiment found that foxtail millet is not a desirable cover crop for this system of no-till overwintered vegetable production. It appears that physical properties of the grass residue negatively affected onion plant stand and overall yield. Millet in a mixture with cowpea either reduced onion yield or had comparable yield to cowpea as a monocrop. While MIL did reduce weed interference in 2006-07 compared to COW there was no significant difference in 2007-08. In addition, foxtail millet appears to be a poor choice for a midsummer seeded cover crop in eastern North Carolina due to its susceptibility to foliar disease.
Onion bolting reduced marketable yield in both years of this experiment. Flower initiation in onions is influenced by onion variety, plant size or age, and several environmental factors including photoperiod, temperature, and plant N status. Onion juvenility lasts until the plants reach a certain critical size and is often measured in terms of plant weight or number of leaves (6 to 9 after the cotyledon). A post-juvenile onion plant is receptive to vernalization, which is essential for floral initiation, and the rate of flower initiation increases with plant size. While not quantified in this experiment, decomposition of cover crop residues could have influenced mid-season onion plant size and in turn differing rates of flower initiation. N immobilization from MIL in 2007-08, which had an initial C:N ratio over 37:1 could have contributed to smaller midseason plant size in MIL which in turn reduced the rate of bolting.
Soybean meal shows potential as an effective source of N even when surface applied in cool weather months. Even though the 105 kg N•ha-1 rate appeared to contribute sufficiently to soil N for use as a starter fertilizer onion yield was higher at the 210 kg N•ha-1 rate. Split applications of soybean meal could be an important improvement on the one application management scheme used in this experiment. This study showed that N would be available for plant uptake in less than two weeks after surface applying soybean meal, which facilitates the use of soybean meal in multiple applications tailored to timing of crop plant demand.
Educational & Outreach Activities
Preliminary findings were presented to growers at a Seasons of Sustainable Agriculture workshop at the Center for Environmental Farming Systems in Goldsboro, NC on May 10, 2007. Results of the first field season were shared as part of a session at the 22nd Annual Sustainable Agriculture Conference in Durham, NC on November 10 2007. Masters thesis available online at NCSU libraries.
We provided producers with specific information about summer cover crop selection and rotation strategies as well as examples of the weed pressure and availability of organic amendment nitrogen during the winter growing season following grass or leguminous cover crops.
Areas needing additional study
Refinement of timing and amount of split applications of soybean meal to provide N fertilizer relative to periods of increased crop demand in warm and cool weather (this study was conducted in winter months).
The use of summer cover crops for no-till fall planted crops on certified organic land (this study was managed organically on transitional land).
Economic analysis of this management strategy for small scale organic production.
Other heat adapted annual cover crop species for this management strategy.
Other cash crops for no-till over-winter production following summer cover crops (e.g., garlic).