Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/18/2009
Publication Date: 7/1/2009
Citation: Brennan, E.B., Boyd, N.S., Smith, R., Foster, P. 2009. Seeding Rate and Planting Arrangement Effects on a Legume-Oat Cover Crop in Organic Vegetable Systems. Agronomy Journal. 101:979-988. Interpretive Summary: Winter cover crops are important in organic vegetable rotations and can reduce nitrate leaching, suppress weeds, and add soil organic matter. Legume-cereal mixed cover crops may also reduce reliance on off-farm sources of nitrogen due to biological nitrogen fixation by the legume component. A 2-yr study was conducted on two certified organic sites in Salinas, and Hollister, CA, to evaluate the effect of seeding rate and planting arrangement on cover crop growth and weed suppression with a mixed cover crop of 90% legumes and 10% oat seed. Seeding rates were 112, 224, and 336 kg/ha, or 100, 200, and 300 lb/a were evaluated in a normal one-way drilling versus a grid drilling pattern. Above ground biomass of legumes and oat components, and weeds were determined a several times during the winter cover cropping period. Planting pattern did not affect cover crop growth or weed suppression. Increasing the seeding rate improved weed suppression, and increased oat and legume biomass production early in the season but did not affect total final cover crop biomass production. Above ground biomass of the cover crops was more than 8 Mg/ha or 3.5 t/a and the legume portion of the cover crop biomass declined through the season due to competition by the oats. Seeding at the higher rates is probably cost effective in this region because cover crop seed is a small part of cover cropping costs and higher rates may reduce costs of weed management in subsequent vegetable crops.
Technical Abstract: Winter cover crops are integral components in rotations on high-value, organic vegetable farms in the central coast of California, and can provide benefits such as reducing nitrate leaching, suppressing weeds, and adding soil organic matter. Legume-cereal mixed cover crops may also reduce reliance on off-farm sources of N (N) due to biological N fixation by the legume component. A 2-yr study was conducted on two certified organic sites in Salinas, and Hollister, CA, to evaluate the effect of seeding rate and planting arrangement on cover crop density and canopy development, cover crop dry matter (DM) production, and weed suppression with a mixed cover crop of legumes (Vicia faba, Vicia villosa ssp. Dasycarpa, Vicia benghalensis and Pisum sativum) and oat (Avena sativa). By weight, the mix included 90% legume and 10% oat seed. Seeding rates were 112, 224, and 336 kg ha-1, and planting arrangements were the normal one-way drilling versus a grid drilling pattern. Above ground DM of the legume and oat components, and weeds were determined several times during the winter cover cropping period. Planting arrangement had no effect on any of the variables measured. Cover crop density increased with seeding rate but less than expected. Weed emergence was unaffected by seeding rate, but when weeds were abundant, weed DM was 3 to 7 times lower in the high seeding rate. Increasing seeding rate increased oat and legume DM early in the season at both sites and yrs but did not affect total final cover crop DM. Yr affected total cover crop DM production at both sites, but the averaged across yrs and rates, the total cover crop DM at both sites was more than 8 Mg ha-1. The legume DM portion of the total cover crop DM declined through the season at both sites but varied within sites and yrs, probably due to soil quality and climatic differences. At the end of the cover cropping period, legume DM was approximately half of the total cover crop DM in Salinas, but less than 20% of total cover crop DM in Hollister. Due to the high costs of weed control in organic vegetable crops, increasing the seeding rate is probably cost effective because seed is a relatively small part of the total costs of cover cropping in this region. These results illustrate the complex dynamics between the legume and cereal components in a mixed cover crops and how these can differ markedly between yrs and locations.