The 2012 ASHS Annual Conference

Weed control in organic systems is often solely dependent upon cultivation and use of cover crops due to prohibited use of synthetic herbicides. Cover crops suppress weeds both physically and chemically by altering the soil microenvironment and increasing competition for resources. The purpose of this study was to quantify the effects of eight cover crops on soil environmental factors and weed suppression in a field setting. Cover crop treatments included alfalfa (Medicago sativa L. ‘Vernal’), crimson clover (Trifolium incarnatum L.), Dutch white clover (Trifolium repens L.), annual ryegrass (Lolium multiflorum Lam.), winter rye (Secale cereale L.), soybean [Glycine max (L.) Merr. ‘Viking O.2265’], forage radish (Raphanus sativus L. var. longipinnatus ‘Tillage Radish’), hairy vetch (Vicia villosa Roth.), and two mixtures—crimson with Dutch white clover and winter rye with hairy vetch. Cover crops were seeded at two planting dates in mid summer 2011 in Durham, NH, in a randomized complete-block design with four replications. A fallow (no cover crop) treatment was included in the study. Cover crop and weed density and biomass, percent light transmittance at soil surface, leaf area index (LAI), soil temperature and soil moisture were measured in each treatment in early fall. Significant correlations were found between the percentage of light transmittance, soil moisture and soil temperatures, and total weed biomass. Greater cover crop biomass was associated with reduced light transmittance, lower soil temperatures, and decreased weed biomass. Forage radish, hairy vetch, crimson clover, and the crimson-Dutch white clover mix produced leaf canopies with highest LAI and least light transmittance at ground level. High LAI and lower light levels at soil surface corresponded with lower soil temperature in these plots. Dutch white clover produced the least dense leaf canopy and had plots with highest densities of weeds. Of these crops, forage radish seeded at 28 kg/ha was most effective in reducing total weed density and biomass. More specifically, forage radish was markedly more effective at suppressing crabgrass (Digitaria spp.), a problematic weed prevalent at the field site. Forage radish is a new cover crop for the Northeast and has been touted for N sequestration and soil quality benefits. In addition to these potential benefits, this cover crop appears to be highly effective at suppressing weeds and may be useful in organic production systems.