Relationship Between Cover Crop Growth, Weed Suppression, and Yield of No-tillage Broccoli (Brassica oleraceae L. var. italica)

Abstract

In 1997, no-till fall broccoli was grown at the Kentland Agricultural Research Farm (KARF), Blacksburg, Va., and the Beltsville Agricultural Research Center (BARC), Md., to determine supplemental N requirements above the nitrogen contribution from legume in situ mulches. Treatments were tillage systems [CT = conventional tillage, bare soil, tilled prior to transplanting; NT-BS = untilled bare soil; NT-SB = soybean (Glycine max L.) cover crop; and NT-CP = cowpea (Vigna sinensis Endl.) cover crop; and nitrogen fertilizer rates (0, 84, and 168 at KARF, and 0, 56, 112, and 168 kg·ha–1 at BARC). All plots at both sites were treated with recommended herbicides and drip irrigated as needed to supplement rainfall. Dry weight soybean and cowpea biomass was 6.1 and 4.3 at KARF and 4.8 and 3.5 t·ha–1 at BARC. In N-unfertilized plots at both sites, average broccoli yield was higher in NT-SB and NT-CP than CT and NT-BS. The N contribution from the legume mulches was inadequate to meet total crop demand, since N fertilizer applications increased broccoli yield in all tillage treatments, including the legume cover crops (soybean and cowpea). In N-fertilized plots, broccoli yield was similar among tillage treatments at KARF where weed problems were not severe; however, at BARC, yield in CT was higher than in all no-till treatments. Weed pressure was considerably lower in CT than in no-till plots at BARC. Based on these data and other related no-till studies, two conclusions can be drawn: a) no-till systems are a viable option for production of broccoli when weeds are adequately controlled; and b) uniformly distributed, high-residue levels are required for weed suppression when weed pressure is high and herbicides are either ineffective or not applied.