The use of grass‐legume bicultures grown as winter annual cover crops may provide farmers with additional cover crop management options regarding the availability of cover crop residue N. A 2‐yr field experiment was conducted to determine dry matter (DM) accumulation, chemical composition, and N release from grass and legume cover crops grown in monoculture (rye, crimson clover, and hairy vetch) and biculture (rye‐crimson clover and rye‐hairy vetch). Air‐dried plant material was placed on the soil surface in 1‐mm mesh nylon bags for 1, 2, 3, 4, 6, 8, and 16 wk. Following retrieval, mesh bag contents were analyzed for total N, C, cellulose, hemicellulose, and lignin concentrations. The 2‐yr mean cover crop DM production was in the order of rye‐hairy vetch > hairy vetch > rye‐crimson clover > rye > crimson clover. The greatest cover crop N content (2‐yr mean) occurred with hairy vetch monoculture (154 kg N ha−1), compared with a low of 41 kg N ha−1 for the rye monoculture. When grown in biculture with rye, hairy vetch accumulated more DM and biomass N compared with crimson clover, both as a proportion of the biculture and as DM yield. In general, the order of N release rates (rapid to slow) was hairy vetch > crimson clover = rye‐hairy vetch > rye‐crimson clover = rye. Estimates of N (kg ha−1) released from cover crop residue after 8 wk of field decomposition, averaged over 2 yr, were 24 for rye, 60 for crimson clover, 132 for hairy vetch, 48 for rye‐crimson clover, and 108 for rye‐hairy vetch. Results of this study demonstrate only slight reductions in N release from grass‐legume bicultures compared with legume monocultures.

Published studies focused on characterizing the allelopathy-based weed suppression by rye cover crop mulch have provided varying and inconsistent estimates of weed suppression. Studies were initiated to examine several factors that could influence the weed suppressiveness of rye: kill date, cultivar, and soil fertility. Ten cultivars of rye were planted with four rates of nitrogen fertilization, and tissue from each of these treatment combinations was harvested three times during the growing season. Concentrations of a known rye allelochemical DIBOA (2,4-dihydroxy-1,4-(2H)benzoxazine-3-one) were quantified from the harvested rye tissue using high performance liquid chromatography (HPLC). Phytotoxicity observed from aqueous extracts of the harvested rye tissue correlated with the levels of DIBOA recovered in harvested tissue. The amount of DIBOA in rye tissue varied depending on harvest date and rye cultivar, but was generally lower with all cultivars when rye was harvested later in the season. However, the late maturing variety 'Wheeler' retained greater concentrations of DIBOA in comparison to other rye cultivars when harvested later in the season. The decline in DIBOA concentrations as rye matures, and the fact that many rye cultivars mature at different rates may help explain why estimates of weed suppression from allelopathic agents in rye have varied so widely in the literature.

Grass‐legume bicultures as winter annual cover crops may combine the N scavenging ability of grasses and the biological N2 fixation capacity of legumes to improve N management in crop production systems of the southeastern USA. A 3‐yr field experiment was conducted on a Norfolk loamy sand (fine‐loamy, kaolinitic, thermic Typic Kandiudults). The focus of this research was to examine the differences among legume monocultures and grass‐legume bicultures with regard to early spring dry matter (DM) and N accumulation, and related effects on soil inorganic N levels and subsequent corn (Zea mays L.) yield. Austrian winter pea [Pisum sativum L. subsp, sativum var. arvense (L.) Poir.], crimson clover (Trifolium incarnatum L.), commonve tch (Vicla saava L.), and hairy vetch (Vicla villosa Roth) were grown in monoculture and in bicultures with rye (Secale cereale L.), oat (Arena satira L.), and wheat (Triacum aestivum L.). Aboveground plant material was harvested in early March, late March, and mid‐April. Biomass was separated into component species and analyzed for total N and C concentrations. Averaged over 3 yr, legume component DM accumulation in monoculture and biculture ranged from 0.87 to 2.53 Mg ha−1, with a ranking of Austrian winter pea < hairy vetch < commonv etch < crimson clover. For the same period, the grass component DM accumulation ranged from 1.31 to 2.28 Mg ha−1, in the order rye = oat < wheat. Three‐year mean N accumulation values for the legume component followed the same relative ranking and ranged from 24 to 93 kg N ha−1. Grass factor N content ranged from 18 to 39 kg N ha−1 in the order rye < oat < wheat. For all bicultures, the average C:N ratio over the 3‐yr experiment was >30, suggesting that net N mineralization would occur from the decomposing cover crop residues. Profile soil inorganic N (0 to 90 cm) was greater in legume monoculture than in grass‐legume biculture treatments, indicating the ability of grasses to capture soil N. Corn yield was affected by the treatments in 1 of 3 yr, with greater yields following a legume monoculture than a grass‐legume biculture. Collectively, these results suggest that grass‐legume bicultures as winter annual cover crops have the potential to utilize residual soil NO3 and thereby minimize leaching while adding fixed N to cropping systems in the southeastern USA.

A grass‐legume biculture may be preferred over a legume monoculture cover crop due to the scavenging ability of a grass species, especially when high residual soil N levels are present following summer droughts in the Atlantic Coastal Plain. Rye (Secale cereale L.) and crimson clover (Trifolium incarnatum L.) were grown in monoculture and as a biculture in a 2‐yr field experiment on a Typic Kandiudult to assess cover crop recovery of fertilizer 15N and the subsequent corn (Zea mays L.) uptake of cover crop residue 15N. Potassium nitrate labeled with 10 atom % 15N was applied to microplots at 50 kg N ha‐1 1 wk after seeding the cover crops, which were monitored for recovery of fertilizer 15N. Labeled residue was placed in a new microplot to monitor release of residue 15N and its recovery by corn. Averaged across both years, rye monoculture recovered 39% of the labeled 15N fertilizer compared with 19% in the rye‐crimson clover biculture and 4% in the crimson clover monoculture. Following corn harvest and averaged across both years, total recovery of 15N fertilizer from the original microplots (cover crop, corn biomass, and soil N) was 29% for crimson clover, 75% for rye, 55% for rye‐crimson clover biculture, and 20% for the native winter weeds. In 1993, corn recovery of residue 15N was lowest in the rye monoculture (4%) compared with other treatments (20–35%). Results indicated that a rye‐crimson clover biculture was capable of recovering greater residual 15N than a crimson clover monoculture, but less than rye monoculture.

Increased N‐use efficency and economic savings may result from a better understanding of N release patterns from legume residues. A 2‐yr field experiment was conducted on a Cecil fine sandy loam (clayey, kaolintic, thermic Typic Kanhapludult) to examine the effects of crimsom clover (Trifolium incarnatum L.) growth stage on dry matter accumulation, N concentration, and chemical composition in relation to N release under no‐tillage management. Crimson clover was harvested in the spring at four growth stages (late vegetative, early bloom, late bloom, and early seed set). Air‐dried plant material in 1‐mm mesh nylon bags was placed on the soil surface; retreived at 1−, 2−, 4−, 8−, and 16‐wk intervals; and analyzed for total N, C, cellulose, hemicellulose, and lignin concentrations. Averaged over 2 yr, dry matter production increased from 2.3 to 5.6 Mg ha−1, and N concentration declined from 30.2 to 21.2 g kg−1 as crimson clover matured from late vegetative to early seed set growth stages. Cellulose concentration increased by 66%, hemicellulose by 37%, and lignin by 87% from late vegetative to early seed set. Estimated clover N release at the 8‐wk retrieval was 28, 40, 40, and 54 kg ha−1 in 1989 and 51, 67, 73, and 55 kg ha−1 in 1990 for the late vegetative, early bloom, late bloom, and early seed set growth stages, respectively. Results indicated that allowing crimson clover to attain the late bloom stage prior to desiccation and planting of the summer crop can maximize clover top‐growth N content and subsequent N release.