Grass-Legume Cover Crop Mixtures and Manure Affect Weed Seed Production and Legume Nodulation

Summer legume cover crops can improve soil health and reduce the economic and environmental costs associated with N fertilizers in production of many horticultural crops.  However, adoption is often constrained by high seed costs and ineffective weed suppression compared to non-legume cover crops.  We hypothesized that mixtures of cowpea and soybean cover crops with sorghum-sudangrass or Japanese millet would reduce the risk of weed seed production and increase nodulation relative to legume monocultures, and that these benefits would be greatest under lower N fertility.  In field experiments conducted in North Central NY, legumes and grasses were grown either alone or in various grass-legume combinations with or without composted chicken manure.  Under hot, dry conditions in 2005, both cowpea and soybean were severely suppressed by weeds in monoculture and by sorghum-sudangrass in mixtures, resulting in low legume biomass, and poor nodulation.   Treatments involving sorghum-sudangrass produced 8 MT/ha dry weight and suppressed weed biomass and seed production by over 90% compared to the no cover crop control, but seed production among suppressed Powell amaranth exceeded 20,000 seeds/m².  Under more typical temperature and rainfall conditions in 2006, all cover crop treatments produced substantial biomass (4-8 T/ha), and suppressed weed biomass by over 95% compared to the no cover crop control.  However, mean Powell amaranth seed production was 1,100 seeds/ m², equivalent to 3-5 times the estimated initial Powell amaranth seed bank.  In 2006, cowpea mixtures with Japanese millet stimulated cowpea biomass production and nodulation compared to monoculture, but soybeans were suppressed in mixtures with both grasses.  Composted chicken manure shifted competition in favor of weeds at the expense of cowpea (2005), stimulated weed and grass biomass production (2006), and suppressed nodulation of soybean (2006).   In a complementary on-farm trial, cowpea mixtures with sorghum-sudangrass produced over 7 MT/ha and suppressed weed biomass by 99% compared to the no-cover crop control; however, hairy galinsoga growing beneath the vigorous cover crop canopy was able to produce sufficient seeds (600 seeds/m²) to replenish the existing weed seed bank.   Results suggest that 1) mixtures of cowpeas with grasses can increase legume biomass; improve nodulation; lower seed costs; and reduce the risk of weed seed production; 2) soybean is far less compatible with either grass in mixture; and 3) future costs of weed seed production must be weighed against potential benefits of soil health and reduced fertilizer costs in determining optimal choices of cover crops.