Allelopathic cover crops to reduce herbicide use in sustainable agricultural systems

No matter how one may define sustainable agriculture, use of soil-conserving cropping practices, less synthetic herbicide inputs and better weed control would be compatible components. Previously, these components were considered incompatible, since it was widely believed that soil-conserving practices required increased pesticide use, including herbicides. However, we have shown that environmental and ecological differences between the no-till and conventional tillage can enhance the control of certain weed species in no-till cropping systems. With proper choice and manipulation of cover crops and residues, it is often possible to reduce the herbicides use. Thus, in eliminating tillage, by utilizing the surface mulch and allelochemicals leached from a killed cover crop and using most effective herbicides when needed, weed management has become much more effective in no-till. In North Carolina, we have grown soybean (Glycine max L.), tobacco (Nicotiana tabaccum L.), corn (Zea mays L.), sorghum (Sorghum bicolor L.) and sunflower (Helianthus annuus L.) in killed heavy mulches of rye (Secale cereale L.) without herbicides, other than a non-selective one to kill the rye. Early-season control of broadleaf weeds such as sicklepod (Cassia obtusifolia L.), morningglory spp. (Ipomoea spp.), cocklebur (Xanthium strumarium L.), prickly sida (Sida spinosa L.), common purslane (Portulaca oleracea L.) and pigweed spp. (Amaranthus spp.) has been 80 to 95%. Rye is the most weed suppressing cover crop among several small grains and subterranean clover (Trifolium subterraneum L.) and crimson clover (Trifolium incarnatum L.) the most suppressive legumes. This approach will still enhance agricultural sustainability because; (a) productive top-soil will be conserved, (b) herbicide use (especially preemergence herbicides) can be reduced and (c) herbicides for cover crop kill and postemergence selective herbicides, even if used, have little potential for environmental contamination.