Contribution of Cover Crops to Weed Management in Sustainable Agricultural Systems

Cover crops have become a viable option for sustainable agriculture because of Contributions to soil fertility and improved crop performance. This paper focuses on weed control and summarizes present knowledge of the contribution of cover crops to managing weeds in sustainable agricultural systems. Residue from winter annual cover crops provides early‐season weed suppression but not full‐season weed control. Living mulches that are effective at controlling weeds also will require management to prevent excess competition with the cash crop. Elimination of herbicides is not a realistic objective for using cover crops. Rather, herbicides should be considered a tool for managing cover crops and optimizing their potential for improving soils and sustaining agricultural production. Research Question Cover crops have become a viable option for sustainable agriculture because of contributions to soil fertility and improved crop performance. The contribution of cover crops to weed management is not clearly defined. Ideally, weed control could be improved if a manageable cover crop could replace an unmanageable weed population in the agroecosystem. This paper summarizes present knowledge of weed control by cover crops in order to define appropriate cover crop management in sustainable agricultural systems. Study Description This paper focuses on cover crop use in no‐tillage production systems. No‐tillage systems are emphasized because of greater opportunity for cover crops to interact with weed germination and establishment at the surface of soils. Two major types of cover crops are addressed: winter annual and living mulches. Winter annual cover crops are seeded every year in late summer or fall and terminated before planting the following spring. Living mulches are perennials or self‐seeding annuals that grow during part or all of the cropping season and naturally re‐establish annually. Applied Questions Does residue from winter annual cover crops control weeds? Residue from winter annual cover crops can provide early‐season weed suppression but not full‐season weed control. Early‐season weed suppression is increased by increasing residue biomass. Small‐seeded annual weed species with a light requirement for germination are most sensitive to cover crop residues. Weed control is not complete, however, at natural residue amounts, probably because of heterogeneity in the residue layers. Emerged weeds compensate for lower weed numbers with greater growth per plant so weed biomass later in the season is not influenced as much as early‐season weed emergence. Can living mulches suppress weeds but avoid crop competition? Any living mulch competitive enough to suppress weeds will also be competitive enough to interfere with crop growth and yield. Therefore, research on living mulches has focused on regulating the living mulch enough to prevent competition with the crop but permit sufficient recovery of the mulch for subsequent years. Approaches to regulation have included nonlethal herbicide rates, banded herbicides, strip tillage, mowing, and relay planting. Are herbicides necessary for weed control when using cover crops? Most successful management systems for cover crops require herbicides to control weeds emerging through cover crop residue or to regulate living mulches. Given present cover crops and management systems, complete elimination of herbicides is not a realistic objective. Can cover crops permit a reduction in herbicide use? Winter annual cover crops could permit reduction of herbicide inputs and a shift toward total postemergence herbicide programs for many crops. The early weed suppression provided by cover crop residue should permit crops to become established before weed emergence. Postemergence herbicides could be used to control later‐emerging weeds for crops with viable postemergence alternatives. This approach would contribute to sustainable agriculture by replacing preemer‐gence herbicides, which are most frequently detected in ground and surface waters, with postemergence herbicides, which are often used at lower rates and are less persistent. What is the role of cover crops in sustainable agriculture? The major contribution of cover crops to agricultural systems is probably long‐term soil protection and fertility enhancement, particularly on erodible land. Erosion prevention and soil stabilization is probably the highest priority for maintaining a sustainable agriculture on these sites. In this situation, herbicides should be regarded as a tool for preserving soil resources rather than an input to be eliminated.