Weed Management in Conservation Agriculture

Conservation agriculture (CA) is built upon the principles of minimum soil disturbance, permanent soil cover, and crop rotation. Adoption of CA alters weed dynamics and communities, and thereby entails modifying weed control methods. Zero-tillage (ZT) systems allow weed seeds to concentrate near the soil surface, where they have greater probability of germinating but are also exposed to higher mortality risks through weather variability and predation. Considering no seed input into the system, germinable seed banks under ZT reduce more rapidly than under conventional tillage. Reducing tillage causes the shifting of weed communities from annual dicots to grassy annuals and perennials. Surface residues moderate soil temperatures, thereby altering the emergence of both crops and weeds. The germination and growth of small-seeded annuals are affected by restricted light availability, physical growth barriers, and potential allelopathic effects from surface residue. Crop rotation affects weeds through allelopathy and altered timing of both crop management and resource demands. Crop rotations also aim at the use of different herbicides, preferably with different modes of action to overcome problems of herbicide resistance in weeds. The literature indicates that implementing ZT without crop rotation can result in severe weed problems and that greater rotational crop diversity results in easier weed management. Weed management in CA involves extensive herbicide usage owing to minimal exclusion of the tillage practice; however additional cultural practices include: (i) selecting highly competitive cultivars, (ii) adjusting planting dates, (iii) farm hygiene to prevent the introduction of new weeds, (iv) adjusting planting geometry, densities, and fertilizer placement, and (v) microbial bio-controls. Further research is needed towards (i) acquiring knowledge of the interactive effects of tillage and surface residue on weeds, (ii) the application of models and/or meta-analyses to anticipate weed responses, and to analyse intervention points in CA, and (iii) the weed-suppressive potential of longer (4+ years) rotations.