Role of integrated weed management strategies in sustaining conservation agriculture systems

Conservation agriculture (CA) is an agricultural management practice in which there is minimum soil disturbance, retention of residue for soil cover and rotation of major crops. In contrast, soil in traditional agriculture is intensively tilled to prepare a fine and well-pulverized seedbed. Soil tillage or land preparation is the most energy-consuming operation among all field operations. Compared to traditional agriculture, farmers can save up to 40% of time, labour and fuels in CA. The other benefits of CA include reduction in soil erosion, increased soil moisture conservation, lower surface run-off of herbicides and fertilizers, and improved profits. In addition, the presence of crop residue and the lack of soil disturbance in CA increase biological activity in the soil. However, weeds are the major biological constraints to the adoption of CA. In conventional-tilled farming, weeds can be effectively controlled by tillage operations, which uproot and bury weeds deep into the soil. Due to lack of tillage, weeds grow and flourish in CA if effective weed control measures are not taken. Therefore, attempts to implement CA in many regions in Asia failed due to a yield penalty. In CA, weeds can be controlled by manual weeding and herbicide use. Labour, however, is becoming scarce and expensive mainly because of migration from rural areas to cities. It is seldom that labour is available at the critical time of weeding. By the time labour becomes available, yield losses have already occurred due to weed competition. Herbicides are being widely used to control weeds in CA, but there is hardly any herbicide that can control different kinds of weeds with one or two applications. Furthermore, there are concerns about developing resistance in weeds, shifts in weed populations due to continuous use of a single herbicide, less availability of new and effective herbicide molecules, increased cost of chemical control and issues related to environmental pollution. Therefore, to increase the sustainability of CA, there is a need to manage weeds by using integrated weed-management strategies. These may include use of preventive measures, stale seedbed technique, adjusting crop sowing time, use of crop residue as mulch, narrow row spacing, high seeding rates, weed-competitive cultivars, adopting crop rotation and judicious use of herbicides (Figure 1). Preventing weed invasion in a field is a much cheaper and easier option than controlling a heavy infestation. The use of clean crop seeds and machinery (seeding, harvesting and threshing) is the first and most important step in reducing weed infestation in a field. Crop seeds are mainly contaminated with weed seeds where the size and shape of both species resemble each other. Weedy rice or red rice (Oryza sativa L.) is a perfect example, which spread in many Asian countries through the use of contaminated rice seeds. In addition, field margins and irrigation canals should be free from weeds. The use of stale seedbed technique can significantly reduce weed density in a crop in CA systems. Due to lack of soil disturbance, most of the weed seeds remain on the soil surface in these fields. In the stale seedbed practice, a light irrigation is given to stimulate germination of these weed seeds; the weed seedlings that emerge are then killed using a nonselective herbicide (e.g. glyphosate or paraquat). This practice helps in reducing the weed seed bank size in the soil and the crop emerges in a relatively weedfree environment. Some of the weed species sensitive to the stale seedbed technique are Cyperus iria, Digitaria ciliaris, Echinochloa colona, Eclipta prostrata, Leptochloa chinensis, Ludwigia hyssopifolia and Portulaca oleracea. In addition, weed seeds present on the soil surface are prone to surface-dwelling seed predators. When combined with other weed-management strategies, seed predation may help reduce herbicide use and labour demand in CA systems. Adjusting the time of crop sowing can also minimize weed pressure in some crops. Earlier planting of wheat in North India, for example, gives the crop a competitive advantage over Phalaris minor, a noxious grassy weed species. Earlier in the season, ecological and environmental conditions are not suitable for seed germination of P. minor. The adoption of no-till and early planting of wheat in North India proved profitable to farmers as these helped reduce the problems of P. minor. One of the pillars of CA is retention of residue cover on the soil surface. In addition to moisture and soil conservation, residue as mulch can suppress emergence and growth of many weed species. However, the extent of suppression depends on the quantity and type of crop residue. Cereal crops, for example, produce more residue than oilseeds and pulses. Similarly, a crop grown in an irrigated area will produce more biomass than a crop grown in a rainfed area. Growing a cover