Continuous cropping systems and weed selection

Changes in the weed flora of agro-ecosystems can occur as long-term changes or temporary fluctuations in species composition. The rate at which weed population shifts occur depends on the selection pressure imposed, genetic variability among weed populations, plant characteristics and environmental factors. Agronomic practices associated with cropping systems including crop rotation, tillage, herbicide use, soil amendments, and mechanization of harvesting that impart a range of selection pressures on weed populations are discussed in this review. Widespread use of herbicides has had the greatest impact on weed selection in recent years. Evolution of herbicide resistant weeds presents an enormous challenge to farmers. Development of herbicide tolerant crops has provided another tool for farmers however the selection pressure on weeds and potential impact on weed population shifts will require judicious use of this technology. Simulation models provide an excellent opportunity to predict future weed population shifts in response to management practices. Further insight into future management changes on weed selection must proceed towards an investigation of the processes rather than the outcomes. In particular, this must involve an understanding of the ecological factors and processes that are likely to determine the weed responses to particular management regimes.