Long-term modelling of weed control strategies: analysis of threshold-based options for weed species with contrasted competitive abilities

A simple life-cycle-based demography model was adapted for two contrasting weed species (Alopecurus myosuroides and Poa annua). This model included a seed production function that accounted for population self-regulation through weed:weed interactions. The A. myosuroides version of the model was tested with field data. Long-term simulations of population demography were then performed to investigate the relationship between weed control strategies based on density thresholds and both the frequency of herbicide use and the long-term economic profitability. This study confirms that threshold-based weed management strategies are more cost-effective than spraying every year and may allow important reductions in herbicide use. However. after the first transient years of either systematically spraying or withholding herbicide, the long-term spraying frequency was insensitive to threshold values between 0.01 and 100 plants m(-2). The highest long-term profitability was obtained for the lowest threshold tested, and the profitability decreased rapidly when the threshold was raised above 4-6 and 10-20 plants m(-2) for A. myosuroides and P. annua respectively. The study thus indicates that the exact threshold value is of little importance for the long-term reliance of the system on herbicide, provided that it is reasonably low. For species with low competitive ability, high thresholds may be used in some cropping systems to reduce the spraying frequency for environmental considerations, but those options would also reduce the profitability if no compensatory measures were taken.