Economic thresholds and the case for longer term approaches to population management of weeds

The economic threshold is a concept strongly embedded within the weed manag ement literature. There are some theoretical concerns with applying a stati c approach such as the economic threshold to weed management decision makin g. An improvement is to adopt a population management approach where the in tertemporal effects of decisions are taken into account. The focus should b e on managing weed populations through time rather than minimizing the yiel d effect of weeds in a single season or year. Rather than viewing weeds as an annual production problem, the weed seed bank can be considered a renewa ble resource: stock, and the management goal is to deplete this resource st ock through time. The principles of natural resource economics illustrate t hat including the intertemporal effects of weed control will, for a given s ize of a seed bank, result in a greater level of weed control and a higher economic benefit than if control decisions were based solely on the current period effects. A dynamic economic model was developed of an extensive Aus tralian spring wheat (Triticum aestivum) cropping system to test these prin ciples using wild oat (Avena fatua and A. ludoviciana) as an example. The m odel was solved for a 20-yr time horizon for a population management approa ch and the traditional static economic threshold. The economic benefits fro m a population management approach were significantly greater than those ge nerated by the economic threshold, and the final seed bank was considerably lower. This result suggests that a paradigm shift from thresholds to longe r term population management is warranted.