Economic Thresholds and the Case for Longer Term Approaches to Population Management of Weeds1

Abstract

The economic threshold is a concept strongly embedded within the weed management literature. There are some theoretical concerns with applying a static approach such as the economic threshold to weed management decision making. An improvement is to adopt a population management approach where the intertemporal effects of decisions are taken into account. The focus should be on managing weed populations through time rather than minimizing the yield 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 renewable resource stock, and the management goal is to deplete this resource stock through time. The principles of natural resource economics illustrate that including the intertemporal effects of weed control will, for a given size 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 Australian spring wheat (Triticum aestivum) cropping system to test these principles using wild oat (Avena fatua and A. ludoviciana) as an example. The model was solved for a 20-yr time horizon for a population management approach and the traditional static economic threshold. The economic benefits from a population management approach were significantly greater than those generated by the economic threshold, and the final seed bank was considerably lower. This result suggests that a paradigm shift from thresholds to longer term population management is warranted.Nomenclature: Diclofop-methyl, (±)-2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid; flamprop-methyl, N-benzoyl-N-(3-chloro-4-fluorophenyl)-dl-alanine; glyphosate, N-(phosphonomethyl)glycine; wild oat, Avena fatua L. AVEFA, winter wild oat Avena ludoviciana Dur. AVELU; wheat Triticum aestivum L.Additional index words: Dynamic programming, economic optimum thresholds, optimum dose rate, economics, net present value, modeling, integrated weed management, rigid ryegrass, annual ryegrass, Lolium rigidum.Abbreviations: DP, dynamic programming; EOT, economic optimum threshold; ET, economic threshold; IWM, integrated weed management; NPV, net present value; ODR, optimum dose rate.