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.

Abstract. This study addresses the problem of balancing the trade-offs between the need for animal production, profit, and the goal of achieving persistence of desirable species within grazing systems. The bioeconomic framework applied in this study takes into account the impact of climate risk and the management of pastures and grazing rules on the botanical composition of the pasture resource, a factor that impacts on livestock production and economic returns over time. The framework establishes the links between inputs, the state of the pasture resource and outputs, to identify optimal pasture development strategies. The analysis is based on the application of a dynamic pasture resource development simulation model within a seasonal stochastic dynamic programming framework. This enables the derivation of optimum decisions within complex grazing enterprises, over both short-term tactical (such as grazing rest) and long-term strategic (such as pasture renovation) time frames and under climatic uncertainty. The simulation model is parameterised using data and systems from the Cicerone Project farmlet experiment. Results indicate that the strategic decision of pasture renovation should only be considered when pastures are in a severely degraded state, whereas the tactical use of grazing rest or low stocking rates should be considered as the most profitable means of maintaining adequate proportions of desirable species within a pasture sward. The optimal stocking rates identified reflected a pattern which may best be described as a seasonal saving and consumption cycle. The optimal tactical and strategic decisions at different pasture states, based on biomass and species composition, varies both between seasons and in response to the imposed soil fertility regime. Implications of these findings at the whole-farm level are discussed in the context of the Cicerone Project farmlets.

An analysis of the annual costs of weeds in seven winter crops across Australia demonstrated that the most important 15 weed species cause substantial annual costs in both financial and economic terms. Using survey data captured over the 1998-1999 growing season, the financial cost of these weeds in seven crops was estimated to be AU$1,182 million. The main components of this cost were herbicides (AU$571 million), the competitive effects of residual weeds (AU$380 million), and tillage (AU$206 million) while weed contamination of grain was a minor cost (AU$25 million). Across all regions, the most economically important weeds were annual ryegrass, wild oats, and wild radish, although there were regional differences in importance. An economic surplus analysis determined the annual economic cost of weeds in annual winter crops to be AU$1,279 million. This surplus loss represented 17% of the gross value of Australian grain and oilseed production in 1998-1999. Australian grain producers incurred a major loss, with a reduction in producer surplus of AU$1,047 million. Australian grain consumers had a large consumer surplus loss (AU$229 million), while international consumers suffered a small loss and international grain producers gained a small producer surplus from the higher grain prices.JEL classification: D60, Q11, Q16

Pasture degradation is a major issue in the high rainfall zone (>600 mm) of temperate Australia. Characterised by a decline in the perennial grass component, this degradation is responsible for reduced livestock production and implicated in environmental problems, such as dryland salinity, through changed water-use patterns. This paper reports on a multi-disciplinary research program conducted at Carcoar in central New South Wales, one of 6 sites that comprised the Sustainable Grazing Systems National Experiment. The aim of the experiment was to develop more profitable and sustainable pasture systems by evaluating the impact of changes in the perennial grass component on animal production and water-use patterns. Tactical management strategies were evaluated on naturalised and sown perennial grass pastures and on chicory (Cichorium intybus), using a Merino ewe-based first cross lamb enterprise. Data presented showed that grazing deferment over summer, combined with reduced stocking rate, increased perenniality and reduced annual grass weeds compared with continuous grazing. Livestock performance, however, did not always follow trends in available perennial herbage mass. Naturalised and sown pastures were suitable for raising prime lambs, but only chicory had the capacity to finish lambs to market specification without supplementation. With respect to water-use patterns, sown perennial pastures were more successful than naturalised pastures in reducing leakage of water from the root zone, although this seemed to be related mostly to the abundance of the perennial grass in the pasture. From a sustainability perspective, continuously grazed pastures generated higher net cash flows, but had negative environmental impacts, whereas tactically grazed pastures had positive on- and off-farm impacts but lower net cash flows. The implications of these findings for incorporation into future management strategies for sustainable production in high rainfall environments are discussed.

Seasonal variability is an important source of risk faced by farmers and, regardless of an individual's attitude to risk, there are options to tactically adjust production strategies as the outcomes of risk become known. The objective of this article is to measure the economic benefits of alternative approaches to managing weeds, one of the most serious production problems in Australian cropping systems. A bioeconomic model that combines weed biology, crop growth and economics is developed to value the effects of seasonal variability and the role of tactical responses and sequential decision making in determining an optimal integrated weed management strategy. This shows that there are substantial differences in the measured long-term benefits from deterministic and stochastic simulations. It is concluded that, for research evaluation of technologies that involve complex biological and dynamic systems, ignoring the impacts of seasonal variability, responses to risk and sequential decision making can lead to an incorrect estimate of the economic benefits of a technology. In this case study of optimal weed management strategies in Australia, the size of the error is high. Copyright 2006 International Association of Agricultural Economists.