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.

Invasive animal pests have a wide variety of impacts on the economy, the environment and society. There is considerable information on these impacts for individual cases and regions, and McLeod (2004) attempted to value them nationwide for a whole range of pest animals. However, there appear to be no Australia-wide estimates of agricultural losses measured with the economist's concept of welfare and no national or statewide estimates of environmental loss based on the same concept. In the present report, the direct economic impacts of invasive animals on agriculture in Australia, and the nationwide expenditures by governments and landholders on pest management, administration and research, are estimated. The values of agricultural losses are measured through the concept of economic welfare. The overall impact of pests is calculated here as the sum of the effects on agriculture plus the expenditures on management. The estimates cover the impact on agriculture of four introduced invasive pest animals, namely: foxes, rabbits, wild dogs and feral pigs. The analysis also includes estimates, taken from literature, of the impact of birds on horticulture and mice on grains.