Managing the Herbicide Resource : An Evaluation of Extension on Management of Herbicide - resistant Weeds

Pests and weeds can develop resistance to pesticides and herbicides soon after they are introduced. This is demonstrated by early reports of insecticide resistance (Melander, 1914) and herbicide resistance (Ryan, 1970). In some cases the first observations of resistance to a particular herbicide have occurred around the same time as its local commercial release (e.g., Heap & Knight, 1990). In other cases it has taken decades (e.g., Powles, 2008). Herbicide-resistant (HR) weeds are now widespread (Heap, 2009) and are a complex farm management issue (Pannell & Zilberman, 2001). The rapidly increasing availability of HR crops has added to the complexity and importance of managing herbicide-resistance risks. Although the ecological essence of the problem remains the same (weed populations will evolve resistance to herbicides when exposed to repeated selection pressure), HR crops have introduced new avenues to apply certain herbicides and select for resistance. This has seen HR weeds become an issue of increasing concern in regions where resistance was previously not a major consideration (Cerdeira & Duke, 2005; Owen & Zelaya, 2005). The economic importance of crops with resistance to specific herbicides (Brookes & Barfoot, 2006) has also increased the potential economic impacts of weed resistance to those herbicides. In this article, we examine decision making about herbicide-resistance management at the farm level, focusing on the implications for agricultural extension. The research comes from a major Australian cropping region where there has been long-term experience dealing with widespread and severe resistance to important crop-production herbicides (Owen, Walsh, Llewellyn, & Powles, 2007; Powles, Preston, Bryan, & Jutsum, 1996). A range of factors influencing the adoption of a suite of weed control practices for herbicide-resistance management by grain growers has been identified in this region (Llewellyn, Lindner, Pannell, & Powles, 2007). These are referred to in this article as integrated weed management practices (IWM) and are essentially nonherbicide treatments combined with some herbicidebased treatments that can reduce the selection pressure for herbicide resistance. The article begins with an overview of the resource management-based approach used to consider herbicide use and resistance management decisions in this study. This is followed by results from an empirical study where the influence of extension and learning on aspects of the herbicide-resistance management and IWM adoption problem are evaluated. Implications for improved herbicideand weed-management strategies in the presence of herbicide-resistance risks are discussed. Rick S. Llewellyn CSIRO Sustainable Ecosystems, Adelaide