Julius-Kühn-Archiv (Feb 2014)
Reduced herbicide rates: present and future
Abstract
Applying herbicides at rates lower than the label recommendation has been the rule rather than the exception in Denmark since the late 1980’s. Justifications for reducing herbicide rates can be 1) that the dominant weed species in the field are very susceptible to the herbicide, i.e. even reduced rates will result in maximum effects, 2) that the conditions at and around the time of application, e.g. growth stage of weeds, crop vigour and climatic condition are optimum promoting the activity of the herbicide and thus allows for the use of reduced herbicides rates, or 3) that less than maximum effects are accepted because the weed flora is not considered to have a significant effect on crop yield. “Crop Protection Online-Weed” (CPO-Weed) is a web-based decision support system that was developed to support farmers in their choice of herbicide and herbicide rate. CPOWeed will, based on information on crop development and status and the composition of the weed flora, provide farmers with a list of herbicide solutions often recommending the use of reduced rates. The potential of CPO-Weed to reduced herbicide input has been proven in numerous validation trials. In recent years the use of reduced herbicide rates has been linked to the increasing number of cases of non-target resistance in outcrossing grass weed species like Alopecurus myosuroides and Lolium ssp. The underlying hypothesis is that the least susceptible individuals in the population will survive the use of reduced rates and that recombination will lead to a gradual increase in the resistance level in the weed population. This scenario is only valid if the use of reduced herbicide rates is prompted by acceptance of a lower effect but not if a high susceptibility of the weed species present in the field or optimum conditions are the reasons for reducing herbicide rates. This is an aspect that is often overlooked in the on-going discussion on herbicide rates and resistance. Large weed population increases the risk of selecting resistant weed biotypes because the likelihood that resistant plants are present in the population increases with population size. Preventing the build-up of large weed populations is a key objective in integrated pest management (IPM). If the use of herbicide is combined with non-chemical weed control methods the risk of resistance will be reduced further, i.e. in an IPM scenario the use of reduced herbicides rates will be less likely to promote herbicide resistance even if it is triggered by an acceptance of lower effects.
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