Herbicide resistance in weeds – a threat to effective chemical weed control in South Africa

Abstract

Apart from the last 40 to 50 years, weed control in crops has always been a labour-intensive and time-consuming mechanical practice. Relief came in the form of chemical compounds that could control weeds, in particular selective herbicides. The development of highly effective selective herbicides has resulted in farmers becoming increasingly reliant on chemical weed control. Continuous application of herbicides with the same mode of action, however, selected for resistant mutations within weed populations to become dominant, rendering the herbicides less effective. Herbicide resistant weeds currently occur in 187 weed species distributed over 60 countries worldwide. South Africa is no exception and resistance in 15 weed species to herbicides in seven modes of action groups has been verified. This situation is compelling farmers to consider alternative weed control strategies and cropping systems. The most effective strategy to manage resistant weeds would most probably be an integrated weed management program that does not rely on any one type of weed control method alone. Farmers will therefore have to adapt weed management systems and cropping systems to delay the onset of herbicide resistance or, if it is already present, to manage it effectively in order to keep future crop production profitable and sustainable.