When weeds refuse to die

Since the 1940s when herbicides were commercially released, their use has revolutionized agricultural productivity. Without them, weed control in large-scale farming will neither be economical nor practical, and yield losses will be massive. But the innovation was unfortunately accompanied by the increase in the dominance of resistant weeds. One of the earliest recorded cases of herbicide resistance was in wild carrot in Ontario, Canada in 1957. Although science quickly stepped in to develop herbicides with different modes of action, there has been a steady increase in the number of resistant weeds (see global trends in the figure below). Weeds have learned to adapt and science is struggling to keep up.

Herbicide resistance is the inherited ability of a plant to survive a herbicide application that would kill a normal population of the same species. Herbicide-resistant weeds have developed genetic resistance to certain herbicide groups, or sites of action. It is important to note that herbicides do not cause resistance in weed species, rather they inadvertently favour resistant individuals that naturally occur within the weed population. Resistance proceeds when the same herbicide, or herbicides from the same group, are applied repeatedly to an area that contains resistant weeds. The susceptible plants die while the resistant ones, favoured by the reduced competition, multiply. With time, only these resistant species will remain and any weed control efforts using that herbicide will be ineffective.

The introduction of glyphosate provided relief from herbicide resistance for 15 years until glyphosate resistance was found in 1996 from rigid ryegrass in an orchard in Australia. Subsequently, several additional glyphosate-resistant weed populations have been identified even here in the Prairies. The increasing risk of glyphosate resistance means that we are in danger of losing the efficacy of one of the most potent herbicides ever produced. Tank mixing multiple modes of action is an important step in preventing herbicide resistance in weeds, and the spring burn-off window is a good opportunity to use a tank mix rather than glyphosate alone.

Management strategies important in preventing herbicide resistance include:

• use herbicides only when necessary, and use them at the recommended rate
• avoid using the same herbicide or herbicides from the same group in the same field, in consecutive years
• use herbicide mixtures that include 2 or more herbicide groups that control the target weed
• practice crop rotation because different crops allow for a wider range of herbicide options.

Herbicides are very important tools for efficient and cost-effective weed management but their efficacy is an exhaustible resource that can be depleted over time. The present challenge is to manage them in such a way that their usefulness is prolonged while science tries to find a way to beat the constant evolution of resistant weeds. The renowned scientist Robert Pyle once said, “…make no mistake: the weeds will win; nature bats last.” For all our sakes, I hope nature slows down a bit.

-Ikenna Mbakwe, PhD, PAg
Head of Research
SynergyAG