The issue of herbicide resistance should be on top of every crop grower’s priority list because it not only affects the environment and sustainable agriculture, but the producer’s bottom-line in the long run. It can be likened to antibiotic resistance in humans, it doesn’t seem like a problem while you are taking the medication continuously. In fact, it seems like a miracle cure and you don’t bother following the prescriptions because the long-term effects are not immediately clear. Only when disaster strikes, and the microbes become resistant to the medication, will the significance of this error be obvious.
It’s the same principle with herbicides. Herbicides are essential to protect crop yield and quality by controlling weeds that compete for essential resources. However, by applying these essential herbicides but not following the product label or adhering to the resistance warnings, it could have dire consequences in the long-term. Resistance occurs because in a population of weeds, a very small number are naturally resistant to certain types of herbicides. Applying a herbicide will control nearly the entire population, except those that are naturally resistant. The survivors lead to the next generation of resistant weeds and soon the multiplication effect is nearly uncontrollable. This happens when applying the same herbicide with the same mode of action repeatedly to the same population of weeds.
Suppose for instance, a farmer did not adhere to the guidelines for managing resistance and the next moment there’s a palmer amaranth infestation, but this time it is resistant to the very glyphosate he’s been applying year in and year out. Imagine the impact of suddenly not being able to use products containing that active ingredient. Glyphosate-based products are an enormous asset in benefiting soil health and conserving water by allowing the farmer to practise no-till farming. This means that the soil remains mostly undisturbed and crop residues are left behind, thereby eliminating erosion. Eliminating soil erosion prevents not only the loss of fertile land, but also reduced crop yields due to a decrease in plant‐available water reserves, degradation of soil structure and the loss of rooting depth. In addition to decreased yields, the farmer now also has increased production costs as the need for conventional tilling increases fuel and labour costs as well as the number of herbicides required.
This is just one example, but in reality, there are over 250 weed species that have evolved to resist 160 different herbicides globally. There is a high risk of developing resistance if a spray programme for weed control relies on products that only have one mode of action, if weed control is only chemical or if the same mode of action is used many times per season. Other factors include no rotation in the cropping system, a high weed infestation and poor control in previous years.
It goes without saying that every farmer should commit to preventing herbicide resistance, as opposed to just reacting when it happens. By applying the principles of integrated pest management, in other words using a combination of chemical, biological, mechanical and cultural weed control methods and not just relying on one, a farmer is already heading in the right direction.
The importance of reading the product label cannot be stressed enough either. Not only does it contain directions for preventing and/or managing resistance, but it also displays the specific resistance management group that the herbicide belongs to, which plays a critical role when planning a weed control programme.
The Herbicide Resistance Action Committee (HRAC) has a wealth of resources, best practice documents and tools available on their website www.hracglobal.com, including a global classification lookup tool. CropLife SA encourages farmers to familiarise themselves with these resources and to do their part in preventing herbicide resistance.