Herbicide-resistant weeds are becoming an increasingly serious challenge for farmers worldwide, threatening crop productivity, increasing production costs and complicating weed control. For South African producers, the solution lies in combining effective herbicide use with sound agronomic practices and integrated weed management.
Despite advances in agricultural technology, weed management remains one of the biggest challenges in crop production. Weeds compete with crops for water, nutrients, sunlight and space, potentially reducing yields and compromising crop quality. When weeds develop resistance to herbicides, controlling them becomes even more difficult.
Hundreds of herbicide-resistance cases have been documented in more than 100 weed species worldwide, highlighting the scale of the challenge facing the agricultural sector.
In South Africa, herbicide resistance is an increasingly important concern for producers. Dr Elbé Hugo, category marketing lead for herbicides at Corteva, has emphasised the need for integrated weed management to protect farm productivity and maintain effective weed control.
Understanding herbicide resistance
Herbicide resistance develops when a weed population evolves the ability to survive herbicide applications that previously controlled it effectively.
According to Hugo, repeated selection pressure is a major driver of resistance. When farmers consistently use herbicides with the same mode of action, susceptible weeds are eliminated, while naturally resistant individuals survive and reproduce. Over successive growing seasons, these resistant weeds can become increasingly dominant, reducing the effectiveness of certain herbicides.
Suspected resistance should be investigated promptly. Leaving the problem unmanaged allows resistant weeds to reproduce and spread, making future control more difficult. Alternative control measures and, where appropriate, formal resistance testing can help farmers identify the problem and make better-informed management decisions.
The economic consequences can be substantial. Herbicide resistance may increase weed-control costs, reduce crop yields and require additional or alternative control measures.
Elriza Theron, advocacy and communications manager at CropLife South Africa, stresses that managing resistance should be viewed as a long-term investment rather than simply an additional short-term production cost.
Dirk Uys, an independent crop protection consultant at Agbridge, notes that wheat producers in the Swartland region have faced persistent herbicide-resistance challenges, particularly with ryegrass (Lolium species).
Although herbicides remain important tools, effective alternatives are becoming more difficult and costly to develop. Uys says this reinforces the need for integrated weed management systems that combine different herbicide modes of action with agronomic practices, including carefully planned crop rotations.
The key, he explains, is to manage the entire production system rather than relying on a single product or control method.
Understanding the different types of resistance
Maryke Craven, technical lead for herbicides at Syngenta, says farmers need to understand the types of resistance present in their fields to select appropriate control strategies.
Increasing the herbicide dose is not necessarily an effective solution, particularly when weeds have developed target-site resistance. Instead, farmers should consider alternative active ingredients with different modes of action, supported by non-chemical control methods.
The movement of farm implements between fields and farms is another important consideration, as contaminated equipment can transport weed seeds and contribute to the spread of resistant populations.
Herbicide resistance mechanisms fall into two main categories: target-site resistance and non-target-site resistance.
Target-site resistance occurs when changes at the herbicide’s specific site of action prevent the product from affecting the plant as intended.
Non-target-site resistance involves mechanisms that prevent the herbicide from reaching its target or reduce its effectiveness within the plant.
Understanding these mechanisms helps farmers and crop advisers develop more appropriate management strategies, particularly where cross-resistance to multiple herbicides may occur.
Integrated weed management offers a sustainable solution
Although herbicides are an important component of weed control, relying exclusively on chemical solutions is not sustainable in the long term, Theron warns.
She recommends rotating herbicides with different modes of action, following label instructions and combining chemical control with practices that reduce weed populations and improve crop competitiveness.
Chris Preston, professor of weed management at the University of Adelaide in Australia, has also highlighted the importance of treating herbicide resistance as a broader farm-management challenge rather than a problem that can be solved by changing products alone.
According to Preston, rotating herbicide modes of action is essential, but it must form part of a wider integrated weed management programme that includes chemical, cultural and mechanical control methods.
A well-designed programme may include:
Herbicide rotation: Alternate effective herbicides with different modes of action, following product labels and resistance-management recommendations.
Crop rotation: Introduce different crops to disrupt weed life cycles and allow the use of varied weed-control programmes.
Cover crops: Use suitable cover crops to compete with weeds and limit the light and space available for weed establishment.
Strategic tillage: Where appropriate, use tillage to manage weed populations and reduce the contribution of weed seeds to future infestations.
Optimised planting practices: Adjust planting dates and crop density to give crops a competitive advantage over weeds.
Weed seedbank management: Prevent surviving weeds from producing and returning seeds to the soil.The objective is to reduce weed populations and their economic impact while preserving the effectiveness of available herbicides.
Correct herbicide application is critical
Correct application practices are essential to slowing the development and spread of herbicide resistance.
Hanlie Geldenhuys, agronomy lead at Syngenta South Africa, says resistance can be accelerated when herbicides are not applied correctly. Poor timing, unsuitable application rates and inadequate spray coverage can allow weeds to survive and reproduce, increasing the risk of resistance developing or spreading.
Farmers should pay close attention to the following practices:
Apply herbicides at the correct time: Target weeds when they are small and actively growing, and avoid applications when weeds are under stress if the product label advises against it.
Follow label rates and instructions: Use the recommended application rate and comply with all product-specific directions.
Ensure adequate spray coverage: Use suitable water volumes and application settings for effective coverage.
Use appropriate adjuvants and water conditioners: Where recommended, these can help optimise herbicide performance under the relevant water and application conditions.
Avoid unapproved tank mixtures: Herbicide combinations that are not supported by label instructions or compatibility guidance may reduce efficacy through antagonism.
Maintain spray equipment: Regularly inspect and clean sprayers, check nozzles and ensure equipment is calibrated correctly.
Use certified seed: Plant certified seed to support crop establishment and reduce the risk of introducing weed seeds through contaminated planting material.
Prevent weed seed production: Control surviving weeds before they replenish the soil seedbank.Poorly maintained equipment, including blocked nozzles, can result in uneven application and areas receiving insufficient herbicide. Such failures may allow weeds to survive and reproduce, contributing to the selection and spread of resistant individuals.
Craven cautions that repeated exposure to ineffective or sub-lethal herbicide applications can undermine weed control and increase management challenges in subsequent seasons.
Protecting the future of weed control
The growing threat of herbicide resistance is compounded by the limited availability of new herbicide products and the time required to develop new active ingredients.
Integrated weed management provides a practical way to reduce dependence on individual herbicides and improve the long-term sustainability of weed control. It combines two or more complementary methods to keep weed competition below economically damaging levels.
This approach is important for several reasons.
First, a weed-control method that is effective and economical under one set of conditions may not work equally well in another field or cropping system. Second, no single herbicide controls every weed species effectively. Third, repeatedly using the same herbicide or mode of action can select for resistant weeds and change the composition of weed populations over time. Finally, relying on only one control method can allow certain weed species to increase.
Integrated weed management also presents practical challenges. These include planning application intervals and waiting periods, working within the range of available registered products, and adapting strategies to local conditions and weed populations.
Nevertheless, the long-term benefits of a diversified approach outweigh the risks of relying on chemical control alone.
For South African farmers, the way forward is clear: monitor fields regularly, investigate suspected resistance, apply herbicides correctly, rotate effective modes of action and combine chemical control with cultural and mechanical practices.
Herbicide resistance cannot be managed by changing products alone. Protecting the effectiveness of available herbicides requires a consistent, integrated approach that manages weeds across the entire cropping system.
