Early action key as herbicide resistance threatens weed control

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According to Corteva Agriscience, recognising these indicators early and adopting diversified weed management strategies can help slow the spread of resistant weed populations and protect long-term crop production.

Recognising the early warning signs

"Herbicide resistance develops gradually and farmers usually see the first indicators long before a serious control failure occurs," says Dr Elbé Hugo, category marketing lead: herbicides at Corteva Agriscience. "The challenge is recognising those warning signs early enough to make management adjustments before resistant populations become dominant."

At field level, resistance often starts subtly. Farmers may notice small patches of weeds surviving in isolated areas after spraying, while the rest of the field appears well controlled. According to Corteva Agriscience, these "weed escapes" should never simply be dismissed as spraying errors without further investigation.

Herbicide resistance develops when naturally tolerant weeds survive herbicide applications that would normally control them and continue reproducing within the field population. This process is accelerated by the repeated use of herbicides with the same mode of action, particularly in minimum tillage or monoculture cropping systems where limited alternative weed control practices are implemented.

Higher-risk weed species

Certain weed species present a greater resistance risk than others. Wild oats (Avena fatua), plantain (Plantago spp.) and fleabane (Conyza spp.) are highly adaptable because they produce large quantities of seed and possess high genetic diversity. Species such as pigweed (Amaranthus spp.) and ryegrass (Lolium spp.) also cross-pollinate easily, accelerating the spread of resistant traits within fields.

According to Corteva Agriscience, farmers should regularly assess whether their production practices may contribute to herbicide resistance. Warning signs include repeated use of the same herbicide programme in the same fields, a gradual decline in weed control over several seasons, surviving weeds appearing in isolated patches, and cases where only certain weed species survive while others remain effectively controlled.

Farmers should also remain aware of resistance problems reported on neighbouring farms or in surrounding areas, as resistant populations can spread over time.

Resistance is not the only cause of poor weed control

However, Dr Hugo emphasises that poor weed control does not automatically confirm resistance.

"Before resistance is assumed, farmers must first evaluate whether application factors may have affected performance," she explains. "Incorrect spray timing, unsuitable weather conditions, poor coverage, incorrect dosage rates and spraying weeds beyond the optimal growth stage can all reduce herbicide effectiveness."

This makes accurate field records increasingly important. Maintaining detailed information on herbicide programmes, crop rotations, weed pressure and spray conditions allows farmers and advisers to identify developing trends over time.

Preventing the spread of resistant weeds

Resistance problems do not remain isolated to a single field. Resistant weed seed can spread through harvesting, tillage and planting equipment moved between lands. Poor equipment hygiene may therefore accelerate the movement of resistant populations across an entire farming operation.

For this reason, biosecurity practices are crucial in weed management programmes. Cleaning machinery before moving between fields and managing suspect fields separately can significantly reduce the spread of resistant weeds.

Where resistance is strongly suspected, farmers are encouraged to collect seed or plant samples for formal resistance testing. Early confirmation allows producers to implement corrective strategies before resistant populations become widespread. According to Corteva Agriscience, prevention remains far more affordable than attempting to regain control after resistance has become firmly established.

"Farmers cannot afford to wait until resistance becomes visible across entire lands," says Hugo. "The earlier a potential problem is identified, the more management options remain available. Proactive monitoring and diversified weed control strategies are essential to protecting the long-term effectiveness of herbicides."

As resistant weeds continue to evolve worldwide, informed decision-making, regular monitoring and integrated weed management practices remain essential to safeguarding South African crop production systems for the future.