There’s no quick way to outsmart stubborn weeds, so rethink

22 January 2024

By: Maryke Craven

There is no magic weed-combating recipe that can fix everything in a single season. Glyphosate has been considered a quick fix in many respects but nature has reminded humans of how adaptable it can be.

If such a “miracle cure” becomes available again, it will only be a matter of time before resistance develops. Resistance occurs when the same product is repeatedly applied, not only in the same season but also year after year. On average, 10 new biotypes are identified worldwide each year, all developing a common trait such as resistance to a specific herbicide.

Today, there are 523 cases of herbicide resistance in 269 species, with weeds successfully developing resistance to 21 out of the 31 existing “mode-of-action” herbicides. In many cases, weeds are not only resistant to one active ingredient but three or more simultaneously. Palmer amaranth is a striking example of resistance to more than one active ingredient.

These weeds have developed resistance to saflufenacil, chlorimuron-ethyl and glyphosate within the same plant population. Since the introduction of glyphosate, few new herbicides have come to market, exacerbating the problem and predicting a bleak future for farmers. While the ideal would be to prevent herbicide resistance on a farm, that opportunity has passed for many farmers, especially concerning glyphosate.

If a farmer discovers that a weed is resistant, increasing the dose on the product label is not the responsible action as it will exacerbate the problem. The responsible action is to notify the product owner (company) of possible resistance so experts can further address it.

Regarding the control of resistant weeds, four basic goals should be pursued:

1. Prevent the spread of seeds.
2. Prevent seed production.
3. Destroy the seed.
4. Follow a reversal strategy to completely get rid of resistant weeds.

Follow an integrated approach consisting of agronomic, mechanical and chemical approaches. Rethink the weed control strategy that can work on a particular farm with its existing, specific systems (tillage and crop rotation). Each farm will have a unique “winning recipe”. Resistant weeds require deviation from tradition, especially in two aspects.

Crops traditionally planted

The outbreak of Palmer amaranth in 2018 led to the discontinuation of cotton cultivation in the affected area. This step was enforced by a lack of herbicides registered for Palmer amaranth control in cotton. Traditional crop rotation systems (or the lack of crop rotation) will therefore need to be reviewed. Also, investigate alternative crops and experiment with them. Including alternative crops provides access to a new range of herbicides that can effectively combat resistant weeds.

Cultivation practices

Farmers who have faithfully applied conservation tillage will need to consider performing a plough operation. Weeds such as thinleaf alder, which are especially problematic with conservation tillage, can be effectively controlled with ploughing.

Dr Maryke Craven works at the Agricultural Research Council’s grain crop department in Potchefstroom. Inquiries: 018 299 6100.