herbicide; resistance; weed

Crop production: Are your weeds herbicide resistant?

Herbicide-resistant weeds are a major concern for crop producers. Farmers fear that, in an extreme case of resistance, they might lose the chemical tools that had previously been effective.

Weed species that can survive and reproduce following exposure to a dose of herbicide that is usually lethal to its wild type are said to be “herbicide resistant”. These weeds are dangerous to crops as they compete with the crop for light, nutrients and water.

In some cases, they also interfere with harvesting, causing lower yields and a decrease in income, according to Hestia Nienaber from the Agricultural Research Council’s Small Grain Institute (ARC-SGI).

Nienaber explains that the irrigation areas of South Africa’s Northern Cape Province, as well as the vineyards and orchards in the Western Cape, are highly prone to herbicide resistance. Small grain producing areas of the Western Cape are also prone.

Farm management practices, such as monoculture production and no-till farming, are 2 reasons believed to contribute to the problem. In no-till farming, ploughing doesn’t disturb the fields, which means that weed seed have the opportunity to sprout every season.

Also read: Conservation agriculture: No-till and controlling weeds


Field observation is the best way to detect the development of herbicide resistance, according to the Herbicide Resistance Action Committee (HRAC). Nienaber agrees, but says this isn’t always practical.

“Farmers have to monitor their farms, but if you farm on thousands of hectares, this is not a realistic solution. In the future, however, this is what it would take, as farmers are increasingly not able to rely on chemicals alone.”

Nienaber says that farmers should eliminate all other possible causes, before assuming that there is herbicide resistance on the farm. Sometimes weeds suspected of being herbicide resistant are not really resistant at all.

Many other external factors may be responsible for poor herbicide performance. Environmental factors, such as rainfall washing off the herbicide after application, can also be responsible.

The HRAC says that living plants growing alongside dead plants are another indication of developing resistance. However, variations in weed growth stage, incorrect herbicide application or crop shielding could also be the cause.
Past experience is another good test. If, in the past, the surviving weeds have always been controlled successfully by the same treatment, or a gradual decline in control has been noticed over a period of years, resistance may be responsible.

Take a look at past spraying records and the herbicide history of the farm. The repeated annual use of the same herbicide – or herbicides with the same mode of action – could be causing resistance.

The HRAC recommends that producers talk to neighbouring farmers to find out if they are experiencing similar problems. If resistance to the same weed, and involving the same herbicide, has been positively established in adjacent fields or farms, then there is a high probability that resistance is the cause.


Since farmers have no control over most environmental factors, they should focus on eliminating other possible causes. Take another look at your spraying process. Check that the dosage and timing are correct.

Ensure that tank mixes are compatible, that herbicides are sprayed using the correct and registered adjuvants, and that tanks are clean before a new mix is made. Also ensure that the sprayer is correctly calibrated and that all nozzles are clean and functional.

The person responsible for herbicide application should also be trained in the correct procedures. If you are sure that your spraying procedure is correct and that no other external factors have influenced the effectiveness of the herbicide, you may have herbicide resistant weeds.

Also read: Wheel sprayer guarantees accurate application of weed-killer


The only way to confirm resistance is to have weeds tested. Collect a seed sample of the weed in question and send it for testing.

“Farmers should try and collect mature seeds. Seedlings will be stressed when they are transplanted in the greenhouse for testing. The likelihood of them dying from transplanting and not from the herbicide is then greater, thus compromising the test results.”

  • Always collect and store seed in brown paper bags.
  • Seeds stored in plastic bags will become mouldy. Ensure that the sample reaches the testing unit as soon as possible via a courier.
  • Label the paper bag or envelope with the name of the field, farm and date of collection. Collecting seeds too early or too late is likely to lead to samples with low viability, according to the HRAC.
  • With grass-weeds, such as wild oats and rye-grass, the best time is when about 20% of seeds have already been shed.
  • Collect seeds over an area of at least 100 m x 50 m within the main problem area, unless the problem is confined to smaller distinct patches.
  • Avoid areas that have not been sprayed.
  • The HRAC says that the sample has to be representative of the problem area, so a few seeds from lots of heads should be collected.
  • Do not collect samples in wet conditions.
  • Collection is harder and seeds of some species can become very dormant.
  • The testing process takes about 6 weeks, after which results will be available.
  • Seed is planted, and after 2 to 3 weeks the seedling is sprayed with a herbicide.
  • The testing technician then waits another 2 weeks for the results of the herbicide application. If the weeds are found to be resistant, then recommendations on alternative control measures can be made.


If farmers have a problem with weeds, they should first determine if they are in fact herbicide resistant or just tough weeds.

When a weed is difficult to eradicate, it might require a different herbicide. Farmers should check if they are applying the right herbicide, at the right concentrations, using the right adjuvant.

This would cause the weed to suffer a setback and give the crop a chance to grow.

“If the correct herbicide is applied and the weed doesn’t respond and continues growing, farmers should consider that they might have herbicide resistance.”

Farmers should then look into using a different herbicide that might be more effective. Also keep in mind that some weeds are naturally tolerant to herbicides.


Adjuvant: This is a term used to describe any substance that is added to the spray tank, (separate from the pesticide formulation) that will improve the performance of pesticides or herbicides.
Herbicide: Commonly known as a weed killer, herbicides are a type of pesticide used to kill unwanted plants.

  • This article was written by Wilma den Hartigh and first appeared in Farming SA.

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