DEKALB is committed to the sustainable use of all its products and technology and, therefore, promotes weed resistance management as a crucial system to obtain sustainability.

Knowledge of how weed resistance develops, what it is and how to refrain it from developing is necessary to effectively manage the resistance of weeds to herbicides.

Weed resistance is the capability of a plant (weed) to survive and reproduce after being exposed to a herbicide that would have been lethal in other circumstance (Heap, 1999). Thus, the weed will not die if sprayed by a lethal dosage that normally would have been sufficient to kill it.

In any plant population, there will be a tolerant component. If herbicides with the same mode of action are sprayed at a non-lethal dosage or on a continuous basis over a long period of time, selection will take place in favour of the tolerant population. This population may grow to dominate the weed spectrum and weed control may be drastically hampered or no weed control may be possible, thus reducing the yield of the crop.

Roundup® has been on the market for more than 40 years and remains one of the safest products to use. Furthermore, it is one of the groups of herbicides that is less susceptible to weed resistance. There are three weeds in South Africa that have developed some form of resistance to glyphosate. These weeds are Lolium rigidum (rigid ryegrass), Conyza bonariensis (hairy fleabane) and Plantago lanceolata (buckhorn plantain), mostly found in the vineyards and small-grain fields of the Western Cape (Breede Valley, Paarl and Tulbagh district), where the herbicide has been used for many years, often at an incorrect dosage.

How do we identify weed resistance? 

Farmers should keep record of the dosage herbicide used per unit of land, the weed spectrum targeted, as well as the results and possible reasons for achieving these results. The history for each unit of land should be compared over years to determine whether resistance developed over a specific period of time.  An assessment should be done over a few seasons to determine if there is any resistance to a specific herbicide, and to determine possible other factors that may have negated the efficacy of the herbicide.

The following factors should be taken into consideration before weed resistance is considered:

  • Determine whether or not the conditions were ideally suited for application of the herbicide, i.e. the wind speed, humidity and air temperature.
  • Did the herbicide reach its target, at which rate and at which growth stage? If the herbicide reached its target, but at a much reduced rate, the weeds may not be controlled at all. Also, if the herbicide reached the target, but at a later growth stage, the weed may not be controlled.
  • Control of weeds will also be much more difficult because of the growth stage they are in. 

All of these factors have an influence on the efficacy of the herbicide.  

It is important to follow the instructions on the label for information on application rates and spraying conditions.

The best method of eradicating, or preventing resistance, is by using an integrated weed control programme. Such a programme should comprise crop rotation, using different herbicides with different modes of action, as well as making use of mechanical cultivation and weed control practices.

At DEKALB we recommend using herbicides with a different mode of action together with Roundup®. In Roundup Ready® maize, Roundup® PowerMAX may be used together with Harness® Extra EC, a selective residual herbicide which controls grasses and certain broad-leaf weeds and contains acetochlor as an active ingredient. Other herbicides are also registered to be used together with Roundup®.  Again the labels must be consulted for further information and applications should be done as stipulated on the label.

Making use of crop rotation as well as integrated weed management programmes will reduce the chances of resistance developing. If any resistance is suspected, please contact your nearest chemical agent or DEKALB representative.

  • Monsanto SA contracted BE at UP (Pty) Ltd in the second half of 2012 to undertake a research and development programme aimed at investigating weed resistance for the broader South African farming community and to undertake the promotion of Good Agricultural Practices (GAPs) in this regard.
  • The project also aims to build capacity and generate new scientific knowledge and under the leadership of Professor Charlie Reinhardt, extraordinary Professor to the Department of Plant Production Soil Science, a team of postgraduate students will complete masters and PhD qualifications respectively, investigating the optimisation of integrated modes of action for weed control in the South African context.
  • “One of the main objectives of Monsanto in contracting the University to do research on this subject, is to be pro-active and we see this as part of our product stewardship in South Africa”, says Kobus Steenekamp, Managing Director & Commercial Manager for Monsanto South Africa.
  • The programme in its current form is dedicated in finding effective ways for dealing with weed resistance to glyphosate in South Africa as well as to advocate effective weed management practices. The programme is still running.
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