Proactive weed management from a global perspective

Dr Gerhard Verdoorn, operations and stewardship manager, CropLife SA
Published: 24 February 2022

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Herbicides are only as effective as the mindset of the individuals who apply them. One serious shortcoming in agriculture worldwide is the failure to follow label directions for safe and effective use of herbicides. Labels are the products of extensive and very costly research aimed at proving the efficacy, human safety and environmental compatibility of herbicides. The key elements of label-directed use to ensure sustainability and to delay resistance development for as long as possible, are the following:

Use the correct herbicide dosage
Suppliers of herbicides are very conscious of the resistance issue, because too little or too much of the herbicide active ingredient will cause shift in weed sensitivity to the resistance side of the curve. It is thus fatal to do as many producers do: add a little extra or double up on the dosage if the herbicide is not effective as expected. A higher dosage removes all the sensitive individuals and rapidly selects the semi-susceptible individuals or resistant individuals. A lower dosage does the opposite: it does not take out the semi-susceptible individuals and leaves more of them in the next generation to add to the resistant number of individuals. Annual weeds can shift to resistance rapidly if the dosage is not 100% on the dot.

Use the correct spray volume per surface area
Spray volume is much more critical in weed control than in insect control. Labels normally instruct the use of high spray volumes per hectare to ensure adequate cover of the target species for contact herbicides, while being essential to ensure total cover and drenching of soil surfaces for soil-applied herbicides. Even if the correct dosage per surface area is applied, too low a spray volume may result in adequate deposition of the herbicide active ingredient on the target, which means that weeds do not receive a lethal quantity of the herbicide which also speeds up the shift to reduced sensitivity.

Add the prescribed adjuvants and/or complementary herbicides to the spray mixture
Herbicides with different modes of action are often available in ready-made formulations, or labels instruct the use of tank mixtures to ensure effective weed control. This is an essential tool in preventing sensitivity shift. A wetting agent, spreader or penetrant are often required to make sure the herbicide active ingredient or ingredients cover the weed surface evenly to afford maximum efficacy, while a penetrant may be a key component of a herbicide tank mixture to penetrate waxy or hairy weed surfaces. Again, the dosage is not only important in terms of quantity per surface area, but the quantity must be deposited virtually 100% on the target. Adjuvants such as those mentioned are part of the producer’s toolkit to prevent sensitivity shift in weeds.

Check weather conditions before spraying
The universal rule – spray when weather conditions are optimal – is too often ignored. Applying herbicides while weeds are covered in dew or shortly before rain, may very well negate the application and leave a stand of weeds that have absorbed just enough herbicide to shift their sensitivity to the herbicide to the resistance curve. Hot weather and humidity play negatively against any pesticide application. High temperatures evaporate water and solvents which prevent adequate deposition of the spray mixture onto the target weeds, while high humidity prevents adequate drying of the spray mixture on target weeds with resultant dripping off and sub-optimal uptake of the active ingredients. As with other factors discussed above, poor weather is a catalyst for sensitivity shift.

Apply herbicides at the correct window of opportunity
The young of any species are less resilient against environmental and anthropomorphic stressors. Young weeds are highly susceptible to herbicides, but as the plants mature, they become much less susceptible. Most herbicide labels instruct the user to apply the product not later than a certain growth stage of the weed to ensure maximum efficacy. If such instruction is ignored, the more mature weeds will not be controlled effectively and resistance is looming in the future.

Do not apply the same herbicide mode of action more than the label-directed number of applications per growth season
Organisms adapt to an environmental stressor by amplifying the percentage of the population with genes that obstruct the attack mechanism of the stressor. This, in herbicide terms, means that a weed will express sensitivity shift against one or more herbicide modes of action if those herbicides are applied season after season on the same weed population or in too rapid a frequency per weed growth season.

Adapt cultivation practices to move away from total reliance on herbicides
Integrated pest management (which includes weed management) dictates that all possible mechanisms must be incorporated into a pest control programme. Mechanical weeding is often frowned upon, but with modern machinery it is possible to at least use an alternative to the incessant spraying of herbicides. Some weeds like the Palmer amaranth and Conyza species (fleabane) may require mechanical removal and total destruction by fire to destroy seeds as well. Weeds that are expressing very high tolerance or resistance to herbicides demand a paradigm shift – and mechanical weeding may be the only option. A desiccant such as paraquat or glufosinate-ammonium may assist in the processing of burning resistant weeds in situ, but this must obviously be done before planting or after harvest if there is a weed flush.

What can South Africa learn from other Mediterranean countries in terms of weed resistance management?
As mentioned earlier, we are far better off than many other countries with smaller numbers of weeds that are resistant to herbicides. Another advantage is that our cultivation and crop spraying equipment is generally much younger and technologically more advanced than that of most European producers. Better equipment offers better application, which in turn acts as a barrier against sensitivity shift. South Africa may be very far from Europe or the Americas, but our agriculture is certainly on par with theirs, if not better. At the 2018 Herbicide Resistance Summit in Europe, Josef Soukup of the Czech University of Life Science in Prague, delivered alarming news about unique resistance cases in European countries. Grass species like wild oats, Italian ryegrass, black grass, Bromulus and several other weeds, affected more than 10 million hectares of cereals with France being the most affected with 50 unique resistance cases. Weeds were often resistant to more than one herbicide mode of action, which leaves producers at a loss for solutions. Weed resistance to glyphosate was most prevalent in ryegrass species and fleabane species. Compared to South Africa, we see a similarity with Italian ryegrass in wheat and Conyza species all over the country. This should ring the alarm bells for South African cereal and maize producers because of their strong reliance on glyphosate.

Our most important take-home message from the European Union (EU) is that we are still at the dawn of weed resistance in the winter rainfall areas, but producers need to implement resistance management immediately if they hope to escape the situation that developed in the EU. Switching between different herbicide modes of action is perhaps the founding principle of preventing sensitivity shift, but all the other elements as discussed above must also be built into an effective and integrated weed management programme. Arguments about costs should be frowned upon because the final question in herbicide resistance management is: ‘Do you want to grow winter crops sustainably over a long period of time at slightly elevated production costs, or do you want to grow winter crops only for a few more years before resistant weeds beat you hands down?’