Herbicide-resistant weeds threaten profitable soya bean production

Weed resistance to herbicides is a newer phenomenon than insect resistance to insecticides and human resistance to antibiotics. Herbicide-resistant weeds came to the fore relatively late, starting in the 1970s and then rapidly increasing during the 1980s and 1990s. To date, there is no solution to this problem. The process by which all these resistances evolve is fundamentally the same, i.e., repeated use of a particular remedy can eventually lead to resistance. Over time (three to five years), chemical (herbicide) selection of biotypes (individuals) occurs in populations with natural (genetic) resistance to the remedy. Persisting with the practice that promoted resistance in the first place can only worsen the situation.

Herbicide mechanism of action

In weed management, the factors contributing most to the development of herbicide-resistant weeds are dependence on a single herbicide mechanism of action, and both under-dosing and overdosing with that sole herbicide or herbicide group (Tables 1 and 2). Glyphosate, the near-perfect or ‘once in 100 years’ herbicide, unfortunately ran into the wicked weed problem through sole and over-use, which became synonymous with the practice of zero-tillage and the technology of glyphosate-tolerant crops. However, based on how many weed species have evolved resistance, glyphosate, with 60 resistant weed species cases reported globally by 2023, is not the leader.

The herbicides (HRAC group 2) that inhibit the ALS enzyme in weeds is the front-runner with 176 weed species (weedscience.orgweedscience.org, the International Herbicide-Resistant Weed Database. This source hosts 534 unique cases (species x herbicide) that involve 273 weed species (156 dicots or broadleaf weeds, and 117 monocots comprising mainly grasses and sedges).

Records show that weeds have evolved resistance to 21 of the 31 known herbicide mechanisms of action and to 168 different herbicides. The scenario becomes more ominous when considering that of the 31 mechanisms of action only around 26 are economically important. Moreover, a vexing reality is that hardly any unique herbicide modes of action have entered the market in the past 30 to 40 years. This demands that we do our best with ‘old’ chemistry (herbicides). Of course, herbicides are not the only weed management tools.

Disastrous herbicide resistance

There are other methods such as mechanical control (e.g., tillage), cultural control (cropping practices), and biological control (employing natural enemies of weeds). The best chance of effectively dealing with herbicide-resistant weeds, and weeds in general, is to combine or integrate weed control methods as far as it is practical and economical. Soya bean in South Africa is relatively well-endowed with herbicide diversity with 26 different herbicides registered (Tables 1 and 2). Maize has 34, groundnuts 21, and cotton a scant ten. The smaller the herbicide diversity (choice) in a crop, the greater the risk for herbicide resistance evolving due to over-dependence on too few modes of action.

An example of the implications of herbicide resistance is bankruptcy among cotton farmers in the United States due to the intense battle with Amaranthus palmeri (Palmer amaranth), first recorded as an alien invader species in South Africa in 2018 by the SAHRI team. The SAHRI research group (up.ac.za/sahri) has since confirmed resistance in Palmer amaranth to glyphosate and chlorimuron ethyl in soya bean/maize cropping systems, as well as the same resistance profile in Amaranthus hybridus (common/Cape pigweed) populations in the Bergville and Winterton areas of KwaZulu-Natal.

Research continues and solutions for the steadily increasing herbicide-resistance problems are being investigated.

Table 1: Number of herbicides registered for use in soybean

Ways to control wicked weeds

Table 2: Registered herbicides in South African soya bean and their international weed-resistance profile (source: The International Herbicide-Resistant Weed Database, 10 January 2025, weedscience.org)

Soybean, due to its ability to rapidly form a dense canopy, can control weeds by withholding light from those that flourish under high light conditions, such as grasses and sedges. For example, yellow and purple nutsedge (Cyperus esculentus, C. rotundus) can be controlled effectively in this way. Providing irrigation is available and shifting planting dates to times (temperatures) that do not favour weeds and do not hamper the crop is also effective. Pre-plant and pre-emergence-applied herbicides are important tools that allow the crop to gain a biomass (cover) advantage over weeds early in the growing season.

Avoid over-dependence (=over-use) of a single herbicide mode of action, such as glyphosate in glyphosate-tolerant crops (maize/soya bean/cotton). This is especially true for zero or reduced tillage systems where the benefit of mechanical weed control is absent or reduced. It is of utmost importance to be aware of both over- (exceeding label rates) and under-dosing, for example using reduced rates of individual herbicides when part of spray mixtures; spraying weeds that are beyond the ideal age/size for effective control; and spraying under conditions that limit or prevent herbicide uptake by weeds.

Be aware that spraying weeds that are too advanced in either age or size effectively is ‘unintentional under-dosing’ due to the dilution factor in such plants – reminder: both under- and over-dosing promote herbicide resistance in weeds. When poor weed control tempts you to exceed label (=registered/legal) rates you must consider the real possibility that there is a resistance problem – make it known and have it investigated according to the prescribed scientific protocol.