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Herbicide-resistant weeds – a global perspective

Resistance is futile. Herbicide resistance is quite predictable. There is nothing mysterious about herbicide resistance. It is a simple, naturally occurring evolutionary response to selection pressure by a mortality agent, which in our case would be a herbicide.

Heap runs the International Survey of Herbicide-Resistant Weeds, weedscience.org, which has been online for 21 years. Scientists around the world upload documented cases of new herbicide-resistant cases. A unique case is classified as a unique species by the site of action (mode of action). For example, a case from Manitoba that has wild oat resistant to Groups 1, 2, 8, 14 and 15 would represent five unique cases.

As of April 4, there are 467 unique cases (species x site of action) of herbicide-resistant weeds globally, with 249 species (144 dicots and 105 monocots). Weeds have evolved resistance to 22 of the 25 known herbicide sites of action and to 160 different herbicides. Herbicide-resistant weeds have been reported in 86 crops in 66 countries.

Globally, there are over 1.4 million fields with confirmed herbicide resistance and approximately 11 new biotypes are discovered every year. Chronologically, the number of cases is on a steep increase. (See Fig. 1.)

The year 1946 saw the introduction of the first modern herbicides – synthetic auxins – which revolutionized weed control in cereal production. The first appearance of a well-documented case of herbicide resistance occurred in 1970. (In hindsight, there were actually several other cases, including one in Canada: wild carrot with 2,4-D resistance.) This first well-documented case was common groundsel from Olympia, Wash., where they were applying triazine (Group 5) between nursery plots. The resistance was of no economic consequence, but prompted researchers in Europe and North America to go looking in corn where triazine herbicides were relied upon for weed control, and indeed they found atrazine- and triazine-resistant weeds in the cornfields of North America and Europe.

Herbicide resistance definition
Resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type.

There are two prerequisites for resistance evolution. First, there must be individual genes conferring resistance present in the population. There must be at least one resistant plant out there. Second, selection pressure must be exerted on those resistant individuals. Both of these factors must be present or resistance will not occur.

The original frequency of resistant individuals can vary enormously and is dependent on the herbicide as well as the weed. For some Group 2 herbicides and kochia, the original frequency may have been as high as one in 100,000 individuals; for Group 9 (glyphosate) and kochia, it may have been as low as one in 100 million. Therefore, all efforts must be focused on reducing selection pressure on resistant individuals that might be present.

Herbicides do not create resistance. If individuals of the resistant biotype are present and we repeatedly use an herbicide to which they are resistant, then we select for that biotype and the numbers build up. Herbicides have just helped select out the resistant individuals (while controlling the susceptible ones). Resistance is detected when a high proportion – usually greater than 30 per cent – of the population is resistant to the herbicide.

Weed seeds in the soil are often greater than 100 million seeds per hectare, and weed seedling populations are often greater than one million seeds per hectare. Scientific estimates suggest that depending on the herbicide group, there may be one resistant individual in 100,000 to one in 100 million. (See Fig. 2.)

• ALS inhibitors – 1 in 100,000
• ACCase inhibitors – 1 in 1,000,000
• Many groups – 1 in 10,000,000
• Auxins and glyphosate – 1 in 100,000,000

North America is leading the way with the number of cases. Western Europe, Asia, Australia and South America are all following the same trend lines. Eastern Europe is likely underreported. We know there are a lot more cases than are being reported. If we look in Asia, one thing I like to point out is that some countries are really on the rise in herbicide-resistant weeds. China, for many years, didn’t have very many herbicide-resistant weeds and they’ve had a sharp uptick in herbicide-resistant weeds recently. That’s because they have had a migration of people to the cities and they don’t have enough labour to control weeds by hand.

Wheat has the greatest number of herbicide-resistant cases, followed by corn, soybean, rice and cotton.

Factors influencing the evolution of resistance include:

1. Initial resistance gene frequency (for the particular weed/site of action combination).
2. Selection pressure (frequency and efficacy of herbicide use).  
3. Number of individuals treated over time. Resistance is a numbers game. The more individuals you treat, the higher likelihood you’ll select for resistance.
4. Residual activity of the herbicide.
5. Genetic basis of resistance (degree of dominance of the resistance trait and the breeding system of the weed).
6. Fitness of the resistance trait.
7. Weed seed production. Weeds that produce more seed are more likely to become resistant.
8. Seed dispersal mechanisms. Weeds such as horseweed spread very quickly.
9. Seed longevity in the soil.

The reason there is so much Group 2 ALS resistance is related to the number of herbicides in that Group and the area treated, and the high numbers of which can result in resistance. There are 56 registered ALS herbicides, more than any other herbicide group, and they are used on a greater area than any other herbicide group. Group 2 herbicides (as well as Group 1 and Group 6 herbicides) are particularly prone to target site resistance (genetic mutations to the target enzyme that prevents the herbicide from binding and inactivating the enzyme).

Glyphosate resistance
North American, South American and, to some extent, Australian herbicide resistance research is focusing on glyphosate resistance. While overreliance of glyphosate in Roundup Ready crops is the main driver of glyphosate resistance, it is not the only cause, and only accounts for about one-half of resistant cases. The others are in orchards, vineyards and on fallow land.

Glyphosate-resistant crops were rapidly adopted in North and South America because they simplified weed control. Glyphosate-resistant crops saved corn/soybean farmers from ALS inhibitor (Group 2), ACCase inhibitor (Group 1) and triazine (Group 5) resistant weeds. But simply relying upon glyphosate alone to control these resistant weeds was a recipe for disaster.

The first case of glyphosate resistance was in 1996, and there are now 34 cases of glyphosate resistance worldwide. (See Fig. 3.) This is with a herbicide that is generally not prone to resistance because there are not a lot of mutations at its site of action. But just through sheer amount of usage of glyphosate, resistance develops, and it is increasing at quite a rapid rate.

Seven weed species (horseweed, Palmer amaranth, sourgrass, tall waterhemp, giant ragweed, Johnsongrass and rigid ryegrass) account for about 99 per cent of the reported area infested with glyphosate-resistant weeds.

The greatest economic impact is probably Palmer amaranth in the southern United States. Farmers are now using up to seven herbicide applications plus hand hoeing at a cost of up to $360 per hectare. Horseweed covers the largest area but is easily controlled with other herbicides. Glyphosate-resistant kochia is one that Western Canada should be worried about.

The biggest resistance challenges:

1. Multiple resistance – starting to get resistance to two or four or even 11 different sites of action, it is very difficult to control weeds.
2. Non-target site resistance – less predictable, very hard to identify.
3. Decline in herbicide discovery – haven’t seen the introduction of a new mode of action for over 30 years.
4. Overreliance on a few herbicide-resistant crops.
5. Farmers not adopting management strategies. Many have no experience in conventional weed control methods.

Any consistent practice to control weeds year after year will result in directed evolution towards survival. In a rice paddy in the Philippines, hand-weeding barnyard grass eventually selected for barnyard grass plants that looked like rice plants. The barnyard grass was resistant to hand weeding because it looked identical to a rice plant at the time of hand weeding.

The solution is to vary weed control practices and destabilize evolution. The whole message for herbicide resistance management is to be completely inconsistent with all your weed control practices.

June 6, 2016  By Ian Heap Director of the International Survey of Herbicide-Resistant Weeds Corvallis Oregon U.S.