2018 - Managing Herbicide Resistance

Part 1 - the current state of affairs

One of the biggest barriers to successful weed management is the growing problem of herbicide resistance, with the development of herbicide-resistant black-grass being of particular concern.

In this week’s hub, Adama’s technical herbicide specialist, David Roberts, looks at how resistance develops and outlines the current scale of the problem.  In next week’s update, David will outline some practical steps that can be taken to reduce the resistance risk on your farm.

Herbicide resistance: the basics

Put simply, resistance to an active ingredient is the ability of a plant to overcome the dose rate that would previously have killed it. Resistance arises through naturally occurring mutations within the genetic structure of the target organisms.

Resistance occurs when a mutation in part of the population confers insensitivity to the active ingredient. Continued exposure selects these insensitive strains and, providing the mutation does not inflict a fitness or vigour penalty, the insensitive population will escalate to a point where field resistance becomes apparent.

In a process known as selection, an outside agent (e.g. a herbicide) exacerbates the problem by reducing the population of the original species but fails to incapacitate the mutated, resistant gene. In the absence of competition, the new form begins to dominate and control is either lost completely or eroded over time.

Modes of resistance

Broadly speaking there are two types of resistance: enhanced metabolism and target site mutation.

Enhanced Metabolism:

With this type of resistance, genes within a target organism (e.g. black-grass) mutate to increase the rate at which they can metabolise the active ingredient. This allows the plant to withstand the herbicide.

This type of resistance usually manifests as a reduction in control rather than a total loss of efficacy of the active ingredient.  For example, the enhanced metabolism may overcome the active ingredient but, when the dose rate is increased, it is once again possible to control the mutated organism. However, further mutations may require dose rates to be increased until they become uneconomic or too great a risk to the environment.

Target Site Mutation:

To exert control, the chemical’s active ingredient needs to act like a biochemical key to a lock at a binding site within the plant’s DNA.  When a mutation changes the shape of the binding site, the active ingredient no longer fits and control is lost completely. In this case increasing the dose of an active ingredient will still not control the weed.

The scale of black-grass resistance

Black-grass resistance was first recorded in the UK in 1982 and is now a major concern particularly as the range of active ingredients is limited.

The most widespread resistance is to Acetyl CoA Carboxylase inhibitor herbicides (ACCase), with resistance confirmed on more than 20,000 farms in the UK.  Resistance to Acetolactate-synthase (ALS) inhibitor herbicides has also been confirmed in the UK. Of these ALS-resistant biotypes, eight out of nine were target site resistant.

This all means that weeds, especially black-grass, are becoming more prolific, with their ability to reduce crop yields increasing as a result.  This impact is measured on a scale known as the ‘competitive index’ which defines the population of a weed per square metre which will result in a 5% reduction in crop yield.

Because of its ability to produce a high number of tillers, ryegrass needs only five plants/m² to cause a 5% yield loss. However, even though black-grass is ranked as less competitive on the index, with 12 plants/m² needed to result in a 5% yield loss, it is a greater problem because its population density can be much higher (it is not unusual to have in excess of 1,000 blackgrass plants/m²).

As each black-grass seed head can produce 100 seeds, the soil reservoir of seeds can be considerable: in fact, control levels of 98% of the population are required just to maintain the status quo, with control levels of more than 98% needed to have any significant impact on the weed population.  Unfortunately, resistance to many herbicides means adequate levels of control cannot be achieved through the use of chemical control measures alone. As a result, it is commonplace to get a 25% yield loss where black-grass is present and the cost of a herbicide programme to control it can be as high as £150/ha.

The origins to herbicide resistance

Earlier drill dates:

The pursuit of higher yields has led growers to drill their winter crops earlier in the season: a couple of decades ago, winter crops were typically drilled in October.  More recently drilling dates have been brought forward with 50% of winter wheat now drilled in September. This change is to the benefit of black-grass populations as there is no longer a fallow period in which the first flush of grass weeds can be destroyed with glyphosate.

Changing weather patterns:

Warmer temperatures and increasingly wet summers provide the ideal conditions for grass weeds to proliferate.  Consequently, there is a greater potential for resistance to develop.

Withdrawal of active ingredients:

Legislation has seen a number of key active ingredients being removed from the market.  With fewer products to choose from, and the withdrawal of active ingredients outstripping the number of new modes of action being developed, growers have had little option but to become over-dependent on a smaller number of key products, again enabling resistance to develop.

Improper use of chemicals:

The improper use of crop protection chemicals – too many applications, too few applications, dose rates which are too high or too low – can also exacerbate the rate of resistance.  Growers are therefore advised to take professional advice before changing the accepted application protocols for any herbicide and to consider using a range of active ingredients instead of relying on just one mode of action.