Understanding the biology of weeds, and more specifically herbicide-resistant populations, remains a critical tool to effectively and efficiently control weeds and ensure optimal crop yields.

Take the UK’s most economically significant weed, Blackgrass. This weed costs UK farmers £400m/year in lost income and 820,000t in yield losses.

That’s the equivalent of 1.2bn loaves of bread – enough to give everybody in Birmingham three loaves every day each year.

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In a high weed density area, the economic cost of resistant blackgrass is about £450/ha, while the cost in a low weed density is about £75/ha.

In a worst-case scenario, where all fields have a high proportion of resistant blackgrass, this could cost the UK as much as £1bn/year.

Dana MacGregor, a molecular geneticist at Rothamsted Research, explains how gaining a greater knowledge of blackgrass biology, and identifying genes underpinning weediness and herbicide resistances, will help tackle the issue.

Variation in resistance

“By understanding the biology of weeds, we better understand what tricks they use to avoid the cultural and chemical controls that are not working as well as they should,” says Dr MacGregor.

Weeds can develop herbicide resistances by either changing the protein targeted by the herbicide, known as target site resistance, or by finding ways to alter or avoid the herbicide, referred to as non-target site resistance.

“Nature finds a way to outwit us when we try to control it, sometimes by just creeping up on us.

“For instance, when an herbicide isn’t as effective as it should be and just a few plants survive, this may not seem like an issue, but one surviving plant can make about 100 seeds/head and has about 45 heads, which can leave quite a legacy.”

However, Dr MacGregor highlights that herbicide resistance is not as simple as weeds being killed or not killed by a particular chemical.

“There is huge variation in the levels and types of herbicide resistance, with cases exhibiting characteristics of both resistance types.

“It’s, therefore, important to determine what types and combinations of resistances are out in the fields, so farmers can better understand why weeds are not being controlled well.”

Reverse genetics and functional validation

To overcome this issue and develop sustainable and effective weed management strategies, it is important to understand the genetic basis of weediness, says Dr MacGregor.

“Only by learning about the molecular components underpinning weediness and how they have been altered in hard-to-kill plants, will we be able to design weed management strategies that work and, more importantly, do not encourage the development of new resistances in the future.”

One way of achieving this is through the use of reverse genetics, which tests the purpose of genes by altering the level of its presence – and therefore its protein – in plants, and monitoring the outcomes.

“Simply put, this technique uses the principle of ‘what can I break or add in order to understand this better?’.

“It enables us to understand what, specifically, in the plant gives it a particular beneficial trait, such as herbicide resistance.

Knowledge is power, and if we are able to demonstrate that a weed was not effectively killed because it has too much of or lacks one particular gene, this can be hugely powerful.”