Potential broad-leaved weed resistance development

One of the key problems in arable farming is that growers have fewer ag-chem options at their disposal, making chemical rotation more difficult so weeds are more regularly exposed to the limited number of herbicides. As a result, more weeds may be able to develop reduced sensitivity to certain chemicals. Dr Cook (below) explains: “For example, the residual pre-emergence herbicide trifluralin was withdrawn from the UK market about 10 years ago, and now we are seeing more weeds emerging, such as the umbellifers.”

One of the biggest risks to broadleaved weed control is resistance to Acetolactate Synthase (ALS) inhibiting herbicides, which have seen an increase in reported resistance cases.

With ALS-tolerant crops available, there is now an opportunity to use these herbicides in most crops in the rotation. They are easy to use and highly effective, with wide application windows. They are very effective on ‘difficult to control’ weeds like charlock in Clear eld oilseed rape and weed beet in Conviso sugar beet.

But care needs to be taken in thinking through weed control strategies in the rotation when ALStolerant crops are used, to control volunteers. “It is important to make sure you use alternative modes of action,” says Dr Cook, adding that the Stewardship associated with Clear eld oilseed rape and Conviso sugar beet is very helpful with strategies.

Positive additions

However, the news on the herbicide front is not all bad. Approval of Arylex (halauxifen-methyl) – a new family of chemistry recently available for use in the UK – gives useful control of a wide range of broadleaved weeds in cereals and oilseed rape, in addition to good control of ALS-resistant weeds such as poppy.

“Increases in resistance can often be difficult to see, especially in cereals,” Dr Cook warns. “Growers often attribute the failure to control weeds to the timing or rate of application, when in fact it is resistance creeping in.”

This is already happening with brome, and Dr Cook recently identified resistance to ALS inhibitors in prickly sow thistle.

“Some weeds have adapted to survive in modern farming, while others which were prevalent are now seen less than they were 20 years ago.”

For example, couch grass, corn marigold, hairy tare and Venus looking glass are all less of a problem in cereal crops than they were at the end of the 1970s, and corn buttercups are now generally only seen in organic crops, she reveals.

“On the other hand, speedwell has been successful this year and willow herbs, which are often more prevalent in horticultural crops, this season have been seen in open spring crops.“

Resistance risk matrix

Predictions as to which weeds are likely to develop resistance are difficult, and there are no obvious links between resistance and weed family groupings.

However, there are certain characteristics which may play a key role and, in 2009, ADAS developed a resistance risk matrix, bringing together the risks from the type of herbicide used with the biological features of the weed.

Class of herbicide, mode of action, whether it is used alone or in a mixture, and how many times it is applied in a season, all affect the potential to develop resistance – especially when one weed is targeted.

Resistance risk is likely to be higher if a herbicide has a single site of action and, although multiple sites of action are likely to reduce resistance risk, if one of the targeted sites is of primary significance, resistance may develop there, she warns.

Looking at the weed itself, many herbicide-resistant weeds produce a high number of seeds per plant, which increases the possibility of a resistant mutation spreading.

Vigour of an individual weed species is also an important factor in determining its resistance risk, as the vast majority of the resistant weeds – such as black-grass – are highly vigorous, but also common chickweed, poppy, fat hen, Canadian fleabane and Amaranthus spp.

Some such weeds are capable of germinating at different times, and producing multiple generations in one year. However, some of these resistant weeds may also show fitness penalties compared to nonresistant types, and competition may decrease seed return of the resistant types.

The effect of seed persistence is less clear, adds Dr Cook, noting that although resistant plants may be able to develop years later, the opportunity for germination while still viable may not occur.

Pollination and other factors

Another important factor not often mentioned appears to be method of pollination.

“Many resistant weed species are cross-pollinators – however, there are some species that are predominantly self-pollinators, but do show some cross-pollination such as common chickweed and Canadian fleabane.”

Other potential factors include mobility, for example wind-dispersed seeds may have a higher risk than larger heavier seeds, but as yet little is known about this. “This may be of particular importance if resistant weeds are already growing in neighbouring fields, farms or habitats,” adds Dr Cook.

“The repeated use of single products on the same weed species has raised the risk of resistance developing.

“The biggest risks to broadleaved weed control are the ALSinhibiting herbicides, as the rate of increase of reported resistance cases has increased dramatically since the start of the 1990s.

“Predicting the next likely resistance risk to the UK would be really useful, but first we need to know where we currently stand.

“The last survey was conducted over 20 years ago, so a new one would provide the means to assess differences in the species of weed now prevalent – helping to provide clues to better understand how they have evolved.”

Manipulating genetics

Future black-grass control may entail manipulating gene expression in weeds to reverse herbicide resistance, research at Rothamsted Research has indicated.

Farmers Guide spoke to Rothamsted Research molecular specialist and lead researcher, Dr Dana MacGregor to find out about her research on understanding more about the genetics of black-grass herbicide resistance.

Herbicide resistance develops partly as a result of interactions between selection pressure and genetics. Some black-grass strains – such as Peldon – can detoxify herbicides before they can harm the plant, which not only challenges today’s chemical controls, but also those of the future. But which genes are responsible?

Dr MacGregor says: “First we need to know which genes control the weed’s ability to resist herbicides.

Essentially, we need the parts list and an understanding of what each component does.

“With a tractor, for example, if you are unsure of what something does – such as the steering wheel – when you remove it, you soon learn what function it served.”

She adds that she has been following a similar approach.

“If you silence a gene, and the plant loses its resistance, then you know that the gene was influencing resistance.”

The techniques she has been using include virus-induced gene silencing. This relies on infecting plants with a barley stripe mosaic virus that carries a small piece of the plant gene of interest in its genome. When the weed fights the virus, it thinks the targeted gene is coming from the virus, and it fights it too. If the targeted gene was required for resistance, silencing it with this method means the plant can no longer survive the herbicide.

“This helps us learn what is underpinning resistance at molecular level.”

So far, the research is looking promising; experiments using the ACCase-resistant black-grass variety Peldon – which has increased activity of the enzyme glutathione S-transferase (GST) – have been done, and lab results indicate viral treatment reduces expression of the GST, making Peldon more sensitive to herbicides, she reports.

“Since GST genes are also necessary for the cereal crops to survive the post-emergence herbicides, and this virus would infect them too, we will not be introducing this virus into the field.”

The team also managed to make previously susceptible weeds resistant to the weed killer glufosinate by introducing a gene for an enzyme that renders the chemical inactive.

“Our work has established a link between specific genes and the ability of blackgrass to circumvent chemical controls, and through this work we gain a molecular level understanding of what allows black-grass to be such a successful weed.”

“Our work has established a link between specific genes and the ability of blackgrass to circumvent chemical controls, and through this work we gain a molecular level understanding of what allows black-grass to be such a successful weed.

However, there is no blueprint for a quick x for cereal farmers, she warns.

“What we have learned will help design crop protection products of the future that will be more effective against this yield-eating weed, because they take herbicide resistance mechanisms into account.

“But, as product development and approval takes around 15 years, the effects of our work are still very much in the future.”

This work was supported by the Smart Crop Protection Industrial Strategy Challenge Fund and the Biotechnology and Biological Sciences Research Council – part of UKRI.