Wild oats are a predominantly spring-germinating grass weed species. Although they will infest winter-sown crops, spring cereals are most at risk. Herbicide resistance in wild oats had long been suspected in Ireland before it was confirmed in 2016. Since then, research has advanced on the nature, prevalence and severity of resistance in wild oats, particularly to ACCase-inhibiting herbicides.

ACCase inhibitors are any ‘dim’, ‘den’ or ‘fop’ herbicides. These are so called because the active ingredient names tend to end in ‘dim’, ‘den’ or ‘fop’. Common products from this family of actives are Axial, Falcon, Fusilade, Foxtrot and Stratos Ultra. Populations of wild oats have been found with resistance to each of these active ingredients.

Resistance mechanisms

Resistance to ACCase inhibitors is typically caused by two major mechanisms.

1 Target site resistance is where a point mutation to the herbicide target enzyme stops the herbicide from binding. This allows the weed to survive herbicide application and reproduce.

2 Non-target site resistance can be caused by a build-up of genes that allow the weed to detoxify herbicides. Effectively, this reduces the dose of herbicide that is “felt” by the weed, again allowing it to survive and reproduce.

Wild oats have developed resistance to two specific herbicides in some fields.

Non-target site resistance can be considered the more troubling of the two mechanisms because it causes general, broad-spectrum resistance. In theory, a weed of this nature could develop resistance due to repeated overuse of a graminicide such as Falcon and become resistant to a completely unrelated herbicide such as a sulfonylurea (for example).

Research carried out in Oak Park has confirmed that both target-site-resistant and non-target-site-resistant wild oats have been observed in Irish fields. Indeed, some populations carry both resistance profiles together. This type of compound resistance poses serious issues for chemical weed control because the issue may not be alleviated by the discovery of new active ingredients.

Resistance identified

Resistant wild oats have been identified sporadically throughout the grain-growing region. In 2017, Teagasc began a randomised sampling campaign in Co Wexford. This was carried out due to the abundance of spring barley grown in the county, plus the fact that a number of resistant wild oat strains had been identified from fields where weed control was considered an issue.

While it must be noted that the results of this survey are not necessarily representative of the wider grain-growing region, we need to pay heed to the findings.

Populations were subjected to the recommended field rate of pinoxaden (30 g/ha) and fenoxaprop (69 g/ha). It was found that roughly 50% of populations tested were resistant to at least one of the herbicide actives. Approximately 8% of populations were resistant to both actives in this study.

Any population that was found to be resistant to pinoxaden was also resistant to fenoxaprop. However, not all fenoxaprop-resistant populations were resistant to pinoxaden. Resistance was positively correlated with the density of weeds found in the field. Simply put, where there was a heavy weed infestation, it was more likely that the wild oats from that field were resistant.

If we consider that every wild oat in a given field has the same likelihood of developing resistance, then fields with more weeds in them have a greater chance of resistance development, particularly where the same herbicide active ingredient is consistently applied.

Slowing the spread

So, what can be done to slow the spread of herbicide-resistant wild oats on Irish farms? We should look to other countries that have had success with containing resistance for answers.

Resistant wild oats have long been a problem in Canada. For many years, weed control was achieved solely through the use of herbicides. On farms where resistance is a big issue, a variety of cultural tools are now used to control wild oats.

Crop rotation is the number one strategy, and the basis of any resistance management programme. Growers are keenly aware of any weed patches in their fields and record where they are so that seeding rates can be increased to out-compete the weed patches. If the infestation is particularly severe, some growers may opt to desiccate that patch of the crop with glyphosate to avoid wild oat seed return.

From the chemical control point of view, it is imperative that if you are going to spray wild oats, recommended rates of herbicides should be used. Research on a variety of different species shows that sustained use of low doses of herbicides leads to the development of enhanced metabolic resistance (non-target-site resistance).

Bear in mind that reduced rates of herbicides refer to the rate of herbicide that actually reaches the weed plant. While turning, the outside nozzles on the boom are moving more quickly than those on the inside, thus less herbicide sprayed. Similarly, care must be taken when spraying browy, unlevel fields.

Even if a grower applies a herbicide in perfect conditions, with great attention to detail, there still may be some plants that survive. Care must be taken to remove these plants from the field before they set seed and become next year’s problem.

Summary

Herbicide resistance in Irish wild oats exists but it is arguably a problem that can still be curtailed through the use of integrated weed management tactics. Although both target site and non-target site resistance have been found, the effects of resistance can still be effectively managed. This requires sound planning and solid attention to detail in our approach to wild oat control coupled with increased used of the many cultural tools.