Study highlights grassweed successes – and concerns

• Autumn pre-emergence strategy works

• Mix of measures are yielding results

• Ryegrass and wild oats are challenge

East Anglia growers are finding success against black-grass – but more problems with ryegrass and wild oats.

What’s more, many farms appear to be missing a valuable trick by concentrating their wild oat control on the spring rather than making the most of the excellent opportunity they have with their autumn pre-ems.

These are among the key findings of the latest national grassweed management study undertaken this summer by Gowan Crop Protection. The survey included more than 50 growers from the eastern counties.

In total, some 260 farmers across the country participated in the study. Together, they manage almost 90,000ha of cropping, with a range of enterprises, establishment regimes and weed management practices.

The study shows black-grass, ryegrass, brome and wild oat infestations continue to be widespread across the region. Fully 94% of growers are having problems with black-grass, for instance, 88% with wild oats, 68% with bromes and 48% with ryegrass.  

More growers are still reporting an increase than a decrease in problems in most cases. However, the proportion seeing increasing blackgrass problems is much lower than it was in the similarly comprehensive national survey conducted by Bayer in 2016 (Figure 1).

“This clearly suggests the integrated approaches growers have been taking to deal with black-grass in recent years are having the desired effect,” says study co-ordinator, Hank King of Gowan.

“Some slight improvement also seems to be evident as far as bromes are concerned. Unfortunately, the same cannot be said for ryegrass and wild oats, though. In both cases, the proportion of growers seeing increasing problems with these weeds over the past four seasons has risen noticeably.

“Equally concerning is the fact that problems with mixed grassweeds rather than just single species are being reported by around a quarter of eastern growers; especially so since control measures that work well with one species may not be the most appropriate for others.”

Key challenges

The 2022 study shows black-grass remains the most significant grassweed issue for most Eastern growers, with nearly a third reporting problems across half or more of their winter cereals area, and almost two thirds across more than 10%.

Wild oats are the second most problematic grassweed with bromes and ryegrass both somewhat less significant in their scale.

Black-grass with wild oats and black-grass with bromes are the most widely encountered grassweed combinations, with wild oats proving problematic in combination with other species for more than half the growers.

“Grassweed problems continue to be most significant under reduced tillage regimes and less so in plough-based establishment systems,” notes Mr King.

“At the same time, noticeably higher proportions of growers than average with both shallow min till and no till regimes are reporting increases in black-grass, brome and ryegrass problems.”

Herbicide resistant black-grass is continuing to cause issues for large numbers of East Anglian growers, reveals the study, almost 70% reporting serious or very serious problems with resistance.

Around 15% are seeing similarly concerning resistance problems in ryegrass, with 7% finding issues that may be resistance-related in bromes.

Underlining the generally improving black-grass control position, rather fewer growers than in 2016 say they are encountering particular resistance problems. In contrast, greater proportions now are finding resistance problems with ryegrass.

Herbicide programmes

Chemical black-grass, brome and ryegrass control efforts in winter cereals continue to be concentrated on pre-planting stubble and pre-emergence treatments, with noticeably greater reliance on pre-ems than in the past.

In all three cases, however, around a third of growers are using a spring herbicide as part of their treatment programme. This increases to over 40% and two thirds respectively where mixed grassweeds and wild oats are the targets.

The winter wheat grassweed herbicide bill averaged just over £85/ha last season, with nearly a third of eastern region growers spending more than £100/ha (Figure 2).

The highest winter wheat grassweed herbicide bills are clearly associated with greater use of shallow or min-tilling; more widespread black-grass problems; particular issues with resistant black-grass, ryegrass and bromes; and mixed grassweed populations.

Just over half of growers are currently using Avadex (tri-allate) as part of their pre-em programmes. Usage is closely related to the scale and severity of black-grass, brome or ryegrass problems. Interestingly, though, this is not the case with wild oats.

“Growers and their agronomists are missing a valuable trick here,” says Mr King. “Avadex was introduced way back in 1961 as a wild oat killer, and long-standing trial and commercial experience shows it is highly effective at controlling spring-emerging wild oats from autumn pre-em application.

“The data shows the vast majority of Avadex users value the herbicide for adding extra grassweed control beyond just black-grass to their pre-em programmes, while large numbers also appreciate it for the protection it gives other herbicides against resistance development. 

“Sadly, though, around two-thirds of growers are failing to appreciate tri-allate’s value in wild oat control. This is a great shame since employing Avadex in the autumn for its additional black-grass, brome and ryegrass control will, in many cases, make spring wild oat treatment unnecessary. 

“What’s more, our liquid formulation, Avadex Factor which is compatible with the vast majority of pre-em herbicides eliminates the need for specialist application equipment and or an extra pass.”

Cultural controls

Alongside their herbicide programmes, growers are currently employing an average of 9.7 of 12 main cultural techniques to manage grassweeds. This is an encouraging increase on the average of 6.3 of the same techniques identified in the 2016 survey.

The six most widely used cultural grassweed controls – each employed by 85% or more of growers – continue to be spring cropping, delayed autumn drilling, stale seedbeds, competitive varieties, rotational ploughing and increased seed rates.

“Underlining their value, all these most widely used cultural techniques scored relatively highly for their
effectiveness on our 1-5 scale,” says
Mr King. Of the 12 main techniques, only cover cropping, grass breaks, fallowing, spraying-off infested crops and hand roguing scored less than 3.0
(Figure 3).

Paying dividends

“We must never be complacent, of course, but the increasing use of integrated weed management programmes utilising the best chemical and cultural controls seems to be paying dividends in allowing growers to combat even the most problematic black-grass.

“The key challenge is to maintain this progress while bearing down on the increasing threat from wild oats and ryegrass as well as bromes – often in combination – which can be even more damaging to crop performance.

“To do so while responding to growing economic and environmental pressures to reduce tillage as far as possible.”