Grassweed control programmes for this autumn need to take account of warmer winters, changing blackgrass behaviour and the increasing threat from brome, say experts.

Where wheat drilling is planned for early October, protection for over six weeks is required from pre- and peri-emergence herbicides and the active ingredients chosen should be put together for that duration.

See also: Trial leads to shift in blackgrass strategy on Oxfordshire farm

Best results come from pre-emergence sprays in most years, so the components should be deployed according to the weed species present and with one eye on the weather forecast.

While most pre-emergence herbicides work best where soil moisture is present, there are seed depth recommendations for some of the newer materials and their use should be avoided if heavy rain is forecast.

Weather conditions

This summer’s spell of dry and hot weather should help with blackgrass control – as weed seed return will be lower. Fresh blackgrass seed should also chit quickly, due to low dormancy, point out agronomists.

But brome is a different issue, as it germinates through until the spring and is often present in mixed populations of different species.

“With blackgrass, the simplest message is don’t drill anything until November,” says Dick Neale, technical manager at Hutchinsons. “But we all know that’s not going to happen in practice.”

His main message for blackgrass control is sequence, sequence, sequence. “There isn’t a single thing that can be done to make a huge improvement – it’s a combination of lots of things done in a timely way.”

Sequences

John Cussans, principal consultant  at Adas, agrees. “Sequences are more robust and give better control, providing the timing is spot on.

Ideally growers should do a tight pre-emergence treatment and then follow up seven days later.

“If the second application is delayed for too long, control will drop off.”

John Cussans © MAG/Richard Allison

His recommendation is that the pre-emergence spray goes on within 48 hours of drilling and the peri-emergence spay about a week later.

“Those are the timings that will give optimal results on most farms,” he says.

John’s other point is that growers who drill wheat in September in the south of the country are unlikely to get good herbicide control. “Earlier drilling is a risk and it puts the herbicides under pressure.”

Product choice

Where both blackgrass and ryegrass are present, cinmethylin (Luxinam Plus) is a good pre-emergence choice, he believes. “It’s very strong on both weeds and it’s always best to put the strongest herbicide on first.”

Adding pendimethalin can work well, but field-to-field sensitivity to the active ingredient can vary.

“Another option to have with it is picolinofen – remember it must have a partner.”

Flufenacet is the other widely used pre-emergence active ingredient and it can be boosted with diflufenican, metribuzin, aclonifen, pendimethalin or prosulfocarb, depending on the weed challenges and spray timing.

© Tim Scrivener

Where Italian ryegrass is an issue, the use of flufenacet + diflufenican + metribuzin has given good results on the Bayer Weedscreen site, says the company’s technical manager Rachel Banks.

She points out that it brings some contact activity too.

Aclonifen can be added to it in a pre-emergence situation only, she continues.

“That gives you four different modes of action, which covers a lot of bases.”

Bayer continues to work on herbicides containing flufenacet, she adds, with several coded products in the system which add to the options available.

In the same vein, the company is working on post-flufenacet strategies, in recognition of possible future alignment with EU pesticide rules.

What about Avadex?

Avadex (tri-allate) adds to blackgrass and wild oat control, agree commentators, bringing synergy to herbicide programmes through another mode of action.

The choice is between granules and liquid – with the latter being easier and more convenient to apply but the former giving better results and delivering more active ingredient.

“In general, autumn herbicides worked well in 2024 and we saw good control,” summarises agronomist Andrew Watson of Niab.

“The spring materials didn’t – the dry conditions affected them.”

Is blackgrass changing?

Blackgrass is appearing later as a result of a more protracted germination period facilitated by warmer soils, believes Dick Neale of Hutchinsons.

It has prompted him to consider drilling a bit earlier – by 1 October – to get the crop up and away and providing good competition.

“Perhaps a stronger crop is the new late-drilling option,” he suggests.

Dick Neale © Hutchinsons

He also highlights the stale seed-bed technique is often less effective than it was.

It has performed well in the past and exhausted the blackgrass that germinates in late September and early October.

“We see 10 times less amount of blackgrass coming through at this timing now.”

Adas’ John Cussans points out that blackgrass survivors are larger and stronger than they were.

Work done in 1990 recorded 4 heads/plant, while that figure is closer to 12 heads/plant today.

“Output is higher, both for the crop and the weed,” he reports. “As a result, we have smaller numbers of more aggressive plants.”

The other influence on blackgrass has been the move to fewer cultivations and wider rows, he notes.

“A system with less stimulus is more variable and unpredictable, so we see more later germinating blackgrass in these.

“It is reflected in herbicide performance too.”

In contrast, combi-drilled wheat reduces the space available for weeds and encourages some nitrogen mobilisation, with full crop canopy being reached quickly.

That helps reduce the tillering ability of blackgrass, he adds.