Advice on managing grassweeds after low pressure season
© Tim Scrivener Successful weed control was a highlight of last year’s growing season, with some of the lowest blackgrass pressures recorded in the UK over the past decade.
However, experts are warning growers to remain vigilant, as successful control in a single season does not directly translate to reduced weed pressure in the following crop.
John Cussans, weed scientist at independent crop consultant Adas, explains how – by and large – the 2024-25 season was an imperfect storm for weeds, with good overall control.
See also: How to maximise the benefits of stale seed-beds
“Every farm is different, but timely seed-bed preparation, followed by a delay in drilling, meant more weed seeds germinated before drilling and were killed outside the crop.
“Soil-acting residual herbicides also gave better performance,” he says.
John’s key message now is that success in a single season is not directly translated into a reduction in the weed seed bank.
He urges growers to be mindful about herbicide timings, matching the right rate and chemistry for the weed pressure
“It’s important to understand the weed populations you’re trying to control,” he says.
“With crops going into ground potentially earlier after an earlier harvest, combating weed pressure at the pre-emergence timing is important.
“Weeds that emerge at the same time as the crop are the most critical to control.”
How big is the seed bank?
Those farmers who got good control last year are, for the first time in many years, facing a season where they could lower the number of viable seeds in the seed bank from previous years.
Research suggests a figure of 5% for how much of the total weed population can be assessed as seedlings, with 95% being in the seed bank. This is an overall figure across species.
John explains that growers should aim for a seed bank decline of about 75% each year and a herbicide efficacy rate of 95%.
According to the AHDB, 95-100% blackgrass control is required to prevent populations increasing.
This should be achieved through a combination of non-chemical control with herbicides to improve overall control and protect efficacy.
© Tim Scrivener
BASF agronomy manager Aliona Jones explains: “It’s logical to want to scale back herbicide programmes on fields which are currently clean.
However, it’s not just about this year’s blackgrass population.
“There is often a legacy from previous years and, with blackgrass seed viable for up to five years, fields that are now seemingly free from blackgrass can rapidly get out of control if we don’t keep the pressure up.”
She says it is important to build on this opportunity by using all available tools, from delayed drilling and stale seed-beds to using the best chemistry at the optimum timing.
Pre-emergence importance
Stuart Kevis, BASF business development manager, warns that a single blackgrass plant can become an infestation within two years.
“With one plant/sq m, that’s 10 heads/sq m which equates to 1,000 seeds/sq m (60% viability) and 6m seeds/ha – it can quickly escalate.”
He highlights the importance of using the right chemistry at the pre-emergence timing.
“If, for example, we have 73% herbicide control at pre-emergence and 60% control at post-emergence, we have an overall control of 89%, which is below the 95% target.
“By simply improving pre-emergence control to 86% and keeping the 60% control at post-emergence the same, we achieve the aim of 95% overall.”
Stuart recommends applying Luxinum Plus (cinmethylin) at 0.7 litres/ha plus Stomp Aqua (pendimethalin) at 2 litres/ha at the pre-emergence timing, for effective control with good herbicide efficacy.
Farmers’ thoughts
East Midlands grower Rob Barlow has been battling ryegrass and blackgrass in fields he acquired six years ago.
Thanks to judicious use of integrated pest management and a robust herbicide programme, grassweed populations are on the decline.
Spring flush
“This year, for the first time, we were able to hand-rogue 80% of the farm,” says Rob.
“The plants that were pulled were largely the result of a spring flush of grassweeds that emerged well after the active window of the residual chemistry applied at pre-emergence.”
Alongside cultural controls such as spring cropping and delayed drilling, Rob and his team tease every percentage of efficacy from the herbicide programme.
“Luximo offers really good efficacy but if we can take it from 95% up to 98% control, we will,” he says.
“Using higher water volumes to enhance soil surface coverage worked well for us last year and is something we are planning to repeat.
“Timing is really important. We had a crop that was sown at the very end of last year.
“Given that the grassweeds were unlikely to chit over the winter, we decided to wait and apply Luximo in February at peri-emergence.
“Control was good, but not as good as we are used to. That has reinforced our thinking that pre-emergence is best.”
Looking ahead, Rob can’t see his zero-tolerance approach changing, even when populations come down.
Disappointing control
Grower David Hurn described how he was lulled into a false sense of security last year on a field with historically very low levels of blackgrass.
“We’d an area coming out of peas and needed to test a new drill before the season got under way,” he explains.
“Given its past and the timing, I chose to apply flufenacet + pendimethalin + diflufenican, rather than Luximo which was applied elsewhere.
“Disappointingly, that field had a smattering of blackgrass, unlike the rest of the acreage where control has been very good.”
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