Take the long-term view for grassweed control

Understanding how tillage and cropping decisions affect grassweed pressure is vital for building durable control. Low and zero tillage systems are already affecting the weed problems farmers face, and herbicides of all kinds need to be carefully integrated into the programme to avoid resistance.

grassweed control on arable farming article on farm machinery website

Advising farmers in Dorset, Wiltshire and Hampshire, agronomist Todd Jex is something of a specialist in regenerative farming and has experience of dealing with weed problems in these systems.

“I have an interest in no-till and regenerative farming, so I tend to attract these kinds of farmers looking for advice. They make up most of my customers, even though they are a minority of farmers in the region.”

Mr Jex identifies reducing fixed costs as the biggest driver behind the move to no-till. But SFI payments, fuel prices, carbon markets, water company payments and labour availability are also contributing. One of the drawbacks of the no-till approach is that it rules out some cultural weed controls. Ploughing most obviously, but it can be difficult to get good establishment with delayed drilling without cultivation.

Added to that, brome problems become more prevalent on farms with minimal or no cultivation; however, that is the lesser evil compared to black-grass or ryegrass. “Historically, the main problem grassweed has been brome; but we are now seeing increasing problems with black-grass and some Italian ryegrass.”

Good stale seedbeds

According to Mr Jex, both straw and machinery have brought black-grass into the region and it is now present on a majority of farms. For ryegrass, the story is different; he thinks it has always been present, but became more prominent once oilseed rape area and frequency declined. Propyzamide (still available) and carbetamide (now banned) were very effective actives against ryegrass.

“Growing rape one in three was very good for ryegrass control, although it wasn’t particularly good in other ways. Now it is on a much longer rotation – roughly every five or six years – so there isn’t as much ryegrass control in the break crop. Growing winter beans is an option to make more use of propyzamide but the practicalities mean it won’t be on the same scale as OSR used to be.

“For grassweed control in cereals, making good stale seedbeds is massively important and reducing cultivation intensity helps too. This season, the rainfall during summer and autumn has meant it was very easy to get good germination and kill.”

In contrast, many recent seasons, especially autumn 2022 were characterised by dry soils which made any sort of stale seedbed difficult. As a result, there was extensive germination in the crop when rain arrived in late autumn.

“At pre-em we use either a flufenacet or cinmethylin base. And then look to follow up with a peri-em; experience tells me it is best to split the programme rather than frontload it all. You get more longevity, and there is less risk of losing the whole programme in one downpour. This season, I have been particularly impressed with the metribuzin products at peri-em following a flufenacet pre-em.”

For Mr Jex, a typical grassweed control programme has four modes of action in autumn and an ALS in spring. “The post-em is important, because no matter how good your pre-em programme, there are still some weeds in spring. For brome control, I go for pyroxsulam, and Pacifica Plus against black-grass.

“On most farms, Pacifica Plus and similar products are still working; plus it takes out wild oats and cleavers, although I think control has started to slip a little recently. Unfortunately, there are no other modes of action in spring which makes things challenging – but we have to do our best to retain efficacy and protect what we’ve still got.

“Timing is very important; anything beyond the 2–3 tiller stage and it won’t work, ideally smaller. But the balance is tricky, you need good enough growing conditions for the herbicide to work but not so the weed has already got too big. Added to that you can go early and get caught by wintry weather in March, but overall spraying early is the right approach.”

Spring germination is another complication for grassweed control. “We’ve always had lots of spring crops in this region, which has probably helped overall, but it means we get more of a split autumn/spring germination, so I aim to get protection from herbicides for as long as possible.”

Reduce seed return

A good benchmark for effective weed control is fewer seeds in the seedbank at the end of the season than at the start. Typically, 95% or more overall control from cultural and chemical controls is needed to hit this target. In wheat, the heavy lifting is done in autumn with delayed drilling or perhaps ploughing followed by the pre-em but there are always some survivors.

“Come spring, there are three options to reduce seed return: a post-em in early spring or hand-rogueing and patch spraying later on,” says Bayer’s Matt Siggs. “Time, cost and availability of labour mean that rogueing isn’t as attractive as a few years ago so is mainly for light infestations and of course patch spraying has a big impact on yield – it kills the crop. Hence in most situations, a spring post-em is the go-to option to reduce seed return.”

In Bayer trials in 2023, applications of Atlantis Star showed a sustained improvement in ryegrass control of 11% compared to a pre-em only programme. This equated to 21 fewer ryegrass plants per m2. Assuming each plant produces 1,000 seeds this means 2,100 fewer seeds per m2 that can potentially germinate in subsequent crops.

Against black-grass, there was a much smaller improvement in control from the post-em, mainly due to very good pre-em performance in the trials. Used as the only herbicide in the programme to simulate a missed residual program in the autumn and firefighting in the spring, Atlantis Star still delivered 40% control.

“In last season’s trials there was very good control from pre-ems, but there was still a benefit to using a post-em on top. Control varies due to resistance and application conditions. But the general pattern is that it controls half of what’s left of the black-grass and ryegrass, cutting seed return in half. Plus, many broad-leaf weeds and wild oats are controlled at the same time,” concludes Mr Siggs.

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