Mode of action diversity – putting it into practice

The 2023 Bayer Mode of Action Survey found that most farmers (83%) believe herbicide resistance management is important on farm (see chart 1). However, good intentions are one thing, but what does that look like on the ground? We meet two farmers who have adapted best practice in different ways without compromising herbicide performance.

Before looking at what is happening on farm, it is worth quickly recapping the principles of good resistance management.

First and foremost, cultural controls are the primary tool to control weeds; use as many as possible in the rotation. Second, when using multiple actives in the herbicide programme, make sure they come from different HRAC mode of action groups. Rotate actives between seasons, and finally, monitor performance and quickly evaluate reasons for poor control.

Cultivation and cost management

Kent farmer Andrew Cragg is responsible for 600 ha of arable crops in Romney Marsh. He is self-advised, being FACTS and BASIS qualified, and has witnessed how changing farming practices have influenced weed problems.

“In the 1990s, regular ploughing and spring crops meant that black-grass wasn’t a problem, even though the silty clay loam soil is very prone to black-grass.

“Since then, the overarching change has been a move to controlled traffic farming, the upshot of this is less tillage.

“Perhaps the biggest problem we have encountered is the inability to control black-grass in second wheats, so the only solution is no second wheat. The decision was helped because the yield difference from first to second wheats went from 0.5 t/ha to 2 t/ha. This wasn’t all due to black-grass, there were other factors too.”

Mr Cragg changed to a five-year rotation of winter wheat, oilseed rape, winter wheat, spring wheat then peas. In the absence of the plough, successive spring crops are an important tool to reset the system and prevent excessive build-up of black-grass numbers.

“We drill wheat in mid-October which makes a big difference, getting a 12-metre drill was one of the final steps for controlled traffic farming to keep all fieldwork in the same tramlines. It means we can drill all the wheat in four days.”

In winter wheat, he has settled down to a standard autumn herbicide programme. Avadex granules are applied with his own applicator after drilling, followed by a pre-emergence of flufenacet + pendimethalin + diflufenican, usually topping up with more flufenacet six weeks later.

“At the moment, I don’t use new chemistry because the system we have is under control, but I am firmly of the view that if something is needed, then it is worth spending time and money on.”

The autumn programme may be followed in spring with an Atlantis-type product to control wild oats too.

Actives are protected throughout the crop rotation. Mr Cragg tries to avoid using herbicides in spring wheat, preferring later spring drilling despite its impact on yield. In oilseed rape, propyzamide has been an important grass-weed active, although he has noticed a drop in performance which may be due to resistance, lack of cold weather, or some combination of the two.

“If you do the same thing every year, some weeds start to creep through, so you may need to adjust the system. Cleavers and crane’s-bill are two weeds we see more of now and have to take steps to manage.”

To stay on top of developments in herbicides, Mr Cragg relies on NIAB TAG to highlight the strengths and weaknesses of new actives and how they complement the actives already available. He is keen for every decision about herbicide applications to be based on product performance and risk.

For example, although hand-rogueing and patch-spraying is used by many farmers, Mr Cragg is not doing it himself. “Our land is relatively clean, and I don’t see the need to pick out every last one. For other farmers it might be different, if they are growing seed crops, or perhaps they just have a mindset to rogue the farm. But these things come at a cost.”

Dealing with difficult conditions Growing combinable crops on 240 ha of land near the Dorset coast, Andy Sprake, farm manager at Zephen Farms, has to tailor his ryegrass control strategy to suit the soil type.

“Ryegrass is the biggest weed problem; we have some black-grass too, but nothing serious. The soil is medium-heavy loam over limestone, and we are on the coast, so it lies wet for a long time after rain.

“For example, on 12 October there was 7 mm of rain, which caused a 3-4 day delay before we could get back out.

“We aren’t able to drill late for grass weed control. This year was fairly typical, we started drilling cereals in late September until early October, followed by the pre-em. Spring crops are problematic as well; the soil is too wet for travel during much of spring, but quickly droughts out when the weather warms up, so there is a very short drilling window for good establishment.”

Because Mr Sprake cannot delay drilling, he tends to leave spraying off stale seedbeds until as late as possible to maximise weed control.

“The downside of this is that it increases the risk of BYDV, which is often a problem in this part of the world. This season we are trying KWS Feeris which is a BYDV tolerant barley to see if that makes things easier.”

Winter crops – wheat, barley, oilseed rape and beans – are the core of the rotation, with an occasional spring crop due to very high weed pressure. Because delayed drilling and large areas of spring cropping are off the table, effective stewardship of herbicides is even more important in this system.

“Pre-em chemistry is the core of the programme for ryegrass control, although we still sometimes use a post-em for brome,” Mr Sprake says. “This is backed up with hand rogueing or patch spraying to remove survivors as the next generation would probably be worse.”

This season he has used Liberator + Proclus + Defy on winter wheat and Liberator + Defy + Anthem on winter barley. In both crops the programme contains four actives, from three different HRAC mode of action groups. Centurion Max (clethodim) in oilseed rape and propyzamide in beans are also still providing good levels of control.

Reviewing autumn 2023 pre-ems

Looking back at the October rainfall map from the Met Office (see chart 2), you might think this article would be discussing the worst autumn since 2019, and how to prepare for drilling spring crops. But the heavy rainfall associated with storm Babet did not arrive until 18 October, by which time many crops had been drilled and received the pre-em.

It has still taken its toll though; torrential rain is likely to have washed some pre-ems away. Added to that, few farmers were able to apply top-up applications to extend protection and potentially bring more actives into the programme. This was highlighted in our online poll of growers last autumn, which showed almost two-thirds of respondents were not going to apply a peri-emergence or top-up herbicide because it was too wet (see chart 3).

Thankfully, stale seedbeds were successful earlier in the autumn to balance out the problems later on.

The situation was different in Scotland though, with heavy rain at the start of October and numerous rainfall records broken. The county of Angus recorded its wettest-ever day on record and overall, it was a difficult month for arable farmers.

The disruption to the autumn herbicide programme means there may be more call for follow-up applications this spring. Either a mesosulfuron product, such as Atlantis Star, or perhaps a residual herbicide to prevent additional weed germination.

Key messages

• Stack cultural controls first, then stack different modes of action

• Adapt resistance management guidance to fit the farm

• Check weed development and herbicide performance now to decide what follow up action is needed.