Growers Grapple for Grassweed Solutions – CPM – Charles Wright

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Charles WrightTechnical Grassweed survey

With grassweed pressures rising and chemical efficacy declining, is there an alternative option to protect against some of the most yield-robbing weeds? CPM explores the tools available in the arable armoury to tackle grassweeds and looks at how an age-old active fits in.

Grassweeds are the bane of many farmers' lives, with rising resistance to chemical treatments and a declining pot of active ingredients to choose from.

It's therefore more important than ever to use the existing actives to best effect, both to maximise grassweed control and reduce the risk of further resistance developing.

Black-grass in particular is causing chaos for growers across the UK, so it comes as no surprise that in a survey carried out by CPM and Gowan that 80% of growers noted it as the key grassweed they're tackling. "Black-grass seems to be growing everywhere and expanding in terms of the areas affected," explains Kuldip Mudhar, development manager at Gowan." There are many ways seeds can be spread from farm to farm, from machinery to manures and bought seed as well as straw and now people are finding it in places they've never had it before.

"It [black-grass] severely restricts yield – in the worst-case scenario it is not possible to grow winter cereals. However, the bigger issue is adding more weed seed to the seed bank causing issues in the following years."

Problematic grassweeds

While black-grass appears to be stealing the limelight in terms of key concerns, growers also revealed that the other problematic grassweeds they look to control are wild oats (71%) and brome species (66%). “Black-grass remains the key grassweed for most growers, but we are seeing an increase in issues like bromes and ryegrasses, and those issues are becoming more severe," explains Will Smith, senior trials manager at NIAB.

“It could be the case that people are maybe taking their eye off the ball a bit on these other species which has left a gap on herbicides – they haven't worked on black-grass and so growers have left them out, which could be the reason why other species are becoming a problem."

There's no denying that concerns are growing around chemical efficacy and resistance, with only a small percentage of growers – 22% - stating that they are certain resistance isn't an issue for them on farm. In contrast, 42% suspected they have issues, but haven't had this confirmed, and 28% have had their resistance status confirmed through testing.

So is it better to test and confirm resistance, or are growers just better off pre-empting the problem?" Growers could really benefit from confirming exactly what resistance they have," according to Kuldip. "If you're throwing chemistry at a problem, but you don't actually know what the issue is then it could ultimately make resistance worse and leave you with a hefty, wasted chemical bill."

Particularly where resistance issues are new to the farm, it may be worth getting this confirmed, adds Charles Wright, agronomist at Farmacy.

Even for those who don't yet believe they have a problem in terms of black-grass resistance, it is advisable to be proactive, notes Kuldip. “Growers can't bury their heads in the sand for too long when it comes to resistance – it’s far more widespread than people realise."

In order to prevent resistance, Charles recommends looking at other methods of control – rather than simply relying on chemistry. "For me, black-grass control is simple – it’s all about putting the right crop in the right field.

"Think about cultural controls: What type of cultivation is most suitable? Will a change to your traditional drilling date help? If you know you've got a problem, I really recommend opting for spring crops, but if fields are suitable for winter cropping then it can be beneficial not to drill until the second half of Oct.

“This kind of systematic approach is worth adopting regardless of whether or not there is an existing issue – it’s all about being proactive rather than reactive when it comes to grassweed control."

In terms of selecting chemistry, the declining number of effective actives means this choice can sometimes seem like a bit of a balancing act for growers, with factors like half-life needing to be taken into consideration.

When asked how important they considered the physical-chemical properties of an active substance, 84% of growers revealed their main priority is to use the most effective product, compared to 16% who said the focus for them was to ensure the product has no effect on the following crop.

"For me, the most important thing is to use the most effective products – there is no point trying to hedge bets around protecting the following crop and missing out on control within this crop," explains Will. "The key thing here, however, is making sure you switch up products and use a wide range of ingredients."

NIAB trials

NIAB has carried out trials work using flufenacet in combination with a variety of different ingredients and the results highlighted that Avadex (tri-allate) was one of the best in terms sequencing the granules or tank mixing the liquid and giving the best control.

Of course, Avadex is nothing new. In fact, it has been around since the 1960s – a record that not many other actives can compete with. According to the survey, 43% of growers had used Avadex Excel 15G over the past three years and 15% had used liquid Avadex Factor over the same period.

So almost 60 years later, what's keeping Avadex in pole position as a key tool in the arable armoury? "Recommended for use in key cereal crops, Avadex offers a wide spectrum of activity against grassweeds like wild oats, black-grass, brome and ryegrasses," says Kuldip. "It introduces a different mode of action to many commonly used products and provides residual activity. As well as this, it can provide a useful effect on a number of important broadleaf weeds."

Despite initial concerns due to the practicality of applying it, Charles believes including Avadex is a must for all growers. "A few years ago, I was quite sceptical as it [Avadex] can be quite difficult to apply. However, with more growers moving to later drilling, the severity of black-grass and the need for an effective triallate product, Avadex has to be part of the programme."

Though traditionally used on cereal crops, 13% of growers had used Avadex on peas, beans, sugar beet or oilseed rape. "In the past, we used to have label approvals or EAMUs on those crops," explains Kuldip. "They don't exist anymore but with actives disappearing, people are looking for other alternatives and they know that Avadex has done a good job."

To apply Avadex granules, specialist equipment is required, which potentially means an investment by growers either through purchasing kit or paying out for a contractor to do the job. According to the survey, 36% of growers use a contractor, compared to just 14% who apply it themselves. “A very important part of using Avadex is applying it correctly – if you don't, you're likely to end up with stripes of grassweeds across the field," says Kuldip.

Given the time pressures faced by farmers, a contractor may be a useful option, he adds.

"If you've got a good contractor who will come at the right time – at pre-emergence – then use a contractor. But for me, getting it on at the right time is critical, so it might be beneficial to apply it yourself. Once the crop has emerged, Avadex's effect is greatly diminished."

Will agrees: "Using a contractor is great from a cost point of view as you're effectively lowering fixed machinery costs and saving the investment of purchasing a piece of kit that you are likely to only use once a year.

"However, given that timing is so critical, it may be a bit more reliable just to apply it yourself."


For those going down the self-application route, the mounting of the applicator can make a significant difference to the product's efficacy. Of those growers who had an Avadex applicator, 30% mounted it on the tractor and the same percentage towed it or mounted it on an ATV.

In contrast, 20% mounted it on the drill and a small proportion (14%) put it on the roller. So where is the best place? "As with all pre-em products, you need a consolidated, firm seedbed, so products are normally applied after the last cultivation," explains Kuldip. “That firm seedbed is key, so l can see why people are applying it via tractor – they drill, roll and then apply – but if you think you've got a good consolidated seedbed, put it behind the roller or drill."

According to Charles, with later drilling there isn't always the guarantee that you can roll post-drilling. "In an ideal world, having a separate vehicle – like an ATV – gives greater flexibility."

Despite seeing good results when applied in this way, Charles does have some concern with those applicators mounted on the drill. "lf granules are applied too quickly, it can affect the spread pattern – ideally Avadex should be applied at lower speeds."

With all the evidence presenting a solid argument for including Avadex in the stack, is this likely to convince growers this drilling season? According to the survey, 24% of growers are probably going to use Avadex this autumn, while 9% definitely will and 39% already use it. This is in comparison to just a quarter of growers (24%) who said it is unlikely. “Using a different active ingredient that is known to be effective is a win-win for growers," says Will.

Understanding Avadex

Avadex from Gowan has been used for grassweed control since the 1960s.

Tri-allate – the active ingredient in Avadex – is a thiocarbamate and works by inhibiting lipid synthesis.

When applied to the soil, the product forms a chemical barrier through which germinating weeds have to grow. They then subsequently absorb the chemical and are killed.

Granules vs liquid – weighing up the options

Deciding whether to use a granular or liquid product can depend on several factors including application equipment, cost and efficacy, but what are the pros and cons when it comes to using Avadex Excel or liquid Avadex Factor?

"The granular formula is a more robust product and can be applied earlier," explains Kuldip. "It has a higher rate of Al - 2250g/ha – which means a good level of control is provided."

Will agrees: "Dose for dose, Excel has more triallate in it. So in terms of tackling black-grass, this will offer better control."

In contrast, the liquid property of Avadex Factor makes for easier application, according to Charles. "Liquid Avadex boasts operator ease and can be put through a normal sprayer, rather than growers having to invest in specialist equipment. It seems to be a good tank mix, but you do need to be aware of mixing products in terms of crop safety.

Obviously, the downfall is the smaller amount of triallate per ha, but if applied in the right conditions it can match the control of granules, adds Kuldip. "Avadex Factor contains 1620g/ha of triallate, so it's 72% of the dose compared with Avadex Excel.

To make the best of it, we position it in slightly later drilled fields or conditions where there is more moisture as we have found it works better that way."

Charles agrees: "Having seen it in trials, it performs nearly as well, given the right conditions."