Scottish growers should be aware of weed resistance to ALS chemistry continuing to grow amongst broad-leaved weeds – in particular in chickweed – according to Scottish Agronomy’s Andrew Gilchrist.

“Nothing lasts forever if you keep using the same product,” he explained. “ALS has been over used for more than 15 years and resistance is continuing to develop in a large range of weeds – consequently, we need to look at adopting different strategies.

"Scottish growers are using a lot more residuals in spring crops, in fact the volume used has gone from zero to more than 60% of total crops, then they top up with other actives such as phenoxies," he said. “In fact, I think phenoxies are scheduled to have a firm future as a backup and partially to replace ALS chemistry, simply due to their completely different mode of action.”

Phenoxy chemistry is nothing new, in fact it has been commercially available for more than 70 years and remains amongst the world's most widely used herbicides. This includes MCPA, 2-4 D, CMPP-P, 2,4-DB, MCPB, 2-4 dichlorprop-P, explained Nufarm’s Iain Allan. “Phenoxies is a general term to describe a group of herbicides that mimic the effect of natural plant hormones called auxins. They are formulated from acids, normally as salts and sometimes, as esters for cost effective broad spectrum weed control in cereals.

"They have a significant role to play going forward in managing resistance because they have the lowest risk of fostering resistance development. Introducing phenoxies to postpone resistance will lengthen the time before resistant prone herbicides become truly non-renewable.”

Such herbicides have the lowest risk of fostering resistance development due to their mode of action. They mimic plant auxins which controls cell enlargement, division and development throughout the plant’s life cycle, and they over stimulate plant cells, causing abnormal plant growth and subsequent death.

So far, the full mode of IAA has yet to be understood in detail, however researchers have confirmed that IAA binds to auxin binding proteins located in the cell membrane, endoplasmic reticulum, the cell nucleus and cytoplasm.

At East Garleton Farm, Haddington, John Shedden said he became aware of growing resistance to ALS creeping in on his 181 ha unit, not only to chickweed and fumitory. “There is definitely a place for phenoxies,” he explained. “We managed to nip the chickweed resistance in the bud with a mix of CMPP plus fluroxypyr and it proved to be the best value for money in term of cost litre per hectare.

“Cow parsley is now proving to be the challenge and so far, we have found that the only chemistry that works is a CMPP/dicamba plus SU mix if you catch the weed at the early growth stage. We’ve found you have to have good contact and apply as a single spray pre-GS30.

“Yes, you have to be careful not to have too many mixes in the tank, otherwise it could get too hot – take advice from your agronomist,” he added. “However, a lot of farmers aren’t aware that the older chemistry has a different mode of action and it can be used as an alternative to combat resistance, furthermore they have a fear it doesn’t mix with some of the latest products.”

Cost control is a priority for him: “We do look at new chemistry, however compared with the phenoxies it doesn’t come cheap to go through the full registration process and reach the market. Furthermore, at the end of the day herbicide application is for us the last resort,” he pointed out.

“We are very, very strict about combine hygiene and share with just three other farmers, and the rotation is still very important – oilseed rape, winter barley, winter wheat, spring barley and grass.”