Care needed with herbicide applications to late-sown crops – Agronomist & Arable Farmer

Pre or post-emergence herbicides in late-sown winter crops could lead to crop safety issues if conditions are sub-optimal. That’s the warning from Duncan Connabeer, a technical support manager for Hutchinsons.

Following autumn drilling disruptions, some might have grabbed any opportunity to get seeds in the ground. However, wet soils may still present a challenge for establishment and weed control.

“Where crops have been drilled or mauled in during late November, rough seedbeds will inevitably reduce the efficacy of residuals, and increase the risk of damage,” he notes.

Mr Connabeer adds that crops that have been broadcast onto seedbed, rather than drilled, should not be treated with residual chemistry. This is not supported by chemical manufacturers and could lead to crop damage due to the seed germinating where the herbicide is active. In the case of winter beans, growers should avoid the temptation to apply pre-emergence herbicides when the crop is emerging or has already emerged.

Further consideration should be given to crops that are water-stressed, he added.

It’s possible that rooting will have been affected, which could increase the risk of crop damage.

“This could be a particular problem in looser soils. If the herbicide is washed down to damaged roots, then damage is still likely. Growers might see harsh herbicide symptoms where they don’t expect them,” Mr Connabeer adds.

Particular care is needed with direct-drilled crops. He advises growers to check seed or rooting depth.

“Where other crops are still to be drilled, we may yet see good conditions for product use, but there may be further tank mix and/or timing constraints,” he explains.

“Much of what, and/or when we can do things, will be greatly dependent on drilling depth, seedbed and soil conditions.”

Growers should also remember that any crop sown after January is considered a spring crop by the Health and Safety Executive’s Chemical Regulation Division, which reduces the herbicide options available.

Many products should also not be used in waterlogged soils, including propyzamide on OSR, although in such cases there is still sufficient time to get sprays on and still expect good efficacy before the application window closes at the end of January.

The mild, wet autumn has been ideal for aphids, so Mr Connabeer recommends a pyrethroid for growers applying post-emergence herbicide on crops that have exceeded 170 day degrees since emergence or the last aphicide application.

He notes that while the latest cold spell will help to slow aphid activity, for those in high-risk areas it may be necessary to treat crops just for aphids if need be.

“Grain aphid (Sitobion avenae) can tolerate temperatures down to -8 degrees Celsius or more, while the bird cherry-oat aphid (Rhopalosiphum padi) is reportedly able to survive -0.5C for 24-48 hours.

“Monitoring shows there was more virus in aphids this autumn, and there are still plenty of them around, so applying treatment at the right time can be a worthwhile insurance policy to mitigate the risks. It only takes a mild day or two, and we’ll soon see more aphid flights, so growers in high-risk areas need to be vigilant.”

Waiting until the spring T0 fungicide is too late, he suggests.

“A lot of the BYDV in the 2022/23 crop was spread in the winter and early spring, before the T0 was applied. Also, remember that once crops reach growth stage 30 moving into 31, you don’t need to treat for aphids because the virus coming in after that stage is less significant and the damage will have already been done.”