Light intensity key to mesosulfuron herbicide efficacy

Farmers should apply mesosulfuron-based herbicides on sunny days to optimise performance, according to research carried at the University of Nottingham.

For many seasons, farmers and agronomists have noticed that sunny weather seems to be linked with higher efficacy from the sulfonylurea herbicide, but up to now, there were no trials to back this up.

To investigate the effect of sun, researchers tested Atlantis Star (mesosulfuron + iodosulfuron + thiencarbazone) on Italian ryegrass, rye brome and sterile brome.

See also: Advice on managing glyphosate resistance this spring

It took place in a glasshouse with LED lighting used to replicate growing conditions from early February.

“This trial shows light intensity has a direct benefit on efficacy in glasshouse conditions at relatively low temperatures of 6C,” says Matt Siggs, technical manager at Bayer who commissioned the research.

Matt Siggs © Bayer

Most importantly, the work excludes the other ways sunlight could increase efficacy.

For example, sunny weather could improve performance through faster drying on the leaf and a higher ambient temperature.

However, often clear skies during the day are mirrored at night which leads to higher frost potential and lower average temperatures, as it can de-wax the crop and increase the chances of transient crop damage.

Applications at higher light intensity equivalent to sunlight (350W/sq m) showed a small, but consistent, improvement in activity compared to those at lower light intensity (100W/sq m), which replicates cloudy weather.

The field rate of Atlantis Star is 0.333kg/ha. In the trial, a reduced rate was used to emphasise the impact of light, so the results shouldn’t be used to predict control in the field, just the relative benefit of brighter conditions.

The least effective herbicide is the one that doesn’t get applied. In practice, there is always a trade-off between getting the application done and waiting for ideal conditions.

Added to that, smaller weeds are easier to control which is another reason not to delay waiting until warm growing conditions in late March or April, as this often compromises control as the grassweeds will have potentially reached stem extension ahead of the crop.

“Once the weeds are sending resources upwards into the leaves and stems, that means the chemistry is not delivered to the roots, and hence we often get complaints of regrowth or suspicions of resistant populations.”

Matt says there is often a spell of bright weather in February.

“We had it in 2023, but not last year. Be ready to spray as soon as conditions improve, and have a plan about which fields to target first.”

Farmers are also advised to avoid making applications to stressed, frosted and waterlogged crops, where the uptake of herbicides is likely to be reduced.

Adding a residual herbicide partner is also an option in the spring to control further germination of grass and broad-leaved weeds.

Trial results show metribuzin products, in particular, can improve overall efficacy against difficult to control grassweeds.