Should you switch to autumn herbicide application to control weeds?

Late summer weed control is real opportunity that’s being missed by many grassland farmers.

A farm survey of 200 grassland farmers carried out by NuFarm in July and August found that only 31% of respondents had considered applying herbicide in the autumn.

The main barrier to autumn herbicide application was lack of awareness, with half of farmers admitting they didn’t realise it was a possibility. 

Revealing the results of the study at a media briefing in Shrewsbury last week (9 August), NuFarm agronomy manager Brent Gibbon said there’s a common misperception that spring is the best time to apply herbicides.

Independent grassland consultant George Fisher says many farmers believe weeds are dying back in the autumn, which puts them off treating, but in actual fact he says efficacy can be good because weeds are sending nutrients (sugars) in the right direction.

See also: How weed control can improve grass DM/ha by over 10%

Both spring and autumn herbicide applications can be equally effective, providing the conditions are right.

However, achieving the correct application timing in the spring can be “very tricky for several reasons” with late summer often providing a bigger window of opportunity, explained Mr Brent.

Below, Mr Gibbon and Mr Fisher advise on why autumn reseeding can be better and offer some tips on how to achieve the best results.

Why apply herbicide in the autumn rather than the spring?

  • Weather conditions are not always suitable in April.
  • Soils take longer to warm up in the spring.
  • In the autumn, all weeds will be actively growing in warm soils, sending nutrients (sugars) from the leaf to the roots (known as translocation) so this ensures the weeds are killed at the roots.
  • Perennial weeds will be at the leafy “rosette” stage of growth following harvest – the optimum stage for herbicide application.
  • You need to allow about two to three weeks for the weeds to grow to the right stage after grazing or cutting the field.
  • You then need to allow a three-week interval between application and cutting or a two-week interval between application and grazing to allow the active ingredients to translocate in the plant.
  • This total of five to six weeks can interrupt harvest and grazing plans at the start of the season.
  • In comparison, in the autumn, harvest is coming to an end and grazing rotations are being lengthened to build covers for the spring, with other fields often available to allow for more flexibility for treating weeds.
  • Minimises the chances of weed competition in the spring and reduces spring workload when farmers are very busy.

What happens if farmers operate a multicut system?

If you’re going to get one more cut of silage (after August/September) then apply herbicide in the spring.

But if you have finished then it’s a good time to apply herbicide.

Should you apply herbicide twice annually?

No, this is not about “double hitting”. It’s about applying it at the most convenient time of the year for you.

What benefits does it offer farmers running rotational grazing systems?

You can select which fields you’re going to graze first next spring and target these for herbicide application in the autumn.

This avoids having to wait weeks, if you treat in the spring, before these fields can be grazed again.

When should you treat, regardless of whether you’re applying herbicide in the spring or autumn?

  • When weeds move to a leafy, vegetative state following cutting or grazing (allow approximately two to three weeks)
  • Soil temperatures need to be 10C for weeds to grow.