Blackgrass dormancy doesn’t equal post-emergence herbicide delays

Does highly dormant blackgrass seed mean it is best to wait until spring to control the weed? Not according to the experts.

On the face of it, the finding from HGCA-funded research that blackgrass seeds are, for the second season running, highly dormant means a more protracted germination period.

For a mainly contact-only material, such as Atlantis (mesosulfuron-methyl + iodosulfuron methyl sodium), that could imply it would be best to wait until spring to ensure all weeds have emerged before application.

But last year’s experiences suggest that is not necessary. As part of the dormancy project, large containers of soil were sown on two different dates (21 September and 21 October) with blackgrass seed. Emergence date was closely monitored.

The results were quite surprising. While emergence was slow, particularly from the later sowing date, almost all of the seed came up in the autumn, with no real evidence of spring germination.

That was in pretty much ideal soil conditions, James Clarke (pictured) of ADAS notes. “Spring emergence might occur when cloddy seed-beds break down or where soil is cultivated or moved again in the late winter or spring. At ADAS Boxworth we have seen high populations of spring-germinating blackgrass in spring crops, but none in autumn-sown crops where seed-beds were reasonable.”


An equally intriguing result was that just 20% of the seeds germinated at either drilling date. What happened to the other 80% is a mystery, Mr Clarke admits. But if that result was replicated in field situations last autumn, it could explain why plenty of blackgrass came up this autumn in stale and early seed-beds, he suggests. “What have emerged might be last year’s dormant seeds.”

Typically, 80% of the blackgrass seeds that lie dormant in the soil will either rot or be eaten over the course of a season. But that would still leave 16% free to emerge this autumn, which in large weed seed-banks could be a fair population, he notes. “It shows it is not black and white.”

An alternative explanation for this year’s September flush could be that the early warm and wet growing conditions overrode dormancy, suggests Nick Myers of ProCam. “Early seed-beds have been literally heaving with the weed this season.”

While it is possible dormancy has been overridden, it isn’t something that has been seen before, Mr Clarke points out.

“We’ve got some ongoing research that should help answer the question.”

That involves a repeat of last year’s germination experiment, this time with seeds artificially manipulated to produce either high- or low-dormancy seed from the same seed batch. “We’re sowing these seeds, so there won’t be any contamination, and we will monitor their germination. But it is too early at the moment to answer the question.”

Whichever explanation is correct, it is good news for growers, who have been able to take the chance to use glyphosate pre-drilling or pre-crop emergence to reduce their blackgrass burden.

With little spring germination, it also means growers shouldn’t wait until spring before applying post-emergence herbicides, such as Atlantis. “It is also worth remembering that late-emerging plants are much less competitive, especially in well-established crops,” Mr Clarke says.

Practical experience from Agrovista trials last year also suggests autumn applications are more effective even in high-dormancy years. Atlantis applications in late November averaged 97% control across a number of different pre-emergence treatments.

Treatments applied in mid-March also achieved 97% control last season, the firm’s Mark Hemmant says. “But spring results in commercial practice and elsewhere on the trial field were much more of a lottery.”

That was also the case the previous season, when early March applications averaged only 65% control compared with over 90% from November sprays.

And then there is the yield penalty from spring applications. Crops receiving autumn Atlantis applications typically yield an extra 1t/ha compared with spring-applied treatment, says Mr Hemmant.

It all adds up to a firm conclusion: Don’t let dormancy affect what you do in the autumn.

Dormancy effect

  • Blackgrass seed highly dormant
  • Surprising flushes seen this autumn
  • Will there be prolonged germination?
  • Evidence suggests post-emergence sprays should not be delayed

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