Dry and cobbly seed-beds in many areas are making some growers consider delaying grassweed pre-emergence herbicide sprays until early post-emergence in wheat, according to agronomists.

“Growers won’t commit to pre-emergence sprays while it remains so dry,” Dan Robinson of Agrivice in East Suffolk said on Monday. Fears over pre-emergence efficacy, crop safety and the possibility of having to re-drill if crop emergence was poor in the conditions were all reasons why growers were reticent, he explained.

“Looking at seed-beds that have gone cloddy and hard I wouldn’t want to risk pendimethalin-based treatments or trifluralin for that matter at the moment.

“And if you end up having to re-drill, and you’ve put on a pre-emergence which you would then have to incorporate, it is even worse. It adds another pressure. If you spend your money on a pre-emergence spray you want it to work.”

Gerald Abel of Farmacy agreed that where seed-beds were dry throughout the soil profile it was probably better to hold off until early post-emergence before spraying. “It is mainly a seed depth and lack of moisture issue.”

Where seed-beds were good, he advised growers sprayed a true pre-emergence. “But they are few and far between this season.”

Some growers had already sprayed pre-ems, UAP’s Chris Bean noted. But he warned that activity from trifluralin would almost certainly have been reduced in warm conditions. “Volatilisation will have been hard and furious on warm dry seed-beds.

“If growers have sprayed pre-ems based on trifluralin or slashed rates of Crystal or Liberator and added in trifluralin to cheapen the mix, they might have done themselves a disservice.”

On dull days or when seed-beds cooled off and became damper the risk of trifluralin volatilising declined, he said.

He was less worried about the dry conditions affecting the performance of Crystal (pendimethalin + flufenacet) or Liberator (diflufenican + flufenacet). “If there is enough moisture for germination there should be enough for the pre-ems to work, and the products should activate when moisture comes along.”

But he too was concerned about seed-bed quality. “Some are good, others look like pebbly beaches. Pre-ems might not work so well if weeds come from around clods, or after rain breaks the clods down.”

That meant he was advising growers to wait until early post-emergence where seed-beds were poor. “I’d look to put Crystal or Liberator on with either IPU or chlorotoluron when the tramlines are visible.” But don’t forget the new 1500g ai/ha rate limit for IPU and if the crop emerges fast in a warm seed-bed it might pay to let the crop harden up a bit before “blasting with IPU”, he added.

* Dry weather causes problems with oilseed rape establishment