Protecting grassweed chemistry

With no new grassweed herbicide chemistry in the pipeline, protecting the efficacy of existing products is crucial, says Tim Whitehead, who manages Velcourt’s 970ha (2397-acre) Vine Farm in Hertfordshire.

While applying “one hit” of products such as Atlantis (iodosulfuron + mesosulfuron)/trifluralin in November or February/March may prove effective that season, you could be storing up problems for future years, he says.

“There’s no new chemistry coming through, so we have to look after what we’ve got.”

Bayer CropScience’s Gary Jobling agrees.

“Most new products at the moment are based on existing active ingredients – you’ve got to put other modes of action into the programme.”

Mr Whitehead says that despite having high levels of resistant blackgrass and wild oats, control last autumn was very effective due to good stale seed-beds and early weed emergence.

Recent LINK project research suggesting that dormancy is related to temperature and humidity during blackgrass seed maturation could explain annual variations, although it isn’t always black and white, he says.

Disced stale seed-beds are sprayed off with Roundup (glyphosate), before drilling and treating with Crystal (flufenacet + pendimethalin) or Liberator (diflufenican + flufenacet) + trifluralin, ideally at the pre-emergence stage, he says.

In most cases, this is followed by Atlantis in November.

“This year it’s done a fantastic job.

There’s always a fear about going in too early, but I was confident that 99% of blackgrass had emerged by November, due to the ideal conditions.”

But resistant wild oats present an added complication as the best control is achieved with a February/March Atlantis spray, he says.

“Some blackgrass control can’t wait until then, so we have to go in with an interim treatment, such as five litres of isoproturon or three litres isoproturon + two litres Hawk (clodinafop-propargyl + trifluralin) in November.

It does a good job as a holding spray if applied early when the grass weeds are small, even though the blackgrass is resistant.”

Spring Atlantis is then only used where necessary and not as routine, he notes.

Fellow Velcourt manager, Stephen Walker, who runs 3000ha (7413 acres) near Stamford in Lincs uses a similar strategy, but aims to apply most Atlantis before Christmas.

“It’s not always possible, as November applications, especially on heavy land can be very difficult.”

Where the main herbicide needs to be delayed until spring, a holding spray such as a low rate of IPU may be needed to control small broadleaved weeds and annual meadow grass, he says.

Growers should not overlook the importance of crop rotation and he has seen good weed control from a first wheat – set-aside rotation.

“We’re now in the fourth year of this rotation and blackgrass populations are noticeably lower.”

Stewardship options such as overwintered stubbles could provide a good opportunity for such practices and may also prove more cost-effective than controlling blackgrass in break crops, notes Mr Whitehead.

He has achieved good control of resistant blackgrass in oilseed rape by applying reduced rates (1 or 1.2 litres) of Kerb [propyzamide] + fusilade or similar.

“It did a fantastic job, which Kerb by itself doesn’t do, but it is expensive.”