10 September 1999


WELL-PLANNED weed management strategies combining selected pre and post-emergence herbicides are the best way of getting on top of difficult-to-control grass weeds in winter cereals this autumn, according to a series of field trials conducted last year.

The trials, on eight different farm sites across the country, were carried out as part of DuPonts annual assessment of herbicide programmes on known resistant blackgrass populations.

They examined a variety of control strategies involving recommended rates of a wide range of commonly used herbicides, including several IPU-based standards. Blackgrass populations averaged 500 heads/sq m.

Regardless of timing, the IPU-based treatments gave control levels of little more than 50% averaged across the eight sites, says DuPont cereals herbicides manager Martyn Rogers. Treatments based around post-emergence flupyrsulfuron-methyl (Lexus) increased control to over 67% when applied at blackgrass tillering and 73% at the 1-2 leaf stage of weed growth. Mixtures with pendimethalin (Stomp) were particularly successful.

Pre-emergence tri-allate (Avadex) further improved the effectiveness of all programmes. The only treatments consistently giving the 90%-plus control required to prevent weed populations building-up through seed return were Avadex followed by mixtures of Lexus and Stomp.

"The results gave us an excellent lesson in the essentials of modern blackgrass control. First and foremost, they underlined the great vulnerability of IPU-based programmes. Relying on IPU + Treflan, for instance, we were pushed to control much more than 50% of the blackgrass on average. And even combining them with pre-emergence Avadex, the best control we could achieve averaged less than 75%, which is nowhere near enough to prevent a year-on-year population build up.

"Nor could we average more than 75% control with any of our post-em only strategies," he says. "Adding a pre-emergence application of Avadex, however, made all the difference, particularly with post-em Lexus/Stomp.

"When we were able to get the Lexus/Stomp treatment on early at the 1-2 leaf blackgrass stage after the Avadex we achieved an average of over 95% control. And we still obtained over 90% control from pre-em Avadex followed by a later Lexus/Stomp + oil application at blackgrass tillering."

Mr Rogers points out that Avadex + early Lexus/Stomp gave the most consistent results across all eight sites. While some post-em only strategies gave 14% control on specific sites and other pre-em/post-em sequences just 28%, on no site was control with this strategy less than 91%.

The 1998 trials clearly demonstrate the value of a well-planned sequence of treatments combining the residual and contact activity of herbicides with different modes of action, Mr Martin maintains. The residual Avadex hits early germinating blackgrass and other grass weeds, sensitising any plants not killed to the contact and residual combination of Lexus and Stomp, which gives good protection through to the spring, he says.

"This strategy provides an excellent way of protecting against the development of herbicide-resistant weeds as much as in tackling existing resistance problems," concludes Mr Rogers. &#42

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