23 October 1998

Is resistant blackgrass threat really on wane?

HAVE herbicide-resistant blackgrass problems peaked, or should growers still fear further appearances on their farms?

While two herbicide makers think the former, an independent researcher disagrees strongly. But all three say growers must remain vigilant to prevent problems flaring up in future.

New cases of resistance confirmed by herbicide maker Novartis have fallen for two years running, says weed expert Pat Ryan. "There appears to be a slow down in the number of new resistant cases."

That may be because farmers who fear resistant blackgrass have already sent in samples, he says. Alternatively, farmers may be assuming all blackgrass is herbicide-resistant and are not bothering to test.

"Whatever the reason there does not seem to have been the rapid spread of resistance across the country that was expected."

Mike Read, of AgrEvo, reports a similar slowdown in sample testing and confirmed cases of resistance.

But independent researcher Stephen Moss, of IACR Rothamsted, disagrees. "I dont think new cases are tailing off at all – numbers are still increasing.

"Less than 10% of farms with blackgrass have been tested, but of those 50% have shown herbicide resistance. And we are still picking up new farms with a resistance problem."

Poor blackgrass control last autumn may have muddied the picture, he notes. Although the number of tests rose, bad weather and application difficulties were to blame in many cases, producing more susceptible results than usual.

Dr Moss would like to see a national survey to pinpoint problems, but that would probably be too costly, he notes. Instead he advises growers to seek a test wherever blackgrass populations are high or herbicide resistance is suspected.

Growers are advised to apply mixtures of full-rate agro-chemicals early in the season, and combine this with stale seed-beds, ploughing and non-selective herbicides. &#42