10 September 1999


REGARD all blackgrass with caution and assume that you have some degree of resistance on the farm, David Stormonth, technical director of Farmacy – the new agronomy company formed by distributor Brown Butlin – advises cereal growers with difficult-to-control blackgrass.

"Resistance to herbicides is now widespread in blackgrass and growers seeking high levels of control should assume a resistance problem," says Dr Stormonth. "Too often a standard basic approach gives rise to a control failure."

Results from in-house resistance testing done at Farmacy over the past two years support the view that resistance is more common than believed. "Weve been using the Rothamsted Rapid Resistance Test on seed samples, which involves breaking dormancy and testing them in the presence of different herbicides.

"Not only does it identify the resistance mechanism present, it gives the results in time for the autumn."

He believes most growers are now aware there are different types of resistance mechanisms and varying levels of resistance. "Enhanced metabolism and target site resistance are already in peoples minds. We know enhanced metabolism is widespread, but tests have shown that target site is also becoming increasingly common.

"Getting seed tested helps with choosing the right treatments. Its also important to recognise that resistance patterns are complex and vary from field to field." That makes standard control programmes more inappropriate than ever.

High levels of control are essential, Dr Stormonth adds. "Dont accept anything less than 98%. Even 98% is only just good enough."

He advises the use of stale seed-bed techniques wherever possible and warns growers moving to earlier drilling dates that blackgrass control may become more of a problem.

"Dont forget that rotations offer the chance to use different cultivation and herbicide strategies. Continuous wheat compounds the problem where blackgrass is concerned, so ideally a break crop should remain in the rotation."

While there may appear to be a huge range of products offering blackgrass control, Dr Stormonth points out that there are only six different chemical groups (see table).

"No one product will be the answer. The best solution tends to be field specific and will be bound by factors such as drilling date, seed-bed and soil conditions, previous treatment success and product preferences."

He outlines three steps to successful blackgrass control, but stresses that each situation should be assessed carefully before deciding on the course of action.

"Start with a sensitiser treatment, such as Avadex or terbutryne. This should be applied before the weeds have emerged.

"The post-emergence treatment should then be a combination of two of the chemical groups – such as trifluralin or pendimethalin mixed with either a sulphonyl urea, a substituted urea or a fop.

Timing is the third critical factor and can be the difference between success and failure, he concludes. "Regard the blackgrass 3-leaf stage as the most important date in the autumn, as this is when the post-emergence spray should be applied. Always aim to complete control in the autumn rather than delaying until the spring." &#42

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