Loss of key herbicides could have wider implications

8 July 2008

CEREAL growers facing the impending loss of key grass weed herbicides isoproturon and trifluralin must think beyond obvious replacements if they’re to potentially avoid far-reaching consequences undermining the sustainability of winter wheat and barley production.

That was a stark warning from a series of weed control seminars held around the UK recently, and with speakers from leading independent agronomy firm TAG and Syngenta Crop Protection examining wider implications of the withdrawal from sale of isoproturon (IPU) and trifluralin this September.

In recent years, these herbicides have been used annually on about 2 million hectares of UK farmland, leaving a huge hole, but according to TAG research and technical director Jim Orson, their loss isn’t the only issue growers have to face.

In the battle against black-grass, resistance to recently-introduced

Farmers must seek to protect remaining herbicide chemistry against resistance with the loss of IPU and trifluralin, believes Simon Parker of Syngenta, so that profitable cereal production can be maintained

sulfonylurea herbicide chemistry is increasing and there has also been disappointment in the control achieved with pendimethalin, widely used in many rotations, he noted.

On top of this, there are also no major new herbicide modes of action in active development, he stressed. So future black-grass control may require additional strategies to manage resistance, including increased emphasis on pre-emergence options, and he asked whether now was the time for the industry to make greater use of cultural methods to ease the pressure on herbicides. “Whatever method you use, it’s going to cost more money,” he said.

Speaking after the event, Simon Parker of Syngenta agreed that growers faced some tough challenges after the loss of IPU and trifluralin.
“They have become weed control cornerstones for UK growers,” explained Mr Parker. “IPU is a key ‘holding’ spray to reduce black-grass prior to applying a main follow-up post-emergence treatment, and has been the major treatment against annual meadow-grass. Meanwhile, trifluralin has formed an important low resistance risk partner for mixing with other black-grass herbicides at both pre- and post-emergence timings.

“As a knee-jerk reaction, there is a danger their loss could lead to increased reliance on alternatives where resistance is already a concern.

“Consequently, not only do the roles of IPU and trifluralin need replacing, but in a way which minimises resistance selection pressure on remaining herbicides, such as sulfonylurea chemistry and pendimethalin, so that profitable cereal production can be maintained.”

To address the situation, Mr Parker said Syngenta had been developing alternative programmes using the herbicide Defy to provide improved pre-emergence activity against black-grass. The company has also developed successful Defy-based strategies for controlling annual meadow-grass without IPU.

He said pre-emergence Defy is not only classed as low resistance risk, but is from a different chemical class to sulfonylureas and pendimethalin, plus a number of other pre-emergence materials, so targets weeds using a different mode of action.

“For pre-emergence use, it can form a valuable cornerstone in mixture with other herbicides,” explained Mr Parker. “We’re suggesting that growers try newly-developed tank mixtures of 5 l/ha of Defy with 0.1 l/ha of diflufenican or 4 l/ha of Defy mixed with 0.4 l/ha of a diflufenican + flufenacet product. Not only are these mixtures trifluralin-free, but neither relies on pendimethalin at the pre-emergence timing – helping to avoid over-use by retaining it for post-emergence only.

“Perhaps more importantly, by using one of the new Defy mixtures, trials have shown it is possible to improve control of black-grass at the pre-emergence timing – by almost 6%. By achieving a greater reduction in black-grass pre-emergence, it can help replace the role of a later IPU holding spray and reduce the pressure facing a follow-up post-emergence treatment, such as a sulfonylurea,” he pointed out.

Similarly, Mr Parker said IPU had been a main component used in about three quarters of all annual meadow-grass with broad-leaved weed treatments. Where black-grass was also present, he said this was usually controlled with the herbicides used against that weed, but where black-grass wasn’t present, it needed specific attention.

“To replace the loss of IPU against annual meadow-grass, we have developed a special lower dose tank mixture of 1.5 to 2.0 l/ha of Defy mixed with 0.1 l/ha diflufenican. This has performed at least as well as an IPU mix – giving up to 99% control. Importantly, it can also be used in winter wheat and winter barley so growers don’t need to remember to use different herbicides. It also controls a wide range of broad-leaved weeds at the same time.”