20 October 2001

Spreading the risk

Its post mortem time on blackgrass. Tom Allen-Stevens joins the A-team experts

IF you havent drilled your winter wheat yet, you may be feeling in a minority this year, with so many growers opting for early drilling. But if you also have bad blackgrass, you should be revelling in your individuality, says the A-team.

Delaying drilling was the biggest factor that helped the team at ADAS Boxworth reduce the blackgrass burden in its Syngenta-sponsored trials. The site is heavily infested with both target site and enhanced metabolism resistant blackgrass. In the plots two cultivation regimes, two drilling dates and a range of chemicals were set against each other.

By the later drilling date (15 November), much more of the blackgrass had emerged and had been sprayed off with glyphosate, notes ADAS weed specialist James Clarke. This left less to control but that was not the only factor that improved overall control.

"We also got a higher percentage kill of blackgrass in the later-drilled plots, which was down to a combination of factors. Firstly the blackgrass was smaller, and it grows more slowly, so its smaller for longer. Its also harder to get high levels of chemical control if you have a high population of blackgrass."

He also recognises that the year and soil conditions favoured control in the later-drilled plots – the damp, cold conditions later on held back blackgrass development, giving the chemical a free rein to do its job. "Enhanced metabolism resistance has less of an impact in small slowly growing plants or when conditions are ideal for chemical activity."

Another factor that improves chemical control, notes Syngentas Jason Tatnell, is growth stage distribution. "If you drill later and spray later, few further plants will be coming through and most will be at the same growth stage. At earlier dates you get a much bigger spread of weed growth stage, so a higher percentage will be too big to control and some may not have emerged."

Generally, a pre-emergence chemical was found to be vital to get good control. "It added up to 1t/ha to the yield, but it also adds flexibility," says Mr Tatnell. "It sensitises the blackgrass which widens the window of application for post-emergence sprays."

Avadex (tri-allate) was the main pre-em on trial, but Crystal (flufenacet + pendimethalin) was also used in one treatment. From the results, Crystal appears to be the winner, but Mr Clarke warns not to jump to conclusions from the limited data. "Crystal did well in one comparison, but it was only one. Both chemicals are very dependant on soil moisture, with Avadex probably having an edge in drier conditions. One is a granule while the other is a liquid, so it is important to consider which formulation suits your system."

The post-emergence chemicals on trial were isoproturon, Panther (IPU + diflufenican), Hawk (clodinafop methyl + trifluralin), Lexus (flupyrsulfuron-methyl) and Stomp (pendimethalin). Toil, a methylated rapeseed oil, was added to Hawk mixes, according to manufacturers instructions.

The best post-emergence treatment was Hawk + Lexus + Toil. Efficacy was improved further if this was applied at exactly the three-leaf stage of the blackgrass, rather than at the recommended 1-2 leaf stage.

"Spot on three leaves is indeed the best timing to apply," confirms Mr Tatnell. "But these were exact experimental applications and if you wait until all the blackgrass has three leaves in the field situation and the weather turns against you before you can spray, you might not get back on until tillering starts."

Mr Clarke notes that all the best mixes included Hawk: "Here is a site with known RR target site resistance, and were still getting the highest level of control from using Hawk-based treatments. In the short term thats a gain, but the penalty is that youre selecting more plants with resistance."

And this brings him to the most important herbicide message to bring out from the trials: "It is essential that you use sequences and mixtures of active ingredients and modes of action when tackling blackgrass, particularly where you have target site resistance."