Although resistance to most grassweed herbicides, including Atlantis, is clearly rising, the 2009/10 season was plagued by other factors that made blackgrass control especially challenging, says Bayer CropScience’s Chris Cooksley.

“Effectively there was no ‘cultural control’ to help reduce weed numbers before the herbicide programmes began – and then the weather disrupted spraying plans right through until April.”

The dry autumn reduced stale seed-beds’ effectiveness, reduced crop competition, and shortened the autumn Atlantis (mesosulfuron + iodosulfuron) application window. Some growers also omitted pre-emergence herbicides because of the conditions, notes Mr Cooksley.

The late, cold spring then delayed spring Atlantis applications until the weeds were big and crops thin and stressed.

However, the difficult season should not be used as an excuse and growers should re-examine the way they use Atlantis, he stresses.

Poor control means blackgrass seed return will be high this year. “So if people wait until spring to apply Atlantis the problem could become even worse.”

Overall Atlantis performance has declined in many areas, especially where it was spring-applied, he acknowledges. “That’s down to a combination of resistance, weed size and variable application conditions.

“But I believe blackgrass size is now more important than climatic conditions.

“Where resistance is a problem Atlantis applied to small weeds will generally give better results than waiting for ideal growing conditions. If you wait until spring the blackgrass may well be tillering.”

Resistance sampling can be misleading, suggests Mr Cooksley.

“Normally samples are taken from herbicide application survivors and so may not represent the field population. However, the trend of increasing resistance is obviously there, so it’s wise to assume it’s present and plan accordingly. Programmes can then be fine tuned where testing identifies the level and type of resistance and the chemicals affected.

“It is essential now to consider a comprehensive programme of pre-emergence herbicide followed by Atlantis applied in the autumn when the blackgrass has no more than three leaves.

“Size is everything in the fight against resistant blackgrass.”

Equally important is using a combination of different active ingredients in a sensible programme, he believes.

“Applying a pre-em herbicide based on flufenacet (such as Liberator) gives you the best possible start to the programme. Adding diflufenican, prosulfocarb or pendimethalin may well provide extra control depending on your resistance profile.

“When planning a programme it’s best to combine different products in the pre-em slot rather than aim for a pre-em herbicide followed by a peri/early post-em treatment followed by Atlantis in late autumn. The three-stage approach runs the risk of pushing the Atlantis application into spring, where control becomes more of a lottery.

“Don’t be concerned about missing late germinators. Most blackgrass is through by mid-November and Atlantis’s residual autumn partner will help control those later plants.”

Three-stage approach may not suit all

The key question, according to NIAB TAG specialist adviser Jim Orson, is whether poor control by Atlantis in many fields this season was due to increasing resistance or the weather.

“The answer, I suspect, is a bit of both. If that’s the case, resistance may be worse this year, so getting application and timing right will be even more important. The key issue for farmers is whether infestations can be controlled with Atlantis now offering lower levels of control than in the past.”

In general he agrees with Mr Cooksley. “But some farmers prefer a three-stage attack – pre-em, peri-em and then Atlantis – as they’re concerned about the crop safety of applying all their pre-ems/peri-ems together. In addition, conditions at the pre-em timing may not be suitable to get the best out of herbicides they applied, so there is a danger of putting all the eggs in one poor basket. Some farmers also like to get a feel for how much blackgrass there is before spending more money on peri-ems.”

Plants with target-site resistance to Atlantis will not be controlled at any growth stage, Mr Orson points out. “So if the situation isn’t clear it’s important to reduce reliance on Atlantis. If high populations are expected and the blackgrass resistance status is unclear, farmers must consider additional cultural control.”

Application needs more care

Many sprayers still travel too fast applying incorrect spray patterns, warns Mr Cooksley.

“You’re trying to hit a small target so some finesse is required. There’s no point planning your programme to the nth degree, but then spoiling it by not getting the spray onto the target.”

“Choose your nozzle to achieve a fine/medium spray quality, and don’t exceed 12kph. Make sure the boom is at the correct height and stable.

“Always use Biopower adjuvant with Atlantis, and avoid complex and antagonistic tank-mixes. Ideally use it with a residual partner only in the autumn, and make sure it goes onto a dry leaf in good drying conditions.”