Growers are being urged not to tank-mix key blackgrass killer Atlantis with any other pesticides this spring to maximise its efficacy.

To encourage growers to follow the new advice manufacturer Bayer CropScience has cut its list of supported tank mix partners, and will now only permit one additional tank-mix partner from an approved list of products.

But the recommendation is likely to prove controversial for growers faced with spraying Atlantis (mesosulfuron-methyl + iodosulfuron-methyl-sodium) at the same time as other key spring inputs.

Adding additional products to Atlantis and its adjuvant Biopower can reduce the product’s efficacy, Chris Cooksley of manufacturer Bayer CropScience explains. About 60% of users in a recent grower feedback survey thought Atlantis efficacy was declining.

“Cutting back the number of products in the tank is the simplest way to boost the products efficacy, Weed size, moisture, crop and temperature are beyond our control, so it makes sense to focus on areas where we can have an influence,” he says.

The advice applies to all of the firm’s mesosulfuron-containing products including Pacifica, Othello, Horus and Hatra.

Even a small drop in efficacy can have a significant impact on seed return and resistance build-up, says Mr Cooksley. “Chlorothalonil in particular should be avoided as it can reduce efficacy by 4%, for example,” he says. “We need to keep the product working as long as possible as there are no viable alternatives on the horizon.”

A limited number of products are supported by the firm. Fungicides form the bulk of the list but there are a number of herbicides and growth regulators included. (See www.bayercropscience.co.uk for a full list)

If growers are pushed later in the season they may be tempted to add other products to the tank-mix, he notes. “It’s their choice if they want to do this, but they do so at their own risk and need to be very cautious.”

However, if blackgrass is a serious problem growers must find the time to spray it separately, he warns. “The weed is changing and is becoming more difficult to control, he says. “Therefore we need to go back to basics.”

Independent agronomist Bryce Rham from Shropshire agrees that it is preferable to apply Atlantis and Biopower alone, but believes later in the season growers will be forced to mix in other products. “I would much rather it was applied on its own, but if there hasn’t been opportunity to put it on by the end of March and other products need to go on I would have to consider recommending a tank mix,” he says. “I think my growers would just laugh at me and do it anyway if I didn’t.”

He is also experimenting with the liquid formulation Horus and believes it could improve efficacy. “I have had good experiences with Othello and hope this will translate to the liquid form of Atlantis.”

But Frontier agronomist Brian Ross from Suffolk takes a harder line and would only add other products to spring-applied Atlantis in exceptional circumstances. “We really don’t want to lose efficacy and our company policy is to apply spring Atlantis on its own,” he says. “If there was a bad blackgrass problem it would be irresponsible to recommend adding other products.

“If I really had to I would consider adding one of the recommended partners, but I’d rather do them separately,” he adds. “I made the recommendation to apply it on its own last year and although some people grumbled, they understood the importance and went along with it.”