What did 28 November 2006 and 2 March 2007 have in common? It is a question that many of the growers and agronomists who visited Agrovista’s Maidwell trials site last summer would like the answer to, because it might just help them avoid the little – or big in the case of 2 March – down turns in Atlantis performance against blackgrass this season.

Agrovista’s wave trial highlighted what some growers have found in the past year – Atlantis performance can be variable. Sprayed across 14 dates in the autumn and spring, its control ranged from 50% to 100%.

The decline in performance into spring was no great surprise to Agrovista technical manager Mark Hemmant. “Historically we’ve always had better results from autumn applications than spring.”

But what were more surprising were the dips in efficacy on certain dates, which then rebounded only one or two weeks later. For example, there was a period at the end of November when blackgrass control from Atlantis dropped to 82%, when just three weeks earlier it had been 96%. One week (8 Dec) later control improved to 92%.

“There was nothing in late November that would have told us not to spray. There had been the occasional frost, but it wasn’t very cold.”

Like The Arable Group’s  Jim Orson (see panel), Mr Hemmant believes leaf moisture played a part in the poorer performance. “We had some very dewy nights, so maybe it didn’t dry so well onto the leaf and impaired performance.”

But the trials do give some pointers for growers looking to improve Atlantis consistency against blackgrass.

1) Apply a pre-emergence spray

It takes out some of the inconsistency in Atlantis performance, Mr Hemmant notes. Atlantis applied on the 29 November, a day when efficacy was less than expected, gave 82% control when no pre-emergence was applied. Applying a weaker pre-emergence, such as Avadex or half rate Trooper + trifluralin, increased control to 88-89%, while full rate Trooper + trifluralin + Grounded increased it to 93%.

“It shows putting on a decent pre-emergence can iron out some of the inconsistency in Atlantis performance that result from the weather.”

On days where Atlantis control was good it mattered less which pre-emergence was used, but Mr Hemmant cautions against using weaker pre-emergence materials.
“You’re reducing your flexibility with timing, and weakening your chances of preventing resistance. Weaker pre-ems potentially expose more blackgrass to Atlantis – it’s all just a numbers game.”

2) Apply in the autumn

Even applying a pre-emergence product isn’t always enough to remove Atlantis inconsistency however. Blackgrass control dropped to just 50% when applied on 2 March in the Agrovista trial, and even applying the full rate Trooper treatment did not rescue the situation fully (75%), Mr Hemmant notes.

Applying Atlantis in the autumn – once blackgrass reaches the two- to three-leaf stages – should also ensure yield is not lost, he adds. “Leave control until the spring and you start to get thinner crops, even if performance remains good.”

In the Maidwell trial Atlantis applications after mid February – once growth had re-started and the blackgrass started to compete more strongly with the crop – lost around 1t/ha compared with earlier applications.

3) Apply in good conditions

Applying in good conditions might be easier said than done. For a start it is not always clear when conditions are not suitable for Atlantis. On every date Agrovista applied an application last season it thought is should work, Mr Hemmant points out. “None of the treatments were designed to make Atlantis fail.”

Using the correct nozzles is another important consideration to get the most out of Atlantis

But a number of lessons have been learnt from commercial experience with the product. Atlantis mainly works through contact activity so it stands to reason that good growth from the weed will help maximise efficacy, as more chemical will be taken into the plant more quickly.

And experience two seasons ago showed the product works better going into cold weather than coming out of it. “It is better if the product has been actively growing for a while rather than if growth has just re-started.”

Clearly, while the wet leaf theory is unproven, it would also pay to be careful about spraying Atlantis onto wet leaves, particularly when there is no prospect of the product drying onto the leaf.

4) Use a partner

If you’re applying in the autumn then standard advice has always been to apply a residual partner, and there are even more good reasons to follow that advice this season.

Longer blackgrass dormancy, as predicted by seed tests, is likely to mean a more protracted emergence period and the use of a residual partner should help take some of the risk out of applying Atlantis a little too early.

But the Agrovista trials also point to the use of Hawk as a possible alternative. Combining residual activity from trifluralin plus extra contact help from clodinafop-propargyl, the product gave around 8% more control of blackgrass than the purely residual Firebird, when applied on a sub-optimum date for Atlantis, Agrovista technical director Craig Morgan says. “It looks like it could help remove some of that seasonal variability.”

There are certain keys to getting best performance from Hawk also, he stresses. The first is to know the resistance status of the blackgrass – there’s little point using it where fop target-site resistance is high. “You’re missing a trick if you don’t know your resistance status.”

Trials evidence also suggests it is vital to use the Hawk nozzle to apply Hawk. Control improved by 20% over a standard 200 litre/ha flat fan nozzle in application trials, Mr Morgan says. “It is an integral part of advice.”

But he wouldn’t necessarily use Hawk over a residual everywhere. “If it’s a nice open autumn, and it is the first or second week of November I’d use a residual. But once we get into days when it is a bit wetter, you’ve got dewy leaves then I think it would be foolish not to use some of this clodinafop activity.”

Atlantis consistency guide

Use a robust pre-emergence Apply in the autumn Use full dose Use correct adjuvant (Biopower at 1 litre/ha even at 100 litres/ha water volume) Use appropriate nozzles a) Hawk nozzles or those that give finer sprays b) Avoid air-inclusion or low droft nozzles Avoid spraying when weeds not actively growing a) e.g. in very cold or dry conditions b) But Atlantis better going into cold than coming out Use a residual herbicide Hawk might be a good partner provided: a) Little or no target-site fop blackgrass resistance b) You use Hawk nozzle c) And when conditions more marginal Consider avoiding spraying when unlikely to dry onto leaf  a) particularly if heavy rain forecast in next 24 hours

Why is Atlantis performance more variable?

Resistance build-up, cold or very dry conditions are all possible – and reasonably well-accepted – reasons for Atlantis variability. But none of them adequately explain why on certain application dates last season, and probably in previous seasons, Atlantis performance was poor, Jim Orson from The Arable Group says.

TAG’s first experience of unexplained poor efficacy from the product came two or three years ago in a trial in Suffolk, Mr Orson says. “It didn’t do very well, yet it wasn’t cold and growth was OK.”

Resistance could also be ruled out as applications on later dates worked well. “We couldn’t understand the result.”

Last season the organisation sprayed several of its blackgrass and sterile brome trials on 2 March. “It was a good spray day, but again we got very poor performance. In one brome trial, which was sprayed late in the day at 5.30pm, control from Atlantis was 0%.

“Again it was difficult to explain these failures, as it was warm and there was good growth.”

But a pan-European discussion on Atlantis variability between members of an informal weed scientist group, where French and Danish results suggested Atlantis doesn’t work as well in high humidity, has triggered a possible explanation regarding wet foliage, Mr Orson says.

On both occasions where TAG experienced poor control the foliage never completely dried, and then it rained heavily the next day. “It suggested that if Atlantis doesn’t dry onto a leaf then it may be susceptible to rain later than would be expected from its rainfastness.”

Commercial experience doesn’t necessarily back that argument, Gordon Anderson-Taylor of Bayer CropScience says. “A large proportion of Atlantis is going on in November, when let’s face it, it is not easy to get a completely dry leaf surface, and we haven’t had a rash of product performance issues.

“Performance issues are not related to a single factor, but a combination of factors. The theory is not unreasonable, but I think in commercial practice we would have identified more problems around the issue.”

Mr Orson admits it is only a theory. “We’re going to try to reproduce the conditions in trials this season to investigate further.”

But he suggests it may pay not to spray Atlantis this season onto wet foliage that has no prospect of drying, particularly if heavy rain is forecast for the next day. “As Atlantis’ performance declines then getting everything else right is going to be more and more important.”

Active Ingredients
Atlantis – mesosulfuron-methyl + iodosulfuron-methyl-sodium Firebird – flufenacet + diflufenican Hawk – clodinafop-propargyl + trifluralin Trooper – flufenacet + pendimethalin