Residual herbicide stacking – the use of more than one active ingredient in tank mixes or close sequence against one primary target weed – is the new buzzword for blackgrass control programmes in wheat this autumn.
It’s not a new concept – many growers were using combinations of flufenacet, pendimethalin and trifluralin, for example, while the latter was legal to use.
But its use in blackgrass control programmes has been brought into sharper focus for this season by Atlantis’s poorest overall performance in any season. That was undoubtedly due to a tricky season characterised by slow weed growth and a wet late autumn, which prevented most growers from applying it before Christmas, and then a very cold, dry spring that meant little active growth before April.
But the rise in blackgrass resistance to Atlantis, particularly enhanced metabolism, was another contributory factor. In Syngenta testing of blackgrass that had survived herbicide treatment, the proportion showing resistance to sulfonylureas has increased from just 16% in 2004 to 95% in 2009.
Rothamsted Research tests have confirmed resistance in over 300 fields in 23 counties. It is not a localised problem.
With that background it is not surprising growers and their agronomists are looking to reduce the reliance on post-emergence Atlantis.
Cultural controls, such as the use of higher seed rates, competitive varieties and delayed drilling on the worst fields, should be considered strongly, but once planted, using combinations of pre-emergence or early post-emergence residual herbicides is the only solution to cutting populations before Atlantis applications.
The starting point for most stacks is likely to be flufenacet treatment. Both Liberator and Crystal, formulated with an additional active ingredient, have proved to be the best of the residual herbicides against blackgrass.
But what to add to increase control. There are plenty of options, including tri-allate, prosulfocarb, diflufenican and pendimethalin, either individually or in combination. And then there is the thorny question of what rates of each.
Avadex might be one popular choice this season, Jim Orson of NIAB TAG believes. “In many trials this year it was the star product.”
That was partly because of the dry autumn reducing the performance of other residual herbicides more than it did of Avadex, which, as a granule, is less effected by volatilisation, he explains.
But the granule formulation does bring some disadvantages. “You cannot tank-mix it with anything, and it needs a specialist applicator, so is a separate pass.”
Even so demand is high, despite a rising price, he reckons.
In Agrovista trials, the product also performed well, the firm’s Mark Hemmant says. “It did a cracking job in our trials when applied at peri-emergence.”
That was usually following a pre-emergence treatment based around a flufenacet product, and followed up by a non-Atlantis treatment in an attempt to investigate whether the product could be avoided.
A key finding in the trial was that the pre- followed by peri-emergence approach could build up useful control, but the follow up treatment had to be timed carefully if the benefits weren’t to be lost, he says.
“Where we delayed our follow up treatment of Cinder + Auxilliary + Galion into the spring control was much poorer than from a late November timing.”
Most blackgrass has some degree of enhanced metabolism resistance, which builds up as it grows, so delaying treatment lessens the chances of it being effective, he explains. “But we also lost the sensitisation effect of Avadex.”
An alternative to Avadex in the peri-emergence slot could be another shot of flufenacet, he adds. Combined with a full rate of Crystal or Liberator at pre-emergence, it allows growers to apply an extra half dose of flufenacet to take the amount applied to 360g/ha.
“By putting a second application on you’re increasing the persistence from the flufenacet. Initial applications are usually made when the soil is quite warm, so will break down faster.”
In trials the combination had not worked quite as well as Avadex, Mr Hemmant says. “Neither product are very exciting individually but they are useful in a programme.”
There is a danger of relying too much on flufenacet, Mr Orson warns. “We’ve seen before if you use one product regularly you can build up product specific enhanced metabolism resistance.
“And we do feel from our trials, and from some in Europe, that flufenacet could be slipping. So loading up with 360g/ha flufenacet over two applications might only be a short-term fix, and growers probably need to look at the use of a broader range of active ingredients.”
A third option is to tank mix either flufenacet product with Defy. In Syngenta trials that has added around 10% extra blackgrass control, the firm’s Iain Hamilton says.
The trials have also tried to examine what rates should be used, and come to the conclusion that in 15 trials using either 0.4 litres/ha Liberator + 4.0 litres/ha Defy or 0.6 litres/ha Liberator + 2.0 litres/ha Defy were broadly similar in control. “There was about a 3% advantage for the 0.4 + 4.0 mix,” Mr Hamilton notes.
In particularly bad blackgrass situations growers could consider full rates of either Liberator or Crystal with Defy, he suggests. Those treatments had typically added another 10% to control on top of the reduced rate options in three AICC trials.
Full rates may be necessary in some situations, Mr Orson agrees. “The easy days are over. But cost is an issue. You’re asking growers to spend maybe £70-80/ha by peri-emergence when he is uncertain about what level of blackgrass will come up.”
Another problem could be crop safety. “Full rates of flufenacet plus Defy need to be pre-emergence, not peri-emergence. We have seen some singeing of the crop at peri-emergence.
“I would also never put on high rates on a wet soil, when it is due to rain again. The herbicide doesn’t seem to lock on to the soil very well in that situation, and can be washed through [causing crop damage].”
The final two options to consider are pendimethalin and diflufenican. Pendimethalin is, of course in Crystal, while DFF is in Liberator, but there is no reason not to use DFF with Crystal and pendimethalin in Liberator programmes.
“Typically there is a benefit of adding pendimethalin to flufenacet even when there are high levels of enhanced metabolism resistance,” Mr Hemmant says. “And DFF in mix does contribute a bit, which is worth having. There is a benefit in using all three actives.”
Broadleaved weed control
An additional benefit of using Defy in a pre-emergence tank-mix is its effect on broad-leaved weeds, AICC agronomist Jonathan Olver says.
Cleavers, in particular, are a key weed for Mr Olver in cereals. “I have zero tolerance for cleavers in cereals as they are easy to control in the crop.”
And he found from using a mix of 0.6 litres/ha Liberator + 2.0 litres/ha Defy that, where cleavers populations were low to moderate, he could avoid having to clear up cleavers in the spring. “Where there was a higher population I did have to come back, but in around 50% of fields I had used the mix I didn’t need to.”
That bonus helps justify the extra cost of the mix, he notes.
He’s also interested in Defy’s activity on cranesbill and brome. In a Lincolnshire trial, there was a clear dose response with Defy in mix with either Liberator or Crystal, mr Hamilton notes. “It is a weed that seems to be increasing.”
The product also has a useful benefit against meadow brome, according to the results of a NIAB TAG trial in Bedfordshire. Full rates of both Defy and Liberator followed up by chlorotoluron + pendimethalin gave 90% control of meadow brome compared with 45% without the Defy.
“Brome is difficult with the restrictions on using more than one sulfonylureas herbicide, so anything you can do to reduce the problem is important,” Mr Hamilton points out.
Those results have helped persuade Mr Olver to switch to higher rate Defy in his mix with Liberator where he has both brome and blackgrass.
Atlantis – mesosulfuron + iodosulfuron
Auxilliary – prosulfocarb + fenoxaprop-ethyl
Avadex – tri-allate
Crystal – flufenacet + pendimethalin
Defy – prosulfocarb
Hurricane – diflufenican
Liberator – flufenacet + diflufenican
Stomp/Cinder – pendimethalin
Reduce reliance on Atlantis
Use several actives against blackgrass
Base on flufenacet
Avadex/Defy/additional flufenacet good options