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Crop Advice & Expertise

Variable black-grass control needs follow up treatment

Pre-emergence herbicide performance on earlier wheat crops drilled into dry soils has been variable this season, leaving some growers a tricky choice .

Pre-emergence herbicide performance on earlier wheat crops drilled into dry soils has been variable this season, leaving some growers a tricky choice between applying post-emergence herbicides or spraying off with Roundup (glyphosate).

Later drilled crops are likely to benefit from more moisture to help activate pre-emergence herbicide performance, but require growers to monitor going into winter to try to maximise weed control to take advantage.

Ryan Hudson, a Velcourt agronomist, tried to adapt his herbicide programmes to work around the lack of moisture in the soil, but has seen varying levels of success.

“Where we now have black-grass problems, in most cases we held off drilling into dry seedbeds and the plan was to go in with fairly robust stacks of residual herbicide including triallate, Liberator (flufenacet + diflufenican) and pendimethalin. However, due to the dry weather we generally broke those back down into just Liberator at pre-emergence with the view that we could follow it up with more residuals once we had rainfall.”
Where residuals were applied in good conditions there was good efficacy but due to the dry conditions some of the earlier pre-em applications have been more variable in performance: “I’ve had a few places where blackgrass has come through but then we’ve had a reasonable amount of rain which has activated the herbicide and washed it into the soil, so we’ve started to see the black-grass reddening and whitening where the flufenacet is taking effect.
“In the areas we applied reduced pre-ems, we have come back with additional flufenacet and a partner product to maintain residual activity against black-grass. Most crops will now have received a full dose of Liberator and a top up flufenacet plus partner product.”

But there are one or two places where a significant amount of black-grass has survived which may need a post-emergence contact herbicide as soon as possible.

“I’ve got a handful of situations where pre-ems haven’t worked so well due to timing or dry conditions and black-grass is coming through at 1 leaf. In this scenario we’d be looking at an Atlantis (mesosulfuron + iodosulfuron) and flufenacet follow up which will go on now while weather is mild and we can get a dry leaf.” Mr Hudson maintains that to avoid problems with resistance it is important to target the plant when it is small at 1 leaf, and more vulnerable to herbicide activity.
“In really bad situations, growers should consider the crop’s viability and seed return, possibly writing the crop off with glyphosate to control the black-grass. You’ve got to be realistic about the level of control you’re going to get, especially if the farm historically has resistant black-grass.”

Waiting to take action will mean weeds are harder to control later on in the season, so Mr Hudson advises acting now. “If farms have got a lot of black-grass coming through they can’t expect post-ems to control 100% of the population. Black-grass is a numbers game - we need 97% control to stand still and based on the high number of seeds produced by one black-grass plant it’s important to be realistic about what can be achieved and think long-term. Don’t just leave the decision until spring when you’ve got a field full of blackgrass and it’s too late to change plans and put a spring crop in.”
With variable rates in pre-em control from farm to farm, Darren Adkins, technical manager at Bayer believes those that held their nerve and drilled later in the month are likely to see better control as the season progresses.

“Those that drilled a bit too soon in early October and saw pre-ems go on in dry conditions are seeing a lot of black-grass around the same growth stage as the wheat. The people that waited until late October saw pre-ems go on with some moisture. It’s a bit early to tell, but I’m expecting them to do quite a lot better.”

Mr Adkins also thinks that a post-emergence treatment might be necessary this autumn. “If you’ve got small amounts of blackgrass, less than 10/m2, an application of Atlantis OD with some residual chemistry like half rate Liberator, or some pendimethalin as soon as the black-grass is at 1-2 leaf would be sensible. If there’s a lot of black-grass and you think the seedbed will still be okay you could spray it off and re-drill.”

For growers considering Atlantis, a new OD formulation is available for the first time this autumn. Atlantis OD is a new formulation which contains the same actives – mesosulfuron and iodosulfuron – as Atlantis WG but delivers them in a more effective oil dispersion (OD) formulation.

“In trials, it has been shown to have a clear advantage over the water dispersible granule formulation – Atlantis WG – for controlling black-grass,” says Bayer’s Development Manager Dr Gordon Anderson-Taylor.

Across several years’ trials, there is a difference of 6% in overall black-grass control between Atlantis OD and Atlantis WG. Although relatively small, any small advantage can count for a lot with black-grass control particularly when pre-em performance is not as reliable due to dry conditions.

“The post-em is the final opportunity to maximise the level of control for the season,” says Dr Anderson-Taylor.  “It’s not just about reducing plant numbers, it’s about reducing seed return for the following season.” And like any herbicide, application is crucial in ensuring effectiveness and longevity of the product.
“With post-ems, we know that there’s a degree of resistance out there, in particular advanced metabolic resistance. The bigger the plant is the bigger its capabilities are to break down herbicides which is why an autumn or winter application of Atlantis should be applied at one to three leaf stage of black-grass.

“This can be difficult because ideally you should apply to a dry leaf surface but when autumn comes along it brings frosty nights and morning dew. During clear days there will be dry leaves out there but probably only for the middle few hours of the day, so people have got to be prepared to catch that ideal spray window where conditions are good to travel.”

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