Potato weed control: changing of the guard – CPM – John Keer

The loss of diquat is causing an adjustment in strategy at the beginning of the season as well as at the end ...

CPM investigates the impact of potato herbicide strategies this season.

There’s been a changing of the guard among potato herbicides with the loss of some well-known active substances, leading growers to develop new programmes and strategies. Good control is still achievable, but achieving it will require a little more thought, according to John Keer, agronomist for Richard Austin Agriculture.

Much has been made of the loss of diquat, but growers are likely to miss it more for its performance as a herbicide, he believes.

“For most growers it was the preferred desiccant but now it’s gone, growers will have to choose between pyraflufen-ethyl and cartentrazone-ethyl. At the pre-emergence timing, the week spectrum to be controlled should be a consideration, but where possible I’ll be keeping Gozai (pyraflufen-ethyl) in reserve as a desiccant,” he says.

Scottish Agronomy’s Eric Anderson agrees, noting that label restrictions for pyraflufen-ethyl will likely be the consideration that influences which contact is applied at the start of the season.

“Carfentrazone-ethyl is registered separately as Shark for weed control uses, and Spotlight Plus as a desiccant, while Gozai has both uses on the same label. As a result, using Gozai as a herbicide (at 0.4 l/ha) will limit the amount of pyraflufen-ethyl available for desiccation if the maximum permitted dose in the season of 1.6 l/ha is to be observed.

“For crops that are expected to be vigorous at the point of burn down, it’s prudent to use Shark pre-em to maximise desiccation options,” he says.

Greater use of residual herbicides

While the loss of diquat is promoting growers to review contact herbicides, the bigger change will be to force growers towards greater use of residual herbicides at the early post-planting stage.

“The loss of diquat means earlier herbicide application and we’ll be advocating spraying at seven to 10 days after planting, once ridges are settled. Contact sprays, such as Gozai or Shark, are limited to no more than 10% emerged, any later and the growth will be seriously checked.

“In a later planting season, like this one, it’s growing time that growers can ill-afford to lose. It’s a complete change in strategy to the one we’re are used to because diquat could be applied much later, at up to 40% crop emergence,” says Eric.

With the dilemma of which product to use as a contact option relatively straight-forward to resolve, thoughts will turn to the residual programme.

“Residuals remain the best means of post-planting weed control and the most cost-effective. I tend to favour three-way mixes with Sencorex Flow (metribuzin) as the base treatment for its broad spectrum of activity. It also has better activity on weeds that escape either Gozai or Shark,” says John.

Those faced with cultivated land that has baked hard and cloddy in the quick-drying conditions this spring may find it more difficult to achieve a good tilth, in which case expectations of performance may need to be tempered, he says.

In these cases, John favours a three-way mix to bolster control. “On the silts, I opt for a rate of 0.4-0.7 l/ha of Sencorex Flow, depending on variety and soil texture, in combination with Emerger (aclonifen) and Defy (prosulfocarb).”

“Emerger isn’t a replacement for the higher rates of linuron (that we used to apply) but, of the recent introductions, it has the best spectrum of control and is more cost effective too,” he believes.

The new herbicide from Bayer is a residual with a different mode of action. As the weed emerges through the soil surface it picks up the active substance on its hypocotyl (in broadleaf weeds) or coleoptile (in grasses).

“Emerger’s best attribute is that take-up by the weed is less constrained by soil moisture than with other residuals. It’s also probably the best product for control of black-bindweed, although post-em treatments will still be required in several situations,” says John.

Another result in this change of approach is the extended period of control required because of earlier residual applications, and this supports the need for strong product combinations, adds Eric.

“The choice of partner products will depend on the weed spectrum present, but because applications are likely to be far earlier than most are used to, the weed picture is likely to be less than complete. This means growers will need detailed knowledge of the weeds present in their fields to inform product selection,” he comments.

While weed spectrum will influence programme design, it’s the variety selection that will limit product choice.

“Once annual meadowgrass (AMG) passes the two true-leaf stage, neither Gozai or Shark will touch it. Where AMG presents a strong challenge then I would advocate Artist (flufenacet+ metribuzin) which will also give good control of cleavers,” he says.

The varietal restrictions of metribuzin and the phytotoxic effects of clomazone are well understood, but there are also considerations with Defy (prosulfocarb) and Stomp Aqua/Anthem (pendimethalin) that will influence programmes, suggests Eric.

“Defy has a shorter half-life so should be applied in mixtures just before crop emergence, while both Defy and Stomp Aqua are seriously constrained in dry soils. Both have gaps and strengths in their weed spectrums. Where fumitory may have previously necessitated the use of Defy, a more effective alternative may be Emerger, which is less constrained by dry soils,” he says.

Mop up grassweeds before planting

In some fields where potatoes will be planted grassweeds, such as blackgrass, ryegrass and bromes, have all developed strongly where conditions prevented much, if any, winter or early spring control. The mild winter has enabled them to become firmly established and tillered. By mid-April they, and difficult broadleaf weeds, will be well into stem extension, making them challenging to control with even the best glyphosate formulations, says Bayer technical specialist, Roger Bradbury.

“Warmer soils will help delayed sugar beet and potato sowings get out of the ground and away more rapidly. But well-established weeds will also benefit and take even greater advantage of them. So late plantings need the cleanest possible start ahead of the in-crop herbicide sprays.”

To achieve the cleanest late-sown start to spring crops, while adding as little to workloads as possible, Roger recommends careful integration of cultivations with the most effective glyphosate treatment.

Specifically, he advises an early glyphosate spray as soon as ground conditions allow, hitting weeds before they get too far into stem extension. This should be followed by the most appropriate cultivation for the soils concerned once they’re fit, and a second glyphosate spray immediately ahead of sowing, if needed, to deal with newly-emerging weeds.

For sugar beet and potatoes, this follow-up spray can be replaced by an approved glyphosate as a pre-em for extra workload flexibility.

“It will be more important to use a modern glyphosate formulation under challenging conditions to deal with weeds already into stem extension,” he stresses. “Their superior rainfastness, from just one hour, and a cultivation interval of as little as six hours gives leeway to deal with uncertain weather and adds extra flexibility to fit in with very tight workloads.”

Using the right rate is also important, he says. While 720g/ha of glyphosate in a modern formulation is normally sufficient for well-tillered grassweeds, where weeds have gone into stem extension he recommends increasing this to 1080g/ha. Rates can be reduced to 540g/ha for the second application which will only be dealing with seedling weeds.

“Taking sufficient care in glyphosate spraying is equally essential,” he adds. “For the greatest efficiency, water rates as low as 100 l/ha will be fine. I’d also suggest a forward speed of no more than 12km/h, with a boom height of around 50cm and a medium to coarse spray for the best coverage.”

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