THE European Union recently announced the withdrawal of metribuzin, with a use-up date of November 24th, and whilst it’s uncertain whether British regulator HSE-CRD will follow suit, there is a strong chance the active could soon be lost in Britain.
One agronomist weighing up its loss in weed control programmes is Agrii’s Andrew Sprunt, who oversees more than 800ha of potatoes across the Scottish Borders, East Lothian and Fife.
Around 10% of this area is seed, with the remainder predominantly pre-pack, including whites like Maris Piper, salad varieties and a small area of processing for various end uses, including crisping.
Across his area, annual meadow grass is an ever-present threat and when considering broad-leaved weeds, targets like chickweed, mayweed, groundsel, black bindweed and knotgrass are of increasing concern.
Weather unpredictability has challenged his weed control programmes in recent seasons, with dry spells through May into June resulting in less-than-ideal conditions for residual herbicide efficacy.
He said some residual herbicides available to growers, including prosulfocarb, pendimethalin and clomazone, require soil moisture to work well. Metribuzin, because of its water solubility, helps prop up programmes in drier conditions and would be very much missed.
“I think a product like Praxim (metobromuron) would help fill that gap, should metribuzin go, as it also works well in dry conditions,” said Andrew. “I’ve already found it handy on light, sandy soils, or where the variety is sensitive to metribuzin, but going forward I see it having much more of a role in all programmes, particularly with its strengths on weeds like chickweed and mayweed.”
These species have confirmed resistance to ALS-inhibitor herbicides like sulfonylureas (SUs) in the UK, and others like groundsel have suspected insensitivity to the same mode of action. In Europe, populations of fat hen also have confirmed ALS resistance.
With post-emergence options in potatoes limited and many relying on SU herbicide rimsulfuron in products like Titus, using Praxim to control these species pre-emergence will benefit the potato crop and the wider rotation, Andrew said.
“Adding in a contact partner like Gozai (pyraflufen-ethyl) to a Praxim-based pre-emergence spray just before potatoes come through will help as well. It’s certainly stronger than Shark (carfentrazone-ethyl) on chickweed, mayweed and poppies,” he said.
Growers have often relied on metribuzin to control volunteer oilseed rape, which is a very competitive species in potatoes, but Andrew said Praxim offers decent control at the 3L rate.
Certis Belchim’s Western technical adviser Beth Jones-Davies says 3L/ha is where growers will see its broadest spectrum of control and when deciding on rates to use, it’s important to look at the conditions at the time of application (higher rates help in the dry) and the weed spectrum present.
“Where you have some of those difficult weeds, like volunteer oilseed rape, then increasing rates from 2.5L/ha to 3L/ha could make the difference,” she said, adding: “From your pre-emergence residual and contact spray, the aim is nice, clean ridges.”
As Praxim has no soil type or varietal restrictions, she’s found its flexibility helpful to growers in her area, who often rent multiple fields across a large geographical spread without detailed information on soil type variability.
“Where you have a mix of metribuzin-tolerant and sensitive varieties, majoring on Praxim will also help simplify things for sprayer operators and growers avoid any potential crop safety issues at emergence,” she said.
Ben Burgess, who advises growers from north Shropshire to south Herefordshire on soil types ranging from blow-away sands to peats and heavier clay loams, has been using Praxim on metribuzin-sensitive varieties like Innovator with good success, partnering it with a prosulfocarb product +/- a contact product like Kabuki (pyraflufen-ethyl) and said it also lends itself to his lighter sandy fields and when a pre-emergence spray is left as late as possible.
“Some of my growers are direct planting on some marginal potato land, so Praxim’s crop safety is also very useful where ridge structure is less than ideal. However, at the moment I tend to use Gribuzz (metribuzin) with Stallion Sync Tec (clomazone + pendimethalin) as it covers most bases,” Ben said.
Like Andrew, he can see this changing in the future, with more Praxim + prosulfocarb mixes applied, or potentially Praxim with Stallion Sync Tec where the field’s weed spectrum dictates.
“It’s competitively priced compared to Emerger (aclonifen), so if and when metribuzin does go, it’s going to be a much bigger part of potato herbicide programmes,” said Ben.
Andrew said although Praxim has a broad spectrum of control, there are small species gaps that tank-mix partners must fill. Whilst metribuzin is available, it remains a key component of his programmes alongside Praxim, with both actives strong on annual meadow grass. 2.5L/ha of Praxim with 0.5-0.7kg/ha of a metribuzin product like Gribuzz would be advised, potentially adding in clomazone if there is a major concern about cleavers.
However, where metribuzin can’t be used owing to soil type and/or variety, he favours partnering Praxim with prosulfocarb products like Defy or Wicket. In these non-metribuzin situations where cleavers and fumitory are present, he tends to use 3L/ha of each.
This is particularly important in seed crops, where growers cannot use post-emergence herbicides.
“You only get one shot at it, so stacking your rates up a wee bit higher will help. Even in ware crops, growers would like to avoid coming back in with Titus as it can cause some crop effect,” he said.
Photo: Weeds like chickweed are an increasing threat on Andrew’s potato farm.
