Impressive results from potato weed control trials
AS potato planting gathers pace, thoughts will soon turn to weed control ahead of crop emergence. Here, The SF looks to answer the question: what are the most eective herbicide options this spring?
SAC Consulting’s annual herbicide trials in 2023 took a slightly dierent approach to normal, by testing products in the absence of a crop as a simple weed screen, but as SAC potato specialist Kyran Maloney explains, the results still provide a valuable indication of how key products perform.
Praxim and metribuzin impress
Twelve treatments were under the spotlight last season, including both established and experimental chemistry chosen specifically for its residual activity, although actives with residual and contact activity were still included.
Randomised 2x2m block trials were set up on a Class 3 sandy loam near Blackburn, Aberdeenshire, previously in a grass ley.
Land was ploughed and cultivated at the end of May, with a three-week gap between the last cultivation and herbicide application, to represent farm situations more closely.
Results showed that once again, products based on metobromuron (Praxim), or metribuzin at a higher dose, proved to be the most
eective at controlling a broad range of weed species, averaging more than 90% control across all weeds.
Mayweed was the dominant species, followed by fat hen, common fumitory, and chickweed.
“It was very dry in Aberdeenshire last spring, which really limited weed growth, and meant we had to delay everything much later than in a real farm situation,” explains Dr Maloney.
“But 22 mm of rain fell between 18-21 June, which created ideal conditions for the expected weed flush, and for the activity of preemergence herbicides.”
At the time of application on June 22, the total weed population was 367 weeds/m2, with most species at the cotyledon stage,
although some weeds that survived cultivations were more advanced.
Assessments a month later showed total weed cover had reached 100% in the untreated plots by 21 July, whereas 90% control had been achieved where Praxim was applied at 3 litres/ ha. Control was slightly higher for metribuzin at 0.75 L/ha and flufenacet + metribuzin at 2.25 kg/ ha.
Product partners
Given its strong performance over multiple years, and costeectiveness, Dr Maloney says metribuzin forms the backbone of many pre-em strategies where there are no varietal restrictions, although should be partnered with an active like metobromuron or aclonifen to cover a rounded weed spectrum.
Where farmers are growing a variety sensitive to metribuzin, Praxim provides a good alternative, as it has no varietal or soil type restrictions, and is moderately soluble - depending on soil moisture - which helps eªcacy, he says.
This may be partnered with other chemistry, such as prosulfocarb, if required.
“The important thing is to assess your fields to fully understand the weed spectrum that needs controlling and then select the most suitable herbicide products.”
Certis Belchim’s James Cheesman agrees, adding that Praxim has shown consistent performance over multiple seasons and conditions.
The product is particularly strong against annual meadow grass, charlock, chickweed, fat hen, groundsel, mayweed, and shepherd’s purse.
A suitable mixer partner can bolster this control, and broaden the weed spectrum further, although choices depend on the target weed species and budget, he says.
“Metribuzin plus metobromuron will be the mainstay of many people’s programmes, but you might also look to metobromuron plus prosulfocarb to broaden control of species like fumitory and cleavers, where required. Praxim is very flexible when it comes to mix partners.”
Contact activity
While residual chemistry forms the backbone of control programmes, adding an active with contact activity may be beneficial where weeds have emerged, and will further minimise competition with the crop through to canopy closure, says Mr Cheesman.
A key option is Gozai (pyraflufen-ethyl), which oers stronger performance against a number of weed species than carfentrazone-ethyl, he says. Those weeds include chickweed, poppy, pansy, groundsel, and mayweed.
“It is vital that growers seize any opportunity to take out broadleaved weeds like these to reduce weed and seed pressure, as we are o®en selecting for species like groundsel and mayweed throughout the arable rotation due to the chemistry being used.
“For example, many winter wheat and veg growers use pendimethalin for grass and broadleaved weed control, but it does little to control groundsel.”
Gozai also oers activity on ALS-resistant broadleaved weeds, providing another potential crossrotational benefit.
For best results, Gozai should be applied before crops emerge, to actively growing weeds that are between the cotyledon and 2-4 expanded leaf stage.