Plotting a new course for weed control in carrots

“Following the EU’s decision not to renew the approval of metribuzin, it is looking possible that the UK will follow suit in the next couple of years, with carrot growers already under pressure to remove it from their herbicide programmes for next season,” explains ProCam agronomist and vegetable technical specialist, George Dewhurst. “If this happens, post-emergence control of broad-leaved weeds in carrots could be reliant on just two actives – aclonifen and prosulfocarb – plus a potential third if an extension of authorisation for minor use is approved.

“With such a limited choice of actives to choose from, and little in the way of independent data to indicate how far inclusion rates and timings can be pushed before crop safety is compromised, ProCam is carrying out its own research to understand how to maximise weed control without impacting crop yield.”

A total of 50 herbicide treatments and combinations were tested by ProCam at Cawood in Yorkshire in 2024, with a continuation of last year’s trial planned for the coming season when the focus will be on weed control without metribuzin.

“The trial has been designed to push inclusion rates to levels that growers and agronomists wouldn’t typically be comfortable using in the field, with the aim of being able to make better recommendations to protect commercial crops in the future,” George adds.

Aclonifen and prosulfocarb were tested at a range of inclusion rates at the first true leaf (1TL) and second true leaf (2TL) timings to target key weeds including mayweed, small nettle, fat hen, speedwell, and groundsel.

“The very strongest treatments, which included a three-way tank mix of aclonifen, prosulfocarb and metribuzin at 1TL followed by aclonifen and metribuzin at 2TL, provided good weed control but at the expense of some noticeable crop thinning and bleaching,” George describes. “Despite this, cropping weight remained higher than average with the three-way mixture at 1TL producing an additional 3.5 to 9.5t/ha compared with a tank mixture without metribuzin.”

In treatments without metribuzin, an amount of crop scorching was still recorded, but crucially, weed counts were reduced and the average cropping weight was still elevated.

“This clearly indicates that it is feasible to apply high doses of aclonifen and prosulfocarb at the 1TL timing without yields being impacted,” George continues. “In fact, weed control was most effective when a full rate of Emerger (600 g/l aclonifen) plus a litre of Defy (800 g/l prosulfocarb) were applied at the 1TL timing, with weed control at eight weeks post-application reaching 95%, some 9% higher than the next best treatment which was Emerger applied on its own at 1TL.”

A downside to applying the full rate of aclonifen and prosulfocarb at 1TL is that it makes them unavailable to control subsequent flushes of weeds later in the growing season, points out George.

“We have therefore extended the trial for another year to further our understanding of how to control late flushes of weeds without metribuzin,” George says. “We’ll be testing an array of inclusion rates and application timings to see if we can uncover an alternative approach to weed control using aclonifen and prosulfocarb, as well as some potential pipeline actives that we think could be of value if approval was to be granted. In doing so, we hope to be in a better position to offer useful advice to growers so that they can get the most out of all available herbicide options as soon as possible.”