Trials show options for carrot weed control

The loss of herbicide products, and in particular the loss of linuron on certain crops, continues to present challenges for vegetable growers, and this is one reason that AHDB Horticulture’s SCEPTREplus programme continues to investigate possible actives for use on crops including carrots and parsnips.

In early July growers from across the UK had the chance to visit the Yorkshire trials site, the third year that such trials have been conducted, which will also host the BCGA Trials Day in October, located in a commercial field just a stone’s throw from Drax power station courtesy of M H Poskitt.

“With the loss of linuron, the future of weed control looked uncertain. However, the trials here are already looking very promising and show that there are alternative options, which may be available soon,” said Joe Martin, AHDB Crop Protection Senior Scientist. “We are generating valuable data to aid decisions on taking products forward for approval for use on farms, developing knowledge that can be applied to keep the horticulture industry productive in the future. AHDB will continue to be at the forefront of investigating alternatives in the face of further changes to regulation.”

A panel of industry experts identified that controlling groundsel, mayweed, fools parsley, bindweed, cranesbill and polygonums should be priorities, alongside invasive weeds including fat-hen, fiddleneck, and knotgrass, and as well as the trials in Yorkshire, a second set of trials is being carried out in Norfolk.

Demonstrating a range of both pre- and post-emergence treatment plots, ADAS Plant Pathology Consultant Emily Lawrence, who is leading the trials this year, said; “In total, we’re working with a number of products which are being tested at different application timings, either alone or in tank mixes with currently approved products.

“The trials have built upon previous years’ work and there are four being carried out on carrots and five on parsnips in 2019. At this stage, we’re seeing promising results with a post-emergence application of aclonifen, which we are working on to get approval. However, I wouldn’t say it’s at a point where it’s a direct replacement for linuron.”

Recently approved for use on potatoes, Emerger also holds off-label authorisation for a pre-emergence use on carrot, root parsnip, root parsley, and other crops, which AHDB gained with working with manufacturer and growers. In the trials a pre-emergence application of Anthem (pendimethalin) at 3.3 l/ha on 12 May, followed by three 0.5 l/ha applications of Emerger (applied at 2-3 true leaves on 20 June, 4-5 true leaves on 2 July, and the third planned 7-10 days later) was the most promising of the ‘early post-emergence’ programmes.

Looking at the plots which were only given a pre-emergence treatment; a combination of 3.3 l/ha of Anthem, 1.75 l/ha of Emerger and 0.25 l/ha of Gamit 36 SC, had not performed as well as expected given its performance elsewhere, but still gave some useful weed control. A pre-em treatment which replaced the Emerger with 0.1 l/ha of Flexidor (coded as AHDB9996) had also not performed well at this site.

Where blackgrass is an issue, Anthem + AHDB9917 could be of interest. A number of the pre-emergence only treatments showed some yellowing of the crop, which ADAS Field Vegetable Research Consultant Angela Huckle said was probably due to the Gamit as the clomazone can be washed down into the soil profile, something which had been seen in previous year’s trials. “It is something to watch out for,” she warned.

Two other combinations of pre- and post-em treatments which looked promising were 3.3 l/ha of Anthem followed by three applications of beet herbicides, coded AHDB9861 and AHDB9898, although both appeared to have checked crop growth slightly. Also of interest was a maize herbicide (coded ADHB9975) which was applied as three post-emergence sprays following the ‘standard’ trial pre-em of 3.3 l/ha of Anthem.

A second herbicide trial compared later three spray treatment programmes to the current ‘grower standard’ of 3.3 l/ha Anthem followed by two applications of Sencorex Flow at 0.3 l/ha. These included two coded products (AHDB9981, and AHDB9993,). “9981 is a contact herbicide which currently has approval on brassicas and onion crops,” explained Emily. “It has produced nice clean plots although the crop has slightly pale foliage and may be a bit smaller than some of the [other plots], but the product has very good efficacy.”

Adding additional chemistry (in the form of 0.5 l/ha of Emerger, 0.05 l/ha of Gamit 36 CS, or 0.05 l/ha of Hurricane SC) to the Sencorex Flow appeared to increase the phytotoxic effects on the crop in all cases, with little improvement in weed control provide by the addition of the Emerger.

Bayer trial plots

As well as the AHDB SCEPTREplus trials, Bayer had their own set of herbicide trial plots, as well as plots looking at the combined effects of Velum Prime nematicide and biofungicide Serenade ASO. “The aim of these trials is to see if there is a benefit from applying both products at the same time,” explained Bayer’s Tim Lacey. “The crops will be grown through and we will compare the commercial yields of each treatment.”

In terms of the herbicide combinations, a single pre-emergence treatment of Emerger at 1.75 l/ha had “done a fantastic job,” he said, pointing out that only a little black nightshade remained in the plot. Comparing the former ‘standard’ pre-emergence application of 1.2 l/ha Datura (linuron) + 2.5 l/ha Stomp Aqua + 0.2 l/ha Gamit 36 CS with one which replaced the linuron with 1.75 l/ha of Emerger had also produced good results.

Following with a post-emergence treatment of 0.15 l/ha Sencorex F + 0.1 l/ha Gamit 36 SC, or Sencorex + 1.0 l/ha Stomp Aqua both gave good weed control, while more ‘belt and braces’ treatments improved weed control but also increased damage to the crop. “We haven’t finished learning the best way to use Emerger yet,” stressed Tim. “Hopefully we can pull our experiences and those of growers and agronomists together, but the key message is that the product is available and it works.”

Sowing depth trial

Alongside the chemical treatments, Shaun Coleman of NIAB demonstrated a trial being carried out as part of AHDB’s Strategic Farm Project to investigate the effects of seed size and sowing depth. Nairobi had been established in nine plots with three combinations of seed size and three different drilling depths.

“The plots were sown in good conditions and it rained immediately afterwards,” commented Shaun, pointing out that this might have an influence on the shallower plots in terms of water availability, although the shallow plots appeared to be the worst in terms of crop establishment based on a visual assessment. “The deep drilling looks good, but it did take longer to come through,” added Shaun.

“The larger seed performed the best across all three sowing depths.” The crops will be grown through to harvest when yield and root count assessments will be conducted. Shaun added that funding has been secured for further trials next year, adding that any growers who would like to see specific combinations or assessments should make these known as soon as possible.