Published on 5th January 2024
Learning from Denmark’s success with spray timing tool
More than any other herbicide timing, the success of spring post-ems is dependent on conditions around the time of application. Following success with a decision support tool in Denmark, Bayer is investigating whether it could work here too. The Cossack Tool is nothing to do with horsemen from the steppe, Cossack is the local name for Atlantis OD in Denmark.
Denmark has a considerable area of cereals with 1.25 million hectares predominantly growing winter wheat and spring barley with average yields a little lower than the UK at 7–8 t/ha and 5–6 t/ha respectively. Denmark sits between 55° – 57° latitude, which is equivalent to the area between Newcastle and Aberdeen in the UK.
As it is surrounded by sea, it has a maritime climate which keeps it mild relative to the latitude, rain falls regularly during the year averaging 600-700mm annually. In short, it has a climate and growing conditions similar to the north-eastern United Kingdom.
“The three main problem grass-weeds in Denmark are Italian ryegrass, black-grass and vulpia (rat’s tail fescue) in that order,” says Soeren Lykkegaard Hansen, Bayer GoTo Market lead for Nordic Countries who also manages a small farm on a part-time basis.
The winter wheat drilling period runs from early September until mid-October. To control weeds there has been more use of stale seedbeds and delayed drilling in much the same way as the UK followed by a pre-em.
“A typical pre-em is prosulfocarb, diflufenican and aclonifen. We don’t have flufenacet and in general there is a more limited range of actives available in Denmark because of quite strict regulation. In spring, we follow up with Cossack or sometimes pyroxsulam, but remember pyroxsulam needs warmer weather so our Cossack tool isn’t suitable for all spring herbicides.”
Soeren Lykkegaard Hansen
Atlantis Star which contains thiencarbazone on top of mesosulfuron and iodosulfuron is not available in Denmark at present. Mesosulfuron resistance, particularly metabolic, is a problem in ryegrass and black-grass so correct application and timing are important to achieve sufficient control.
Denmark has a long and variable spring. Warmer weather can be followed by colder periods so the best time to apply Cossack can fall between late February until early April. To help farmers apply at the earliest possible opportunity, Bayer used temperature data to develop a tool which alerts farmers when growing conditions are right to spray.
“By applying early, you are effectively increasing the AI loading compared to the mass of plant meaning you are more likely to reach a lethal dose, even with a degree of metabolic resistance. Two weeks can make a lot of difference to weed development, so we don’t want farmers to miss spray opportunities.”
“In general, I think farmers and agronomists thought it needed to be a bit warmer to apply Cossack than the data we took from trials. This is understandable, because they want to make sure they get efficacy, so possibly add 1-2°C on to the label recommendations to be sure. This tool gives them reassurance that they can spray earlier but also prevents them applying when it is unlikely to work. Overall, it increases the chances of a good result.”
The model analyses temperature on the potential day of application, two days before and three after, suitable conditions are needed on all these days to ensure active weed growth. It presents the results using a traffic light system by postcode; dark green means good spraying conditions, light green - fair conditions and orange - unsuitable conditions.
For good conditions, average daily temperature must be above 4°C, for fair conditions slightly less. Very low temperatures stop all weed growth; If the minimum reaches -4°C or below then then spraying is not advised until sufficient warmer weather has elapsed to restart active growth.
Danish application Map 7/04/23
“As far as I understand, the team in the UK are calculating their own parameters in their trial version of the tool which may be slightly different. In our market research, over half of farmers who had tried it, described it as a high value tool. We can see from visits to the website that we get more views as the weather improves. Good spray windows can be short, it is pays to be ready to go at the first opportunity because as weeds grow, they get harder to control.”
Mr Hansen points out that farmers shouldn’t be concerned when the map is orange because weeds are not really growing. The worst situation is warmer weather coupled with wind and rain which can leave farmers powerless to intervene as was the case in March 2023 in some parts of the UK.
Berkshire agronomist Tony Bayliss was pleasantly surprised by post-em efficacy on black-grass last spring. But he emphasises that the right conditions are essential for it to be worth applying.
“It was a pattern of good control on several farms, even where there is some metabolic resistance. When planning last year, I questioned why we are spending on mesosulfuron + iodosulfuron products. But Atlantis Star, in particular, took me by surprise about how well it worked. The addition of thiencarbazone seemed to make a difference. In some cases, 80% control and the other 20% was struggling and remained stunted with small heads until harvest.”
Mr Bayliss encourages farmers to apply as early as possible and last spring conditions were perfect at the start of February. “The earlier you apply, the better. We applied some as early as 1 February on a clear, sunny day. I’m a big believer in high UV levels when you apply so the plant takes in a large dose rapidly. And, in February weeds are still small which also helps. If you have back-to-back bright days, then I think the efficacy goes up again.”
Applying early in the season, there are some potential compromises. Mr Bayliss suggests being pragmatic and that the leaf needn’t be totally dry at the time of application provided it is a sunny and drying. Clear skies can also mean low night-time temperatures which may deter farmers from applying. But for Mr Bayliss, unless there is a hard frost, he still recommends applying to take advantage of high UV.
“There are often days in February when it is sunny and reaches 8, 9 or 10°C, and they often come together. If the opportunity arises with this type of weather, get on and do it.”
He also noticed improved broad-leaf weed control where the post-em went on early but urges growers to be mindful about following crops, particularly oilseed rape.
This season, most crops were drilled and got a pre-em as planned but heavy rainfall from late October meant residual top-ups were missed in lots of places. As a result, many farmers will be considering whether a post-em is worth including this spring.
The field experience of February 2023 being a good one for mesosulfuron post-ems is confirmed by The Met Office Monthly Weather Report. You can read the report in full using the link but some of the most salient points are included below. Data for England is presented as this corresponds to most of the arable area likely to receive a February post-em.
Compared to 1991 – 2020 average
Series length (years)
Mean daily temperature
Mean minimum temperature
As the table shows, all the key measurements point to it being a good month for post-em use with sufficient warm weather to kick off active growth accompanied by many dry, sunny days to help application. Closer investigation of the data backs up Tony Bayliss’ observation of very successful applications earlier in the month.
The start of February was warmer with high pressure and light winds from the south and west. In hindsight, the first few days of February were practically ideal conditions for Atlantis Star applications, although the month as a whole was good. This is especially true in comparison to March which was the 3rd wettest on record in England making sprays impossible. This underlines the importance of seizing the first good opportunity to get on with a post-em, because you may not get a second chance.
We highly recommend:
A highly-effective herbicide for control of grass-weeds and broad-leaf weeds in winter wheat. Atlantis Star is a coformulation of three ALS-Inhibitors (HRAC Group 2) with foliar and some root activity