Dow AgroSciences are advising growers to avoid adding the residual herbicide carbetamide to propyzamide (Kerb Flo) in oilseed rape, as the same level of weed control can be achieved by using Kerb Flo alone or in sequence, or mix with a contact graminicide, without any increased risk to water.

Risk to dringing water

Andy Bailey of Dow AgroSciences explains that they are discouraging growers to tank mix Kerb Flo with carbetamide, as it could lead to an increased risk to drinking water. "We are very aware of the potential risk of residual herbicides getting into drinking water. If any of these herbicides, propyzamide or carbetamide, continue to be found in water, then we could easily see additional mandatory measures such as dose rate reductions or restricted application timings being enforced or, the worse case scenario, a complete ban. Without herbicides such as propyzamide, the active ingredient in Kerb Flo, growing a profitable oilseed rape or, for that matter, cereals would be very difficult, if not impossible."

He points out that by doubling up on the residual components in a tank mix, you are, in effect, potentially escalating this risk. "To be honest we have a wealth of trials data to show that overall you are not getting improved weed control either."

Andy supports this view by reporting on black-grass trials in which Kerb Flo and carbetamide were applied alone and also in mix at different dose rates of between 25% to 100% of the label recommendations. "The first conclusion from these trials is that Kerb Flo always outperformed carbetamide at any dose. The full dose rate of Kerb Flo at 2.1 l/ha gave around 85% control of black-grass, whereas a full dose rate of carbetamide of 3.5 kg/ha gave 60% control. The second conclusion was that a tank mix of Kerb Flo with the contact herbicide Laser (cycloxydim) gave the best control of over 92%, similar to the mix of the two residuals at full doses of each. In another set of trials a sequence of Aramo (tepraloxydim) and Kerb Flo gave 97% control of black-grass. Here the contact herbicide provided the “pre-treatment” and enhanced the overall level of weed control."

Andy points out that by reducing the rate of Kerb Flo and augmenting it with a reduced rate of carbetamide, the activity of Kerb is compromised and a bigger range of variability introduced.

"I would argue that if you can achieve the same result, but with much less residual, it makes sense to go down that route and so reduce any risk of water contamination. Maintaining careful Stewardship of residual herbicides in rape is vital to ensure long term availability of these herbicides and every manufacturer, agronomist and grower should be taking their responsibility seriously."

Dow AgroSciences takes its stewardship responsibilities very seriously and invests significantly in maintaining important chemistries for the UK farmer, in light of increasing legislative controls. "For example, wide ranging research work on how run-off occurs on treated land and what happens under different cultivation regimes indicates that min-till reduces the level of propyzamide found in water courses. A second investigation conducted at Allerton showed that 20% of propyzamide was lost via run-off, which can be reduced by 50-58% by buffer strips, whilst 80% came through the drains. For this reason we would advise establishing a grass margin of at least 6 metres between field and watercourse, or if that was not possible, switch off the outer section of spray boom when passing near a watercourse. This is just an example of the investment needed to protect propyzamide for the future."

How to optimise Kerb control

Andy advises growers to focus on getting the best out of Kerb Flo by timing its application right. "To optimise Kerb’s performance, there are three areas to consider – temperature, soil moisture and weed seed depth. Soil temperatures need to be low - around 10ºC and falling, so that longer persistence is achieved throughout the germination period. Good levels of soil moisture are needed to help the herbicide distribute evenly in the top few cms of the soil and soil moisture deficit should be less than 50mm and falling. A rough guide to the soil moisture is 80% field capacity, before drains are flowing; soils must not be waterlogged or saturated. Care must be taken to avoid any risk of contamination to water and importantly all aspects of good Stewardship applied. With regard to weed seed depth, seed should not come from any deeper than 5cms as the black-grass would then be germinating below the zone where the Kerb Flo concentrations are high enough to give good levels of kill. If application conditions are right, farmers can expect percent of black-grass control in the high 90’s, a level that very few herbicides can achieve."