A cautionary tale of broad leaved weed resistance

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Most UK cereal growers with blackgrass know only too well that herbicide resistance in this weed is now widespread and impinges significantly on how this weed can be controlled. In the UK broad-leaved weed resistance is generally regarded as an insignificant problem, with the exception of some Scottish growers who have been using SU’s in spring and winter cereals for many years.

“But in Denmark 15% of chickweed samples have been found to be resistant to sulphonyl ureas, 5% of poppies and 1% of mayweeds. Shepherd’s purse has also been found to be resistant to SU in sugar beet in 2012. Resistant chickweed was first identified in Denmark in 1991 in a farming system based on a monoculture of spring barley with reduced tillage and treated with SU every year. Weed expert Solvejg Kopp Mathiassen of the Department of Agroecology Aarhus University in Denmark points out that one of the reasons for the increasing problems with ALS herbicide (SU) resistance in Norway and Denmark is the pesticide tax on this group of herbicides is low as they are considered to be of low environment impact. Consequently the use of these herbicides is widespread and this has put heavy selection pressure on weeds,” suggests Dick Dyason, technical manager for UK and Ireland for Nufarm.

An effective weed resistance strategy is to minimise the use of the offending herbicide group or mix it with another herbicide with a different mode of action. For example mix the standard SU with a phenoxy herbicide such as Mircam Plus (MCPA, mecoprop-p and dicamba), High Load Mircam (dicamba + mecoprop-p) or Duplosan (mecoprop-p) or one containing bromoxynil such as Maya.

Standard phrases now appear on Scandinavian herbicides; “Repeated use of the same product or different products with the same mode of action might result in development of resistance. In order to avoid resistance, it is recommended to alternate with products with other modes of action or to use tank mixes of products with different modes of action but both with activity on the target. “

Dick Dyason explains that if UK growers continue to rely of SU’s alone, then the situation for broad-leaved weed resistance could become similar to that in Scandinavia. “15% may not appear to be that high, but it is 1 out of every 6 fields.”

In the UK, the main resistant weeds are poppies, in the eastern counties, the Midlands, Lincolnshire and Norfolk and chickweed in Scotland and Northern Ireland. There are over 40 farms in 13 counties with resistant chickweed. There are over 25 farms in 9 counties with resistant poppies and there are 5 cases of resistant mayweeds in 3 counties. Many more weed species could soon develop resistance to ALS inhibitors unless care is taken to manage the problem from now on. “Phenoxy herbicides are less likely to develop resistance to such an extent due to their multi side activity,” points out Dick.

Phenoxy herbicides are still widely used in spring crops. Nufarm are experts in phenoxy technology with key brands such as Compitox Plus (mecoprop-p), Depitox (2,4-D), Duplosan (mecoprop-p), Easel (MCPA), Mircam Plus (MCPA, mecoprop-P and dicamba). High Load Mircam (dicamba + mecoprop-p) and many more all, for broad-leaved weed control in cereals.

Maya is a specially formulated bromoxynil herbicide making it an ideal mixer herbicide, with hormones and with SU’s, adding in fat hen, knotgrass, orache and bindweed as well as the more specialist cleaver herbicides such as fluroxypyr and florasulam+fluroxypyr. Its use will help in the management of the increasing problem of broad-leaved weed resistance. It is very crop safe and fast acting.

Maya slots into any broad-leaved weed programme simply and in a straightforward fashion. It has a wide window of application – in cereals from 2 leaves up to second node detectable, so there are plenty of spray opportunities. It is most effective on seedling weeds. It can be applied in 200 litres of water but this can be reduced to 100 litres/hectare for the lower dose rates.


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