Need for Residual Herbicides in cereals and rape

This autumn residual herbicides should be top of the arable advisors' list of products, according to Interfarm UK Ltd. There are three good reasons for this :- long grass-weed germination expected, moist soils and the continued need for resistance management.

Black-grass seeds shed this summer are believed to have a high level of dormancy. Recent information from the Sustainable Arable Link Project, published by ADAS, suggests that the average germination of black-grass seed immediately after harvest was lower than normal. Dormancy will decline over the autumn which means that black-grass will germinate over a longer period of time rather than the usual sudden flush. Residual products are needed to cover the longer window of weed germination that we are anticipating, explains Dr David Stormonth, Technical Manager for Interfarm UK Ltd.

Secondly, soils contain more moisture this autumn than for some years, due to the very wet summer. This will enable good crop establishment and high herbicide activity. The condition of the top surface layer of soil may vary locally but moisture levels from sub-surface layers will find their way to aid the crop and weed growth. Active weed root systems in moist soils mean good uptake of residual herbicides.

Lastly, residual herbicides have different and generally more complex modes of action on weeds compared with herbicides which operate mainly via leaf uptake. This gives growers the opportunity to use highly effective weed control programmes, especially if they are dealing with resistance issues in both grasses and broad-leaved weeds, which are now commonplace throughout in the UK.

"We have become used to choosing herbicides for autumn-sown crops by their mode of action, by their resistance risk, by the growth stage of the target weed and crop, by tank mixability with other products and by sequencing with other products. The residual nature of a product may be part of the overall selection process, but it is not always at the top of the list," says Dr. Stormonth.


Good examples of residual herbicides are Treflan (trifluralin), Plinth (pendimethalin) and Flomide or Propose (propyzamide). Treflan is an ideal low-cost starter for both cereals and oilseed rape, contributing useful activity on grass-weeds, including black-grass, and also broad-leaved weeds, and enabling growers to have some control of their weed problems right from the start of crop establishment. Treflan, the original trifluralin product, has the most developed label compared with some later imitators, giving growers the opportunity to use Treflan in many of their cropping herbicide programmes. Plinth, a long-lasting residual non-leaching dinitroaniline group compound, is well known for its solid performance in cereal black-grass herbicide programmes. Both Treflan and Plinth are essential components to consider when using more recent herbicide introductions especially from the sulphonyl urea group. They can be used alone or in tank mix with other herbicides.

Propyzamide (Flomide and Propose) has established itself as the standard residual for oilseed rape against which all other rape herbicides are measured. Propyzamide will effectively control all strains of black-grass as well as a wide range of other grass-weeds such as wild-oats, meadow-grass and rye-grass. It does not easily leach, unlike some residuals that could put crop safety at risk. It should be applied to moist soil as early in the crop's life as the label allows (crop three-leaf stage).

With no known resistance occurring, propyzamide products have enabled growers to keep winter oilseed rape in the rotation, giving an autumn-sown break crop opportunity for black-grass control, without having to resort to less profitable spring cropping, adds Dr Stormonth.

"Autumn 2004 looks like giving growers and advisors alike the opportunity to use some of the most powerful and cost-effective residual products to their best effect. Interfarm has recognised the importance of these residual herbicides for the UK grower this autumn. Treflan, Plinth and Flomide/Propose are available from their selected distributor outlets."