Cultivations and rotations can offset increased blackgrass resistance to herbicides

As blackgrass becomes increasingly resistant to herbicides, Britain's cereal growers need to extend their armoury beyond chemical weapons, says a leading weed biologist.

Tests are currently underway on blackgrass samples collected from fields where comprehensive weed control programmes were used this year. The results will not be known for a while, but the fear is that they will reveal that blackgrass populations with both forms of herbicide resistance – enhanced metabolism and target site – are increasing. Last season alone, 24 sites were identified where Atlantis resistance was confirmed.

The prolific seeding of blackgrass means one or two resistant plants can quickly become a serious infestation.

"In a competitive wheat crop, each blackgrass plant produces around five ear-bearing tillers and each ear contains, on average, 100 seeds. So just three surviving plants in a square metre produce 1,500 seeds," says Dr Stephen Moss of Rothamsted Research.

"This productiveness explains why an average of 99.8% blackgrass control is needed, just to keep the weed infestation levels static."

Besides poor herbicide performance, minimal tillage on many farms adds to poorer control of resistant blackgrass from pre-emergent herbicides due to an increase in organic matter near the soil surface. Poor pre-em performance puts post-emergent products under more pressure with possibly greater selection for enhanced resistance.

Faced with these increasing pressures, growers must consider all the cultural and rotational options available. For those intent on winter cereals, the options include ploughing before early sowing or delaying planting and making use of glyphosate and stale seed bed techniques. For some spring cropping may be an option, otherwise opting for autumn sown break crops will allow alternative products that do not have resistance issues to be used. These include oilseed rape and field beans.

"In rape and beans products such as trifluralin and especially propyzamide offer the ability to achieve high control levels of even resistant strains of blackgrass," says Dr Moss.

Dow AgroSciences, which markets Treflan (trifluralin) and Kerb Flo (propyzamide), has experienced increased sales as farmers adopt this approach to overcome the increasing headache of resistant blackgrass. For the past five years, the company has been sponsoring trials, mainly at ADAS Boxworth to identify how to make the most of these products in the agrochemical armoury.

"All the trials data, as well as on-farm experience, points to a programmed approach beginning with minimum cultivations," says Dow AgroSciences' Rene Pollak. "In really heavy infestations, best results come from beginning the control programme with Treflan pre-emergence. Then Kerb Flo plus a graminicide will give close to 100% control even when blackgrass populations exceed 500/m2."

Soil moisture has proven critical to optimise Kerb's performance. Dow AgroSciences recommends farmers be patient in the autumn and wait until the top 5cm of the soil profile is moist.

"Each year, Dow AgroSciences invests heavily to develop an ever-improved understanding of how to make the most of Kerb which is an increasingly important weapon in the fight against the spread of herbicide resistant blackgrass," says Mr Pollak.