Sustaining winter cropping under threat from herbicide-resistant black-grass (Alopecurus myosuroides)

Abstract

Black-grass (Alopecurus myosuroides) is an increasing problem in UK arable cropping systems due to the dominance of autumn sown crops and the high frequency of herbicide resistance. More integrated weed control strategies are needed and the aim of this project was to better quantify the impact of three non-chemical methods for control of resistant black-grass: (1) time of autumn sowing of winter wheat; (2) impact of different crop seed rates; (3) impact of spring sown, in comparison with autumn sown wheat. These were investigated in a series of five field trials and associated modelling work both in the absence of herbicides and in combination with a robust herbicide programme involving both pre- and post-emergence herbicides typical of that used in wheat crops.

Delayed sowing of winter wheat by three weeks from mid/late-September to early/mid-October reduced black-grass infestations by 33% on average. In addition, heads and seeds plant^-1, were reduced by an average of 49%. The most important finding was the better black-grass control achieved by the pre-emergence herbicides (240 g flufenacet ha^-1 + 60 g diflufenican ha^-1 plus 1600 g prosulfocarb ha^-1) when applied in later-sown crops. This mean benefit, an additional 26% control (above the 47% achieved at the first sowing date), was substantial. Most of these benefits came from delaying sowing by about three weeks to early-mid October, with relatively smaller additional benefits from delaying sowing until late October/early November.

Increasing seed rate helped suppress black-grass by up to 28% However, it was clear that higher seed rates were less effective at reducing black-grass than delayed autumn sowing, which was a much more effective strategy overall due to a combination of reduced black-grass plant populations and better herbicide efficacy. Crops sown at low seed rates (175 seeds m^-2), or which established poorly, were more vulnerable to black-grass and it was concluded that avoiding low-density crops must be a key objective. Spring sown wheat appeared to be a good solution due to the substantial (92%) reduction in black-grass plants emerging compared with September-sown wheat and the lower seed production per plant. These factors more than compensated for the modest control (55%) achieved by pre-emergence herbicides (pendimethalin+prosulfocarb) in spring wheat.

These findings show that delayed autumn drilling and use of higher seed rates, in conjunction with a robust herbicide programme, can make a very useful contribution to reducing the threat from herbicide-resistant black-grass. However, these measures alone will not be sustainable in intensive arable rotations if resistance continues to increase, especially to the pre-emergence herbicides. The results also show that spring cropping has much potential but can be a difficult option, especially on the heavy soils which greatly favour black-grass. There are no easy options or ‘quick fixes’, but a return to more balanced rotations and a move away from ever-earlier drilling must be the longer-term goal – a reversal of the trends that have occurred over the last 40 years.