Herbicide resistant weeds:the current situation in the UK

Resistant weeds, especially black-grass (Alopecurus myosuroides), are widespread in the UK. Farmers need to reduce their dependency on herbicides and use more non-chemical methods of weed control.

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UK arable agriculture – the background to the resistance problem

Herbicide resistance is a direct consequence of the cropping and weed control system used. Of the 4.7 million ha of arable crops grown in 2016, 39% was wheat, 24% barley, 12% oilseed rape and approximately 3% each for legumes, potatoes and sugar beet. Cereals are the dominant crop and about 66% of all crops are sown in autumn. Wheat yields are high, averaging 8.6 t/ha in 2014. Winter wheat is now sown earlier - in 1972, only 5% of wheat crops were sown in September but now this has increased to 40 - 50%.

Pesticide use is high; the average winter wheat crop is treated 2.5 times with herbicides (3.5 products and 5.7 active ingredients). Fungicides are applied 3.4 times and crops receive about 200 N/ha. Grass-weed herbicide use in arable crops accounted for 9.9 million spray ha in 2012 (Table 1).

Table 1. Grass-weed herbicide use in arable crops in the UK (FERA, 2013)

Herbicide class	common names	a.i. example	% of total spray ha
ALS inhibitors	e.g. sulfonylureas	mesosulfuron	28%
ACCase inhibitors	‘fops’ ‘dims’ ‘dens’	pinoxaden	17%
Oxyacetamides	-	flufenacet	17%
Dinitroaniline	-	pendimethalin	15%
Others include metazachlor, prosulfocarb, propyzamide, chlorotoluron, triallate, carbetamide, flurtamone			23%

Earlier sowing of winter cereals has favoured autumn emerging annual grass weeds such as black-grass and Italian rye-grass (Lolium multiflorums) and farmers have become increasingly dependent on the high resistance risk ALS and ACCase inhibitors (Table 1). The inevitable consequence of this over dependence on herbicides has been the evolution of herbicide-resistant weed populations (Hull et al., 2014).

Occurrence of resistant weeds in UK arable crops

Black-grass is the most important herbicide-resistant weed in the UK. Some level of resistance occurs on most of the 20,000 farms where herbicides are used routinely for its control. ACCase inhibiting herbicides no longer give reliable control due to widespread target site (mainly 1781 mutation) and enhanced metabolic resistance. Mesosulfuron+iodosulfuron (‘Atlantis’) was introduced in 2003 but its performance is declining due to both target site (197 and 574 mutations) and enhanced metabolic resistance. Resistance also occurs to all the pre-emergence herbicides available in the UK (flufenacet, pendimethalin, prosulfocarb, flupyrsulfuron, diflufenican, triallate, chlorotoluron), although resistance tends to be partial and increases slowly. Declining performance of post-emergence herbicides has resulted in greater use of pre-emergence herbicides with many farmers using mixtures (‘stacks’) of 3 – 5 different active ingredients. High efficacy is dependent on adequate soil moisture, good seedbeds and favourable weather conditions, and increasing reliance on pre-emergence herbicides is not likely to be a sustainable strategy long-term.

Italian rye-grass and Avena spp. (wild-oats) are two other grass-weeds in which resistance occurs widely. ACCase and ALS target site resistance is much less common than in black-grass although enhanced metabolic resistance occurs. The frequency of different ACCase target site mutations differs markedly between species. For example, ‘fop’ specific ACCase mutations are more common in wild-oats with the consequence that ‘non-fop’ herbicides (e.g. pinoxaden, cycloxydim) often remain fully effective. It is important not to make generalised statements about the best herbicide strategy to adopt based on resistance characterisation of a single species. Resistance in both weeds is likely to increase in future.

Chickweed (Stellaria media), common poppy (Papaver rhoeas) and scentless mayweed (Tripleurospermum inodorum) are three broad-leaved species in which resistance to ALS inhibitors, especially sulfonylureas, occurs. ALS target site resistance is the main mechanism, and alternative herbicides (e.g. fluroxypyr on chickweed, pendimethalin on poppy and ioxynil/bromoxynil on mayweed) remain effective. Loss of such herbicides due to EU regulatory action would greatly increase the threat posed by resistant broad-leaved weeds.

Non-chemical control methods

Farmers need to adopt more non-chemical control measures against to compensate for declining herbicide performance as a consequence of increasing resistance. The potential of such methods for black-grass control is summarised in table 2.

Table 2. The efficacy of non-chemical methods of control of black-grass based on a review of over 50 field experiments (Lutman, Moss et al., 2013)

	% control achieved
Method	Mean	Range	Comments
Ploughing	69%	-82% to 96%	Rotational ploughing has considerable benefits
Delayed autumn drilling	31%	-64% to 97%	The later the better – but increased risk.
Higher seed rates	26%	+7% to 63%	The higher the better – but lodging issues
More competitive cultivars	22%	+ 8% to 45%	Useful, but marginal effects
Spring cropping	88%	+ 78% to 96%	Challenging on heavy soils; limited herbicides
Fallowing/grass leys	70–80 % per year (of seedbank)		Absence of new seeding critical

The most effective methods (ploughing, delayed autumn sowing, spring cropping and fallowing/grass leys) are not attractive options to most farmers due to the increased risks involved, financial penalties and unpredictable outcomes. However, if resistance is seriously reducing herbicide efficacy and no new herbicides are available, what is the alternative? There is now greater awareness amongst the more technically orientated farmers and agronomists of the increasing risk posed by herbicide resistance and lack of new herbicide solutions. Consequently, more farmers are now integrating herbicide use with effective non-chemical control methods but a long-term strategy is required if herbicide resistant weeds are to be controlled effectively.

References

Food and Environment Research Agency. FERA. 2013. Pesticide usage survey.

Hull R, Tatnell L V, Cook S K, Moss S R. (2014). Current status of herbicide-resistant weeds in the UK. Aspects of Applied Biology 127, Crop Production in Southern Britain, pp. 39-48.

Lutman, P.J. W. et al. (2013). A review of the effects of crop agronomy on the management of Alopecurus myosuroides. Weed Research 53, 299-313.

UK Weed Resistance Action Group. See website here for information and leaflets on herbicide resistance