THE issue of herbicide resistance in blackgrass has been causing concern for a couple of decades.

Historically confined to the South, over the past five years it has spread to the North of England and Christine Lilly, of Frontier Agriculture, believes that resistant blackgrass strains are now present on the majority of farms in our region. She urged growers to take immediate action to reduce infestation.

A survey has shown that some 80 per cent of farms have reported a loss of blackgrass control due to chemical resistance, said Miss Lilly. The first step is to learn how to accurately identify a blackgrass plant, as some growers are still unsure and there continue to be cases where infestations have remained undetected. A 95 per cent control rate is required to prevent an increase in populations, she said.

A range of measures can be taken to reduce resistant blackgrass populations, including a commitment to using a wide a range of active ingredients in sequence. It is believed that the repeated use of the same products is one of the main factors responsible for the emerging problem.

“Over-reliance on herbicides which fall into the *ALS and *ACCASe groups has caused the development of widespread resistance to post-emergence herbicides,” said Miss Lilly. “Growers who are planning to implement changes should therefore place increased emphasis on residual spray programmes; residual herbicides should still be applied in the autumn.

“It is also worth noting that winter barley is more competitive than winter wheat and that the use of glyphosate in stubble is another way in which to tackle weed numbers.”

Delayed drilling was another important weapon in the fight against blackgrass, although Miss Lilly admitted that the practice was not always easy to achieve in the North.

She also recommended a move to spring cropping in fields with high blackgrass populations to achieve greater control, as autumn crops were unlikely to realise their full potential if they continued to be sown.

Changes to agricultural practice had influenced the rate of resistant blackgrass spread, she added.

“There is a trend towards the increased use of contractors, coupled with a rise in the number of farmers who share machinery,” she said. “This has led to seeds from resistant strains of blackgrass being carried from one farm to the next. All of the machinery that is brought on to the farm should be thoroughly cleaned before arrival and diligent contractors are already taking precautions.”

The rise in growing costs and diminishing crop returns have previously encouraged growers to pursue close rotations, with wheat and oilseed rape frequently grown to the exclusion of other plant types. Where practical, it was more effective to include a diverse range of species and this would also help to broaden the range of applied active ingredients.

For growers who routinely use low inversion systems, one year of deep ploughing could help to bury blackgrass seeds. However, researchers at Frontier have found that blackgrass is more prevalent in the direct-drilled plots, compared with plots that are ploughed at the firm’s trial site in Nottinghamshire. This is despite the fact that ploughing disturbs the soil and brings buried seeds up to the surface.

Miss Lilly predicted that it was unlikely that resistant strains of blackgrass could be wholly eradicated on farms.

“Nevertheless, populations can be controlled to a level where they have only minimal impact on yields. The aim should be to reach a point where blackgrass can be hand-rogued and there are examples of producers who have met this goal. The key is to be proactive, rather than reactive,” she said.

Blackgrass research

Researchers at Rothamsted Research in Hertfordshire have uncovered a number of facts relating to blackgrass:

*The number of heads per plant varies widely; mainly depending on crop competition, with two to 20 heads/plant typical in wheat crops

* Each head contains about 100 seeds and a population of 500 heads per square metre can produce 50,000 seeds per square metre.

* If uncontrolled, blackgrass populations can increase by more than 30-fold/year

* The survival of buried seeds is 20 to 30 per cent/year. Therefore after three years’ burial, only a fraction of seeds will be viable, although this may still represent considerable numbers

* Given sufficient moisture, about 80 per cent of blackgrass germination and emergence occurs in the autumn (September to November).

Blackgrass resistance detection

Until recently, the only method of detecting resistance was to send off seed samples to a laboratory. However, Newcastle University, working in conjunction with contract research firm Mologic and Frontier, has developed a ‘pregnancy test’ style diagnostic tool which can be used on farm and will give results within five minutes. The device, which analyses leaf samples, costs about £40.

Blackgrass crop losses

The estimated loss resulting from herbicide resistance in strains of blackgrass in wheat is 0.4-0.8 tonnes/ha at a plant density of 12 to 25 plants per square metre.

*ALS - Acetolactate synthase

*ACCASe - acetyl-CoA carboxylase