Arable farmers in the South West are being urged to adopt a zero-tolerance policy on black-grass to safeguard winter cereal production.

Black-grass levels are increasing in many parts of western England, says Paul Haynes, BASF agronomy manager for the south west.

“Farmers in this region usually operate more of a mixed rotation than the eastern counties, which has aided black-grass management. Growing medium-term grass leys certainly helps.

“But, in the more arable areas of east Somerset and Dorset, black-grass control has gone from bad to very ugly on some farms.”

Most black-grass originates from seed in straw imported from the east, says Mr Haynes. “It can then spread from farm to farm on machinery, particularly combines and balers, and through farm yard manure.

“These new populations are highly resistant to most contact herbicides.

“A robust zero tolerance approach is critical to safeguard winter wheat and oilseed rape production in the long term on affected farms. If you find black-grass, assume it will be difficult and be relentless in controlling it.”

Results from BASF trials show just 10 black-grass plants/sq m will typically produce 100 heads/sq m. “While that doesn’t sound very much, that level of infestation across a 10ha field would produce enough seed in volume terms to fill eight one-tonne bags, compared with just 1.5t of wheat seed drilled.

“Ten black-grass plants/ha can reduce yield by 13% and 50 plants/sq m can halve yield.”

Prevention is better than cure, says Mr Haynes. “There is much more awareness of the need to clean machinery, especially contractors’ combines and balers. More farmers are keeping straw within a ring fence or getting it from a clean farm – they don’t want to import more seed.”

Cultural and chemical control techniques should be combined to achieve effective control and minimise development of resistance.

“Cultural methods need to provide a solid base so that herbicides can finish the job and provide sufficient control. In general, pre-em herbicides have a low resistance risk, so it is increasingly important to get the most out of them.”

Establishing a stale seed-bed, by stirring the top 50mm with a light cultivator and rolling, once the combine has done its work and then spraying off resulting black-grass flushes with glyphosate, more than once if necessary, is key to reducing populations before drilling the next crop.

Correct cultivations are also important. “What many people perceive as min-till typically consists of discs, deep legs and a packer that mixes seed through the profile, which is the last thing you want,” Mr Haynes advises.

“If you know where the seed is, you can manage it. Shallow cultivations will keep seed in the top 50mm. This, perhaps combined with rotational ploughing every five or six years, is key.”

Winter wheat drilling should be delayed until at least mid-October where possible. Spring cropping offers more opportunity to control black-grass out of crop, perhaps following an over-wintered cover crop on heavier land to help timely establishment. A succession of two spring crops may be necessary in some cases, he adds.

With the continuing decline in performance of contact herbicides, growers need to concentrate more on autumn control, he says. Pre-emergence spray programmes should be based on full rates of flufenacet, pendimethalin, and diflufenican as Crystal + Hurricane or Liberator + Stomp Aqua.

“This provides three different but complementary modes of action for grass and broad-leaved weed control. For black-grass that is difficult to control, use Avadex granules first. These have a different mode of action and help sensitise weeds to other pre-em chemistry.”

Preventing seed return is a key component of a zero-tolerance approach, he adds. “There is a very powerful argument to spray off affected crop areas, or whole crop them for forage.”

Hand roguing can be very useful for low populations. “However, this must be thorough or enough black-grass can escape unseen to cause a bigger problem down the line,” says Mr Haynes.