Black-grass

Black-grass is one of the biggest challenges many growers face on their farm. This page brings together a range of key information to help tackle the problem.

Key points:

• Minimising seed return is essential for achieving control.
• A combination of cultural and chemical control methods is needed to control black-grass.
• No two farms are the same, so the approach needs to be tailored to the conditions in the fields on that farm.
• It is important to look at the costs and benefits of controlling black-grass across the whole rotation.

Key tools and publications:

Information sheet 30: Black-grass: solutions to the problem

Video Dr Stephen Moss: Herbicide resistance lost battles and war winning tactics

Information on HGCA black-grass research

The majority of black-grass plants now emerge within crops, rather than before sowing when they could be more easily destroyed.

Drill date:

• 80% of black-grass emergence happens between August and October.
• 50% of winter cereals and virually all oilseed rape is sown before the end of September.

Herbicide resistance:

• Multiple-herbicide resistance now occurs virtually on all farms.
• No new herbicides are likely to become available.
• Some existing herbicides may withdrawn for regulatory reasons.

• Population dynamics: more than 95% control needed to prevent weed populations increasing.
• Emergence pattern: 80% of plants emerge in early autumn.
• Emergence depth: plants emerge from seeds within 5cm of the soil surface.
• Seed longevity in the soil: Average seed decline is 74% per year.
• Competitiveness: aim for fewer than 5 plants per square metre to minmise yield losses. 

About 80% of black-grass emerges in autumn, so spring-sown crops tend to be much less affected and have given a consistently good reduction in field trials.

Spring barley is more competitive than spring wheat but there is little information on the impact of other spring crops and the effect of sowing dates.

In addition, establishing crops in spring can be difficult, especially on heavy soils and herbicide choice is more limited.

Ploughing reduces the risk from grass weeds by burying freshly shed seeds below 5cm depth from which seedlings are unlikely to emerge.

Black-grass seeds are relatively non-persistent in the seed bank (70-80% decline per year) so usually fewer old, buried seeds are brought up to the surface, especially if ploughing is done on a rotational basus, once every 3-6 years.

Shallow non-inversion tillage tends to favour black-grass as freshly shed seeds are retained in the soil surface from where they can readily emerge.

It does, however, avoid bringing large numbers of buried weed seeds back to the soil surface, so is preferable where little seed has been shed in the crop just harvested.

Delayed autumn sowing has three benefits:

• It allows more weed to flush and be sprayed off before sowing.
• Residual pre-emergence herbicides can be 25-30% more effective in later drilled crops as soil conditions are more favourable to good activity.
• Black-grass emerging in later drilled crops tends to be about 40% less competitive, based on yield losses per 100 black-grass plants/m2 

The following factors favour competitive crops that are better able to suppress weeds:

• Higher seed rates >300 plants/m2
• More competitive crops or varieties
• Narrow row spacing
• Improved drainage
• Good seedbeds 

Seed persistence data and farm experience show that a 1 year break is not sufficient to reduce high infestation levels. After 2 years though, less than 10% of seeds are likely to remain. During breaks it is essential to minimise seed return. 

• Spraying off black-grass in winter wheat with glyphosate in the first week of June will prevent viable seed return.
• Consider spraying the same areas for 2-3 years to maximise reductions.
• Cutting or spraying in May or later June is likely to be less effective.
• Minimise spread of seeds and plants in combine harvesters, balers, cultivation equipment, straw or manure.
• Hand rogueing is feasible at low weed populations and is particularly recommended in fields where black-grass is just starting to appear.

Video from the HGCA Agronomists’ Conference 2013