Black-grass

Relying solely on herbicides for control of black-grass is not sustainable in the long term because of increasing resistance. This publication provides information on non-chemical control.

Black-grass is one of the biggest challenges many growers face on their farm.

This page brings together key information to help tackle the problem.

Key points:

> Minimise seed return

> Both cultural and chemical control methods are needed

> No two farms are the same

> Look at costs and benefits across the whole rotation

Why is black-grass an increasing problem?

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 emergences between August and October

> Yet 50% of winter cereals and virtually 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 Some existing herbicides may withdrawn for regulatory reasons.

Visit the Weed Resistance Action Group page

5 things to remember about black-grass

Population dynamics: 97% 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

Spring cropping (88% control)

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 (69% control)

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 drilling (>31% control)

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

Competitive crops (22–26% control)

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

Fallowing/grass ley breaks (70–80% control)

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

Preventing seed return and spread of resistance genes

> Spraying off black-grass in growing crops with glyphosate in the first week of June will prevent viable seed return

> Cutting or spraying in May or later June is likely to be less effective

> Consider spraying the same areas for 2-3 years to minimise black-grass populations

> 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

Is black-grass a threat to Scotland?

An overview of the situation - longshort