Damage

Black-grass, Alopecurus myosuroides, can seriously reduce crop yields through competition for nutrients, especially nitrogen. Although on an individual plant basis black-grass is only moderately competitive, very high populations can occur (many hundreds of plants/m2) which can reduce yield by >70% in serious cases. On average, black-grass populations of 12 – 25 plants/m2 cause yield losses of 0.4 – 0.8 t wheat /ha but losses can be much higher in conditions which favour black-grass (e.g. uncompetitive crops).

Know your enemy

Black-grass is an annual grass-weed propagated solely by seeds and, if uncontrolled, weed populations can increase by >10 fold/year. For successful long-term control, seed return must be minimised. The following five key characteristics underpin successful non-chemical management strategies:

  • emergence pattern – 80% of weed emergence occurs in early autumn
  • plants can only emerge successfully from seeds within 5 cm of the soil surface
  • seed longevity in soil - 75% decline/year. (About 3% of seeds will still be viable after 3 years burial)
  • seed shedding pattern in wheat - mid-June to mid-August with peak in July; most shed pre-harvest
  • population dynamics – >95% control needed to prevent weed increasing
  • a competitive weed – aim for <5 plants/m2 to minimise yield loss and seed return

Herbicide resistance

Since first being found in Oxfordshire in 1982, herbicide-resistant black-grass has now spread to virtually all of the estimated 20,000 farms in 37 counties where herbicides are applied regularly for black-grass control. Populations show resistance to a wide range of different modes of action, with both ACCase and ALS target site resistance (TSR) and non-target site resistance (NTSR), especially enhanced metabolism, now widespread. The frequency of multiple resistance can be demonstrated by results from tests on 122 black-grass samples collected by BASF and tested by ADAS in 2013: 98% showed resistance to at least one herbicide, with 75% resistant to mesosulfuron + iodosulfuron, 84% resistant to cycloxydim, 66% resistant to pendimethalin and 46% resistant to all three herbicides.

Management recommendations

Herbicide use must be integrated with greater use of non-chemical control methods to reduce the reliance on herbicides. The potential of such methods for black-grass control (from Lutman, Moss, Cook & Welham, 2013) is summarised below. The wide range of control level for ploughing can be explained by considering where in the soil profile the blackgrass seed is and this will depend on the cropping history at a location. If it is near the top from recent seed shed ploughing may help to bury it. Conversely, if the blackgrass seed is lower down ploughing may make the situation worse (which is why the control range includes negative values). This needs to be considered when deciding whether to plough or not. Delayed autumn drilling can reduce blackgrass but if it interferes with establishment of the main crop there may be more holes where the blackgrass can come in. Therefore a balance needs to be found and drilling should not be delayed too long to ensure the main crop comes up well. Spring cropping and wider rotations are the most reliable ways of controlling blackgrass.

% 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

Hand roguing up to 90%? Can help prevent seed return but labour intensive
Mechanical weeding 50-80%? New equipment required
Spraying off patches in wheat in early June
up to 100% Glyphosate worst patches
- also removes the wheat

* preliminary data at Rothamsted suggest that cultivars with taller seedlings such as Duxford and Gallant may be more competitive - please send us feedback if you have your own observations about this.

Barley is much more competitive than wheat and in particular hybrid barley:

hybridbarley2.jpg#asset:400John Pawsey is testing rotational grazing on his farm - this involves drilling a grass ley with a competitive ryegrass variety and a red or white clover and then grazing livestock for 1-3 years to prevent blackgrass going to seed. This approach should mean that there is a more manageable blackgrass population when the plot is returned to cropping. He is also trialing a mechanical weeding system.

Andrew Ward posted an interesting article describing his 'zero tolerance' policy for black grass that involves spraying off the worst affected patches and hand pulling the rest on the CropTec blog


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