Straw movements and black-grass: what you need to know

Straw movements and black-grass: what you need to know

The exceptionally dry conditions this season have, inevitably, reduced forage and cereal straw production in traditional livestock areas. This has resulted in a larger than normal movements of straw from cereal growing areas in the south and east of England to the western and northern areas of the UK.

In addition to the increased costs and logistical difficulties this causes, there is also the potential for this straw to spread weed seeds, with particular concern about black-grass.

The amount of weed seed retained at harvest time varies between host crop and grass species. The figure is low for black-grass in wheat, between 2% and 20%, rising to 50% in winter barley. Generally, bromes retain more seed, with rye grasses retaining as much as 70%, even in wheat.

Most weed seeds will end up on the ground after harvest, but some will be retained and incorporated into baled straw. Although the percentage of seeds retained may be small, high seed production by grass weeds means straw from fields with even moderate grass weed populations may contain large numbers of seeds.

What happens to weed seeds in straw used for animal bedding or feed is largely speculation. No research has been conducted with problematic UK grass weeds. Some work conducted in North America, with other species, has shown high numbers of weed seeds can be present in farmyard manure.

Although composting manures reduces seed viability, seed kill is dependent on the weed species, as well as temperature and moisture in the compost windrow. Anaerobic digestion can also reduce seed viability but may not kill all seed. It is, therefore, reasonable to assume that weed seed brought in with straw will retain some viability when the straw is subsequently spread on land as farmyard manure and also that slurry may also be contaminated, while straw fed in field will contribute seed directly to the soil seedbank.

Black-grass populations through the main cereal growing areas of England are widely resistant to the post-emergence ALS and ACCase inhibitor herbicide groups. In more peripheral regions, such as northern and western England, the presence of resistance is more sporadic. Enhanced metabolism of some other herbicide groups has also reduced their effectiveness. Herbicide resistance in UK brome species has not been confirmed, though a small number of populations now show reduced sensitivity to some herbicides. Resistance to ALS and ACCase inhibitor herbicide groups has been confirmed in around 500 UK ryegrass populations, particularly in parts of Essex and Yorkshire. Black-grass seed imported onto farm in straw from the main cereal growing areas of England is, therefore, highly likely to be resistant to post-emergence herbicides, with a lower risk from ryegrass seed and brome seed.

Actions to minimise the spread of herbicide-resistant weed seeds in straw:

  • Avoid buying straw from regions where herbicide resistance is most widespread
  • Avoid feeding potentially contaminated straw in field
  • Maximise the time FYM or slurry is stored before spreading, as seed viability will decline
  • Spread potentially contaminated FYM or slurry on grassland, in preference to cropland
  • If potentially contaminated FYM or slurry is spread on cropland, plough to bury weed seed

Actions if new populations of grass weeds, particularly black-grass, are observed:

  • Identify any newly establishing grass weed populations as soon as possible
  • Adopt a zero-tolerance approach – do not allow seeding
  • Use hand-rouging to remove individual weeds
  • Spray glyphosate to remove patches of weeds

If larger populations affect large parts of or whole fields, then more general control methods should be applied: cereals.ahdb.org.uk/blackgrass

In such cases, it is worth testing seed to see what level of herbicide resistance is present.

Never adopt a wait-and-see approach, as experience shows a small problem with grass weeds can become a large problem rapidly.

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Paul Gosling

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