Crop weeds: weed management at harvest

Page last updated: Wednesday, 17 February 2021 - 10:38am

Please note: This content may be out of date and is currently under review.

Grazing crop residues

Grazing weed-contaminated crop residue can be a cost-effective way to control weed growth. Animal digestion of weed seeds prevents a large proportion from entering the seedbank.

A pile of dumped chaff in the paddock, ready for grazing or for burning later.
A pile of dumped chaff in a paddock. Can be grazed in-situ or burned in the following autumn period.

Benefits from grazing crop residues for weed management

  • Grazing reduces the number of weed seeds added to the seedbank.
  • Grazing can be used to dispose of, and gain value from, weed seed contaminated fodder.
  • Weed seeds can provide a significant proportion of the nutritional value when stock graze crop residue (note that the feed value of the residue will be variable).
  • Post-harvest grazing may reduce crop establishment problems through reduction in stubble burdens.

Other factors to consider

  • Seed burial through trampling may enhance weed germination pre-sowing. Using a knockdown herbicide and delaying sowing can then capitalise on this.
  • Seed of desirable plants (pasture species) may be distributed in faeces.
  • Grazing livestock can distrubute weed seeds across a paddock.
  • The impact of grazing on weed numbers in the seedbank is dependent on the biological features of the weed. Grazing is successful in reducing weed seed numbers in palatable weeds and where the seeds can be easily eaten and digested. However, seed palatability varies from weed to weed. The presence of awns, thorns or biochemical traits makes some weeds less attractive to grazing animals than others.
  • Seed location: Stock must be able to access seed to ingest it. Seed still in the head, or in chaff dumps or feed troughs, is easier to access than seed lying on the soil surface.
  • Seed size: When shed from the seed head, small seeds are more difficult for animals to graze. Small seeds are also more likely to survive ingestion and digestion.
  • Hard seeds: A high proportion of hard seeds will remain viable after digestion. The digestive process can also break seed dormancy, encouraging the germination of seeds shed in faecal matter.
  • Livestock trampling tends to bury weed seed, which can decrease the efficiency of burning as a means of killing seeds. Depending on the weed species, burial may also increase germination rates.
  • Grazing may cause an increased risk of soil, water and wind erosion, increased soil compaction and potential toxicity issues for sheep, for example, lupinosis and annual ryegrass toxicity.

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