Crop weeds: weed management at harvest

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

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Grazing – actively managing weeds in pastures

In crop based rotations, a 2-3 year pasture phase may significantly reduce weed seedbanks to manageable levels before returning to a cropping phase. Pasture weed management requires maintaining desirable legume and grass species while keeping weed numbers under control.

Grazing, in association with other tactics, may be used to help reduce weed numbers. Grazing can be coupled with hay and silage making, mowing and pasture spray-topping for increased weed control. Well managed grazing will increase legume composition and improve feed quality.

Issues to consider when controlling weeds in pasture

Grazing pressure

Even though your newly established pasture may look lush and inviting, avoid the temptation of grazing too soon.

High grazing pressure is necessary to ensure weed control. Insufficient grazing pressure (particularly in spring when the seed heads of some grasses may be less palatable than legume species) will not result in weed control. However, high grazing pressure can increase the proportion of broadleaf weeds like capeweed and erodium. Stock avoid these weeds because it is difficult for them to graze species with a flat rosette. Using a herbicide to cause these species to curl their leaves will give stock better access to them, but grazing pressure must be high to ensure that stock eat these broadleaf weeds.

Timing of grazing is critical

High grazing pressure in autumn will physically remove small weed plants, and short periods of intense grazing are generally recommended to minimise damage to non-weed species. However, optimal grazing management depends of the ecology of the pasture species and weeds in question. For example, silver grass species are best managed by light grazing pressure in autumn, and light grazing pressure will also avoid the development of bare areas where weeds may establish at a later date.

High grazing pressure in spring will reduce weed seed production, but will also reduce seed production by desirable pasture species. Sheep and cattle will preferentially graze the small heads of annual ryegrass. However, sharp awned seeds from grasses like brome grass, barley grass and silver grass are not palatable to stock. So intensive spring grazing to control these species should commence prior to emergence of the seed head.

Grazing can be used in conjunction with herbicides (spray-grazing) to effectively manage weeds

Spray-grazing refers to the use of sublethal rates of selective herbicides (often phenoxy-based) to increase the palatability of broadleaf weeds for preferential grazing. It is usually undertaken in autumn or early winter and is especially beneficial for the control of erodium, capeweed, Paterson’s curse and wild radish. The use of phenoxy-based herbicides causes the flat weeds to curl up and thus become more accessible to livestock.

High stocking rates (up to four times the normal rate for the area) are required for spray-grazing to work effectively. Weeds that are not killed by spraying alone will recover in 2-3 weeks and exhibit normal growth if they are not grazed heavily after spraying.

Burial of seeds

Livestock movement will bury some weed seeds. In some cases this will enhance germination and subsequent weed control. However, this may alternatively allow the seeds to escape other weed control techniques like burning. Consider what weed species are in the field, and how burial will affect subsequent weed control.

Manage grazing to avoid the risk of livestock transporting weed seeds

Practicing good farm hygiene techniques will assist in minimising weed seed transfer.

  • Move stock to holding areas after they have grazed weedy fields.
  • Keep new stock in holding areas for at least five days to empty any seeds in the gut before allowing them to graze other fields on the farm.
  • Keep stock in containment areas when hand-feeding with imported feed.
  • Alter shearing schedules to ensure that fleece length is short when grasses are shedding seed (this will also reduce vegetable fault in fleeces).

Timeline for the implementation of tactics for management of annual grass weeds in pastures. Note that for Western Australian wheatbelt pastures, a dry matter maintenance target is >800kg of plant material per hectare.

Timeline for the implementation of tactics for management of annual grass weeds in pastures

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