One of the tactics for reducing annual grasses and retaining desirable species in pastures is spray-topping. This tactic involves applying a non-selective herbicide at a critical time to target weed seed set, followed by heavy grazing.
The composition of a medium term pasture changes over time. The pasture may be legume dominant in year one, but without intervention pastures become dominated by annual weeds (generally grasses). Typical grass species include annual ryegrass, silver grass, barley grass and brome grass.
These grass weeds cause:
- a build up of weed seeds in the seedbank
- reduced availability of nitrogen from pasture legume input as weeds use nitrogen reserves
- a build up of cereal root disease
- increased risk of eye injury and meat contamination in sheep and increased vegetable faults in wool (resulting from the grass weed seeds).
One of the tactics for reducing annual grasses and retaining desirable species in pastures is spray-topping. This tactic involves applying a non-selective herbicide at a critical time to target weed seed set, followed by heavy grazing. Both paraquat and glyphosate can be used for spray-topping. Spray-topping pasture is possible because annual grasses become much more sensitive to non-selective knockdown herbicides during flowering. So low rates of herbicide can be used to sterilise the grass seeds, with limited effect on desirable pasture legume species.
Benefits from spray-topping
- Strategically timed spray-topping significantly reduces weed seed set, reducing the weed seed bank.
- Both paraquat and glyphosate can be used for spray-topping, allowing better management of herbicide resistance development.
- Spray-topping is a very cost effective tactic to reduce weed seed set.
- An increased proportion of legume in the pasture, resulting in improved feed value of the pasture and increased livestock production from grazing.
- Well-planned spray-topping can be used to set up pastures for high-quality forage conservation (hay or silage) during the following spring.
- Spray-topping is ideally used in the season before fallow initiation, reducing grass weeds and the risk of cereal disease carryover into the following winter crop.
- Spray-topping is very effective against barley grass (Hordeum spp.), reducing injuries to lambs’ eyes, skin and carcasses.
Issues to consider when spray-topping
- The timing of herbicide application is critical to the success of spray-topping. It must after the time when seed heads are fully emerged from the boot and before the seeds reach the dough stage (timing is slightly dependent on the herbicide used).
- Spray-topping is not an alternative to fallow spraying.
- Spray-topping alone cannot control a wide range of grass species simultaneously, because the specific timing of spray-topping will vary between species (depending on when each species produces seed heads).
- Grass weed levels determine the management 'fit' of spray-topping - monitor the pasture as grass burdens increase in response to increasing nitrogen levels.
- Winter cleaning or fallow spraying may be used to finalise the pasture phase before cropping commences.
- Spray-topping can reduce seed set in pasture legumes if the stage of development of the legume pasture coincides with the development stage of the target grass weed. See more information on the effect of spray-topping on pasture legumes.