Spray-topping pastures

Page last updated: Wednesday, 26 April 2023 - 10:26am

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Spray-topping or pasture topping is the application of a sub-lethal rate of herbicide when grasses are coming into head and flowering. The aim is to reduce the production of viable seed and the seedling population in the following year. The grass is not killed by the herbicide application. Spray-topping is normally carried out in preparation for the following season's cropping program.

When to spray-top and what to use?

Paraquat (for example, Gramoxone®) is more appropriate if the target grass species has developed past the flowering stage. Tillers that have emerged will be desiccated rapidly, but any late developing tillers or heads which were still covered by the flag leaf will continue to grow and produce viable seed. Delaying paraquat applications until the milky-dough or dough stage of the weed seed can help to avoid regrowth and tiller escape. It may be better to schedule two applications to target various grass species in the paddock. Paraquat always works better if applied in the late afternoon or on a still overcast day.

Glyphosate needs to move through the plant to the site of action and takes time to control all tillers. Delaying application of glyphosate until the milky-dough stage (of the seed) will reduce the level of control as many of the seed heads will begin to mature before the herbicide is effective.

Barley grass and brome grass

Glyphosate should be sprayed when the seed heads emerge and before the dough stage of the seed. Once beyond flowering, paraquat should be used as it will provide better control of seeds in the dough stage. As barley grass sends up seed heads over an extended period, earlier control with glyphosate could be a good option.

However, this early timing is very likely to coincide with the flowering of pasture legumes. Pasture legume seed set can be significantly reduced if spray-topping coincides with early to mid-flowering. An alternative for barley grass is to use two applications of paraquat (three weeks apart). The interval will vary with seasonal conditions.

Use one application to target early tillers and a second application to target later tillers. Obviously the cost of treatment is increased and trials have shown that pasture legume seedset is reduced more when two applications are made rather than one.

Silver grass

Glyphosate should be applied at the early head stage and paraquat a little later just prior to haying off. The seeds of silver grass are small and it is often difficult to determine different stages of seed development. Additionally silver grass can progress from flowering to hard seed in about a week if the weather is warm and soil is starting to dry out. So it is important to treat silver grass paddocks first in the spray-topping program to ensure effective treatment.

Annual ryegrass

It is important that glyphosate be applied during flowering and when all tillers are in head. Paraquat should be applied a little later when most heads have seeds in the dough stage.

Follow-up paddock treatment

Heavy grazing after spraying helps to reduce the emergence of late tillers (refer to product label for withholding periods). The amount of rainfall and/or stored soil moisture after spray-topping will influence the amount of regrowth. Stock will select regrowth as it is green and palatable. If there is more regrowth than can be managed by available stock the paddock should be resprayed.

Spray-topping may result in increased digestibility and protein levels in the sprayed pasture. This is thought to be a result of halting grass growth and the movement of carbohydrates and/or proteins to seed heads. Increased palatability may also occur with stock selectively grazing sprayed areas. While this will reduce potential seedset further it can also lead to soil erosion. Monitor grazing over summer to ensure that paddocks do not become overgrazed and at risk of wind erosion.

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Alex Douglas
Sally Peltzer