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Grazing strategies for reducing the weed seedbank

A pasture phase in a crop rotation provides many benefits, including the opportunity to reduce annual grass weed populations and manage herbicide resistance.

Tim Condon, Senior Farm Advisor with Delta Agribusiness says that although livestock have long been an integral part of mixed farming operations there are some well-known strategies for managing annual weeds that need to be brought back into focus and implemented specifically for herbicide resistant weeds.

“The key is to have a weed management strategy that spans the whole pasture phase, not just the year before returning to cropping,” he says.

“For example, during the recent drought years in NSW, weed management in the pasture phase took a back seat on many properties. These farmers now have an opportunity to use their livestock to significantly reduce the seed bank of grass weeds like annual ryegrass, brome grass and barley grass, as well as vulpia and wild oats.”

“In this system, achieving very high levels of weed control is possible using a coordinated strategy involving heavy grazing, herbicides and possibly fodder conservation or slashing.”

During the pasture phase Mr Condon suggests spray topping to reduce weed seed set. When the grass weeds are at the late vegetative stage, livestock, usually sheep, can be grazed heavily at around 30 to 40 DSE/ha to reduce the bulk of the pasture and stimulate even grass seedhead emergence, making it easier to time herbicide application.

“Both Gramoxone and glyphosate are registered to spray top barley grass and ryegrass in pasture,” he says. “Gramoxone has a narrower window at flowering in which it is effective. Having the majority of the weeds flowering together is very important to gain the full benefit of this herbicide. Glyphosate is effective over a slightly wider window either side of flowering.”

Spray topping generally reduces seed set of the targeted grass species by around 80 per cent. Once the grazing withholding period is over livestock can return to feed on the high quality pasture, which is also free of grass seed that would otherwise cause eye, wool or skin problems for sheep.

Winter cleaning is the alternative, again using livestock to remove the bulk of the pasture before applying herbicide. “Winter cleaning relies on very heavy grazing of the whole paddock to reduce the pasture to less than 500 kgDM/ha,” he says. “This must be done around July while the annual grass weeds are still young and palatable.”

Sheep_grazing_compressed

Once the sheep are removed an application of Gramoxone, in combination with other herbicides depending on the pasture type, will achieve a very high level of annual grass weed control. Growers should consult product labels or seek advice on suitable options from their advisor.

“After six to eight weeks the pasture will recover and be almost free of annual grass weeds,” he says. “In the last year of the pasture phase the winter cleaning may be followed by a spray fallow over spring and summer to conserve moisture and remove any surviving weeds before planting canola or wheat in autumn.”

A similar level of control is possible using mechanical options such as cutting for hay and silage or slashing. Again, grazing heavily beforehand is still useful to ensure uniform head emergence to maximise the removal of seedheads before the seed has set.

“In different seasons or locations different options will have advantages over others,” says Mr Condon. “Planning to prevent seed set for at least two seasons prior to cropping is very effective in driving down the weed seed bank during the pasture phase.”

“Another excellent option going into cropping is a triple-knock strategy using glyphosate for initial control, then grazing heavily and quickly, at around 50–60 DSE/ha, to reduce the bulk of any survivors, then finishing off with an application of Gramoxone.”

“These are broad strategies to get started but there are several ways to go about grass weed management in the pasture phase,” says Mr Condon. “There are also a number of possible herbicide combinations that are worth investigating with a local agronomist.”

Using livestock to assist with weed control often requires additional management input and may not always line up with the best use of the pasture for grazing. This sacrifice is rewarded through with a significant reduction in the weed population going back into cropping.

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