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Delay spraying stressed weeds after rain

WEEDSMART DRY SEASON SPECIAL

Dry season agronomy is difficult. In affected areas there is a reasonable chance that pre-emergent herbicides applied at or before seeding will not work as well as they usually would, even when it does rain. This fact, combined with the significantly reduced crop competition in most paddocks, will mean weeds will have the opportunity to grow in greater numbers.

Knowing that spraying moisture stressed weeds in dry conditions is less effective, many growers will be looking for any sign of rainfall as an opportunity to quickly apply post-emergent sprays in an attempt to reduce weed seed set.

Australian Herbicide Resistance Initiative communication lead, Peter Newman says waiting for new leaves to grow after a rainfall event is likely to give better results than spraying just a few days after rain.

“When plants are stressed, one of their survival mechanisms is to thicken the cuticle of their leaves,” he says. “This reduces moisture loss from the leaf during hot, dry periods and also reduces the uptake of post-emergent herbicides.”

“These leaves remain thicker and waxier, even when it rains and the plant freshens up,” he says. “Waiting for new leaves to emerge after rain will result in a much better level of control and help minimise weed seed set. The use of the correct adjuvants and spray quality to counteract the increased waxiness of the leaves will also improve herbicide efficacy.”

However, waiting for new growth also has its problems if dry conditions return. “These decisions are not easy, but if rain has fallen and more rain is forecast, perhaps waiting for new growth of the weeds will give the best results,” he says.

Department of Agriculture and Food WA research conducted by Dr David Minkey and John Moore in the 1990s demonstrated the impact of moisture stress and low humidity on herbicide efficacy. Their research showed a 20-fold difference in efficacy of glyphosate sprayed on weeds of the same age under favourable and stressful environmental conditions.

“Unfortunately, spray events are going to be difficult to time and the results are probably going to be less than optimal. This is out of the grower’s control in most instances,” says Peter. “Given that there is a good chance of more weeds surviving in-crop weed control efforts, implementing some form of harvest weed seed control will be an even higher priority this year.”

“High numbers of annual ryegrass is a concern but we know that we can get back on top of a ryegrass seed bank in just a few years,” he says. “Wild radish however produces seed that remains viable in the soil for 5 to 10 years so it takes much longer to drive weed seed numbers down if this weed blows out.”

If faced with a crop failure, spraying out early could be a good option and for crops that are harvested consider a low cost harvest weed seed control option, such as chaff lining, to minimise the potential impact of a weed blow-out.

Chaff lining involves placing a chute on the rear of the harvester that concentrates the chaff-only fraction into a narrow band between the wheeltracks of the header. The straw is chopped and spread as usual. The chute can generally be built on farm at a very low or even nil cost.

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