19 April 1996

Oat herbicides need full whack

to beat drought

By Robert Harris

IF dry conditions persist this spring, wild oat herbicides will need to be applied at or near the full recommended dose to achieve a good kill.

Drought stress causes physical and metabolic changes in the weed which boosts its tolerance to chemicals, says John Caseley of IACR Long Ashton. The widely used fops and dims are especially vulnerable, he notes.

"Although other environmental factors are important, drought stress is the key determinant affecting the efficacy of wild oat herbicide treatments. Plants growing under such conditions show a high degree of recovery to low dose applications." That explains why many spring treatments fail, he adds.

Physical changes brought about by a lack of water over a long period affect all chemicals, he explains. Leaves tend to be smaller and more erect, making the plant a more difficult target to hit. That means less chemical is applied to each plant.

Leaf surfaces are waxier and better able to repel spray droplets, so less active ingredient is retained. And thicker cuticles delay penetration, meaning that the small amount remaining on the leaf is more prone to being washed off if rain follows soon after application, he adds.

Even if a normally lethal dose does eventually enter the leaves, drought-stressed plants can still survive, says Dr Caseley.

"Most fops are esters. They have to be converted to acids before they can be translocated from leaves to kill the meristem," he explains. But plant metabolism is greatly reduced during periods of stress, so chemical conversion takes place at a much slower rate. That gives the plant time to detoxify the chemical. Delivery of fop acids and dims to the target site is also reduced.

Unlike the physical effects which take time to develop, metabolism is reduced soon after the plant goes into drought stress. That means even plants which appear normal can be hard to kill, he warns.

"If drought stress is imposed two to three days before spraying, the slower metabolic rate can still affect some herbicides even though plants are not wilting. It doesnt take much – a couple of bright, sunny days with a good breeze can quickly put plants in dry soils under stress."

The safest bet with drought-stressed crops is to apply chemicals at the recommended rates, Dr Caseley believes. "There is a real risk of reduced control when using lower doses. But you can still achieve good control at full dose most of the time."

Those growers who think they can get away with less than the full rate should use an adjuvant to boost chemical uptake by the plant. "When you reduce the chemical dose you reduce the dose of the formulation component too. Its very important to make up for that shortfall."

In extreme circumstances it may pay to substitute fops and dims with a chemical like difenzoquat (Avenge 2). "It does not need to be activated in the plant so it is more effective under stressed conditions," he says.


&#8226 Drought stress causes:

– Fewer, smaller, more erect leaves.

– Waxier, thicker leaves.

– Slower conversion of fops to active form in plant.

– Reduced delivery of fops and dims to target site.

&#8226 Apply chemicals at or near full dose.

&#8226 Maintain adjuvant at correct level.

&#8226 Consider alternatives to fops and dims.