Weed alert: Summer storms create ideal conditions

Storms and warm weather through November have created ideal conditions for a flush of summer weeds and the GRDC is urging growers to control weeds in fallow and ahead of summer planting to maximise effectiveness, preserve precious stored soil moisture and protect the yield potential of future crops.

GRDC crop protection manager Ken Young said early weed control could potentially deliver significant production benefits for growers.

He said GRDC research investments had quantified the value of stored soil moisture in northern farming systems with trials showing even an extra 20mm of stored soil moisture could be worth an extra 0.5 tonnes a hectare in additional yield.

“At this time of year the difference between effective and ineffective weed control is only a matter of days, not weeks,” Dr Young said.

“Weeds can be the difference between inefficient and efficient water storage during the fallow. Generally speaking if summer weeds are allowed to establish in the fallow, they will start extracting water from depth – more than 100mm depth – within about 12 days of establishment. That’s water that could be used to grow a following crop.”

Dr Young said it was important growers had spray application equipment well maintained, used robust herbicide rates in accordance with product labels and adhered to best management spray application recommendations on water rate, environmental conditions, droplet size and boom height.

“Control can be very effective if plants are sprayed when they are small but leaving them too long can be costly, requiring additional control measures,” Dr Young said.

“As most growers know there are some key factors that impact on the efficacy of summer herbicide applications including adjuvant selection which is critical to the performance of most active ingredients.

“Adjuvants may improve herbicide efficacy by reducing evaporation, increasing droplet survival on leaf surfaces and increasing penetration of herbicide active ingredients into the plant.

“Efficacy is also impacted by temperature and humidity, travel and wind speed, droplet size and viscosity of spraying liquid so it is important from a cost/benefit and sustainability perspective to adhere to best management recommendations on spray equipment and conditions.”

ICAN senior consultant Mark Congreve said when looking at the effect of temperature alone, glasshouse testing has shown higher rates of glyphosate were needed to control barnyard grass under 35/30oC day/night temperatures compared to the same population grown under a 25/20oC temperature regime.

“Typically, registered label rates are selected to cover this range of field variability, so often this effect may not be noticed where label rates are robust,” he said.

“However, if other factors are also reducing field performance, for example low levels of glyphosate resistance, then problems can arise.

“As would be expected, to achieve consistent results with knockdown herbicides applied in summer, we want to maximise the amount of herbicide that moves into the leaf.”

To achieve this, he said growers need excellent coverage with the right droplet size, reduced spray evaporation, and to keep the spray droplet in a liquid phase on the leaf surface for as long as possible.

He said some herbicides penetrate the leaf more easily, so are less affected by harsh summer spraying conditions while the properties of other herbicides mean they are relatively slow to penetrate the leaf and are hence are more susceptible to droplet evaporation, or rain wash off should this occur soon after application.

Advisors wanting to dig into the science of how knockdown herbicides penetrate the leaf and move through the plant; which herbicides are most prone to performance issues under summer conditions; what practical steps can be undertaken to maximise uptake and how herbicide resistance adds additional complexity to the challenge can go to www.grdc.com.au