Consider factors hindering the effectiveness of glyphosate

Author: Alistair Lawson | Date: 09 Feb 2016

Close-up of boomspray

Sprayer setup and using good quality water are critical to ensure full rates of glyphosate are getting to the weeds. This includes using the right spray quality-water rate-boom speed combination.

The surge in glyphosate-resistant ryegrass over the last 20 years has been exacerbated by poor application technique delivering marginal doses of herbicide.

University of Adelaide weeds researcher and director of Plant Science Consulting Dr Peter Boutsalis believes that had he been asked 10 years ago whether glyphosate resistance with Group A and B herbicides would increase exponentially as it has, he wouldn’t have thought it possible.

However, in recent years he has also seen a large increase in not only ryegrass samples testing as glyphosate resistant but, worryingly, also brome grass.

Dr Boutsalis says a common reason for poor weed control when using glyphosate is herbicide resistance, ranging from weak resistance to strong resistance. Plants with weak resistance are often controlled with maximum label rates of glyphosate, Dr Boutsalis says, but relying on a herbicide-only option will develop resistance to very high rates.

“Additionally, maximum label rates also help counteract poor application technique, improves the control of older and/or stressed plants and improves control when poor quality water is used or plants are covered by dust,” he says.

Besides using the maximum label rate of glyphosate, Dr Boutsalis has some further tips for growers wanting to achieve more from their glyphosate applications.

He says spraying glyphosate in the morning can result in greater uptake than spraying it in the evening. Pot trials have shown that glyphosate activity can be reduced as ambient temperature increases. Plant stress caused by frost, drought, waterlogging, temperature, nutrition or pest damage, as well as the growth stage of the weed, will have an impact on efficacy.

“Experiments have found that the optimum daily temperatures for glyphosate activity on ryegrass range between the low teens and mid-twenties,” he says. “These findings have been observed in unstressed two to three-leaved ryegrass growing in pots.

“Glyphosate usually has a greater effect in young actively growing plants, whereas on larger plants, higher label rates are required to maintain good control.”

Dr Boutsalis says it is also important growers set up their sprayer correctly and use good quality water so they are not applying sub-lethal rates of glyphosate. This includes getting the right spray quality and water rate for the job at hand.

More information

Dr Peter Boutsalis

0400 664 460

Follow this link to read Dr Boutsalis’ full paper

Follow this link to read the GRDC Spray Water Quality Fact Sheet 

Follow this link to read the GRDC Pre-Season Sprayer Checks Fact Sheet

GRDC Project Code UA00124, UA00144

Region South