The surge in glyphosate-resistant rye-grass during the past 20 years has been exacerbated by poor application technique delivering marginal doses of herbicide.
University of Adelaide weeds researcher and Plant Science Consulting director 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 rye-grass samples testing as glyphosate-resistant but, worryingly, also brome grass.
Dr Boutsalis said a common reason for poor weed control when using glyphosate was herbicide resistance, ranging from weak resistance to strong resistance.
Plants with weak resistance were often controlled with maximum label rates of glyphosate, Dr Boutsalis said, but relying on a herbicide-only option would 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 said.
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 said spraying glyphosate in the morning could 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 rye-grass range between the low teens and mid-twenties,’’ Dr Boutsalis said.
‘‘These findings have been observed in unstressed two- to three-leaved rye-grass 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 said it was also important growers set up their sprayer correctly and used good quality water so they were not applying sub-lethal rates of glyphosate. This includes getting the right spray quality and water rate for the job at hand.