Better control annual ryegrass and avoid herbicide failure

Commonly suspected herbicide resistance turned out not to be the case. A greater emphasis of assessing ARG populations for their herbicide resistance status, via reputable testing bodies, is, therefore, an important step in better designing long-term weed control programs to protect valuable herbicides like glyphosate.

Grain Orana Alliance (GOA), an agronomist and farmer-based research and advisory group is undertaking research on assessing options to improve ARG fallow and pre-sowing herbicide control.

Ben O'Brien and Maurie Street lead the research in conjunction with other agronomists and farmers. Trials were conducted in the Narromine, Parkes and Alectown districts in 2017/18. Research undertaken in 2019 is yet to be fully analysed and will add to the strength of the data.

Several of the trial sites had suspected ARG weed populations with resistance to glyphosate, but laboratory testing found that turned out not to be the case.

Resistance status had been based on poor previous ARG control by glyphosate.

Ben O'Brien and Maurie Street suspect a lot of poor ARG herbicide controls may be related more to factors such as application set up, water quality for spraying, spray timing, weed stress factors to name a few.

Ben O'Brien also stressed that knowing glyphosate resistance status of a given ARG population is important for determining glyphosate rate requirement for better control, as well as for choice of other herbicide or agronomic control options if resistance is substantial.

Overall the research assessed various glyphosate (active) rates, various glyphosate products, with and without surfactants, plus or minus various other surfactants/adjuvants, paraquat, group A products like clethodim, products used as "spikes" to glyphosate, other herbicides and double knock strategy.

Where ARG weeds were young and actively growing and with no to low rates of glyphosate resistance, glyphosate at 675mls/ha generally provided high level control. Occasionally lower rates performed well but not consistently so. Only in one trial with ARG having low glyphosate resistance did 675mls/ha not perform as well as the next higher rate assessed, 1350mls/ha.

Wetters, surfactants/adjuvants overall did not greatly enhance glyphosate performance at the 675mls/ha or higher active glyphosate rate. Some wetters improved glyphosate effectiveness at lower glyphosate rates but results were generally not as good as the higher glyphosate rate on its own.

Products with built-in wetters sometime provided marginally improved control compared to those with no wetters. In stresses conditions, wetters sometimes improved control at higher active glyphosate rates (675mls/ha or higher).

Group A herbicide clethodim provided excellent ARG control in a couple of trials where it was assessed. It is an alternative to glyphosate where ARG glyphosate resistance occurs. But like glyphosate, its resistance preservation will require careful management to preserve its effectiveness.

Paraquat at 2.0 l/ha commonly proved to be a good alternative to glyphosate and is a useful herbicide tool to use as part of a herbicide rotation program to preserve glyphosate effectiveness.

Ben O'Brien stressed the importance of high-water rates and effective weed coverage. This and other GOA research has shown paraquat as an effective standalone knockdown and commonly improved control when applied as a double knock. This is especially useful where glyphosate resistant ARG occurs. Products combining paraquat and amitrole have also shown good ARG control.

Addition of various products to either glyphosate or paraquat sometimes provided small improved levels of ARG control, but sometimes a useful practise to allow targeting of other weeds.

Further details of this research are available on the GOA website www.grainorana.com.au

Next week: Guides to grazing management post-drought.