Increasing herbicide resistance in annual ryegrass has thrown up some challenges for Bec and Ash Marshall on their 3300ha farm at Normanville, Victoria, leading to a new approach to their farming system.
“Acknowledging that the solution to our ryegrass problem was not going to come in a drum was the first step in planning a long-term strategy to drive down weed numbers across our farming enterprise,” says Bec Marshall.
“Back in 2009, we were following a simple but profitable crop rotation of mostly cereals and a little vetch hay in a good year, herbicides were working well, and we had low grass weed numbers.
“We added lentils to the rotation in 2011, which gave us the option to use crop topping to target grass weed escapes. We were reasonably confident that we were doing enough to keep grass weeds under control with broader use of pre-emergent herbicides and opportunistic hay production.”
After a string of drier seasons, the Mallee’s wet start in 2017 signalled a significant turn of events for the Marshalls with their usual weed control program compromised, newly purchased paddocks bringing higher grass weed numbers, and testing revealed that the ryegrass population was developing resistance to trifluralin, clethodim and glyphosate.
“It was clear that we needed to re-assess our farming system and develop a new long-term strategy for ryegrass control,” Mrs Marshall said.
“In 2018, we introduced oaten hay to the rotation, and although we have a love-hate relationship with hay, there is no doubt that it has made the biggest difference to the grass population in targeted paddocks.”
Hay has been profitable in its own right the last few years, but the Marshalls will keep it in their rotation just for the weed control effect and the risk management benefit of deep stored soil moisture. They maintain that if a crop is ‘sown for hay, it’s cut for hay’, with no backing out at the end of the season.
The second tactic introduced in 2018 was to boost crop competition in their barley phase using higher seeding rates and to choose more competitive varieties such as Compass and more recently, Leabrook – and Planet in good years with sufficient stored moisture.
“Although it is hard to quantify the effect of increased crop competition on grass weeds, we can see the reduction in weeds in our more competitive barley varieties and know it is worthwhile,” Mrs Marshall said.
The next tactic employed since 2018 is a more diverse pre-emergent herbicide program that includes mixing and rotating modes of action at every opportunity. The main aim is to prolong the effective life of trifluralin, the most cost-effective pre-emergent herbicide option in their no-till farming system.
“Herbicide resistance testing is essential to planning an effective herbicide program for the farm. You have to know what still works and what will be ineffective to apply. The most expensive herbicide to apply is the one that doesn’t work,” she said.
“We use new products to increase diversity and herbicide mixes to minimise resistance. Trifluralin is rarely applied on its own these days unless paddocks are exceptionally clean.
“We find that pre-emergent herbicides provide a good return on investment in our system. In the oaten hay phase, we give the paddocks a complete break from pre-emergent herbicides.”
More recently, the Marshalls have needed to respond to escapes of glyphosate-resistant annual ryegrass from some fence-line areas into their crops. They have introduced the double-knock for all glyphosate applications in crop and non-crop areas.
They are targeting small weeds with robust rates on their fence-lines, irrigation channels and check banks, as these are their most difficult areas for weed control. In-crop, they apply glyphosate to terminate their hay crops before cutting and then apply paraquat after the hay is off.
In addition to these newer practices, the Marshalls continue to use narrow windrow burning for harvest weed seed control, albeit infrequently; crop topping all legumes; long grass break phases (3–5 years) in their rotation; and diversity of crops to vary the timing of planting and harvest operations, and herbicide modes of action.
Their crop rotation is now much more diverse and includes dryland wheat, barley, vetch (hay), oaten hay, lentils and canola, and faba beans – on irrigation only.
“We have seen the benefits of resistance testing and implementing tactics that drive down weed numbers fast,” Mrs Marshall said. “We thought we were doing enough before, but it only took one year for the weed seed bank to really blow-out.
“We want to do everything we can to avoid that happening in the future and stopping seed set is the key to keeping the seed bank numbers low across the farm.”
The WeedSmart Big 6 is an integrated weed management program that growers can apply across all cropping systems, including chemical and non-chemical tactics to place downward pressure on the weed seed bank and reduce herbicide resistance risk.
For more information about the WeedSmart Big 6, visit: www.weedsmart.org.au
