Build a robust program around residual herbicides

Random weed surveys conducted in 2020 revealed that annual ryegrass is still susceptible to all pre-emergence herbicides, but there is a definite shift toward increasing frequency of resistance to herbicides used in crops to control this significant weed, particularly in cereals.

Ryan Pratten, independent agronomist with Muldoon Pratten Ag Consulting in Narromine, says the advance of herbicide resistance has been slower than in higher rainfall zones. But the problem is now impacting on-farm cropping decisions in the Central West.

“We are losing the efficacy of several key herbicide groups to manage ryegrass, leaving clethodim as the only viable in-crop treatment on some farms,” says Ryan. “Also, 14 per cent of ryegrass samples collected were resistant to glyphosate, which is a major concern that can’t be ignored.”

In-crop herbicides registered for control of black oats in cereals are also starting to weaken across the region, and milk thistle is now widely resistant to the SU group of herbicides.

After five consecutive high-rainfall winter cropping seasons, limited in-season spray opportunities and softer finishes, Ryan is seeing the impact of heavy weed pressure on farms.

“This has created a perfect storm on many farms in the region, and it will take real dedication to bring those weed seed banks back down to manageable levels where we can contain the risk of herbicide resistance,” he says.

To meet this challenge, Ryan is working with his client group to build robust weed control programs. These programs include pre-emergent residual herbicides applied before or after sowing their winter crops, along with strategies that vastly reduce the number of weeds that survive and set seed.

“Residuals must be integrated into a bigger system and should not be used on their own in high weed seed bank situations,” he says. “Even the best result will still leave 10 to 20 per cent survivors, representing a high risk for herbicide resistance unless other WeedSmart strategies are used to eliminate seed set in those survivor plants.”

Ryan’s recovery strategy begins in the summer fallow period. He suggests adding residual chemistries to the tank mix with paraquat when applying the second knock on fleabane and windmill grass over summer to take the pressure of glyphosate. Optical spot spraying can also be part of this strategy to eliminate any remaining weeds after applying the residual.

Next, a diverse crop rotation underpins an effective herbicide program, allowing growers to implement the WeedSmart Big 6 strategy to mix and rotate herbicide mode of action groups. This is essential when it comes to integrating pre-emergent herbicides into the program.

Ryan suggests a 5-year crop and pre-emergent herbicide rotation such as wheat (using pyroxasulfone, Group 15 [K], e.g. Sakura as the base), canola (propyzamide, Group 3 [D], e.g. Rustler), wheat (bixlozone, Group 13 [Q], e.g. Overwatch), barley (prosulfocarb + s-metolachlor, Group 15 [J, K], e.g. Boxer Gold), pulse (carbetamide, Group 23 [E], e.g. Ultro). He says there are opportunities throughout a crop rotation like this one to implement many WeedSmart Big 6 tactics to keep weed numbers low and herbicides working well for longer.

While appreciating the value of some synergistic pre-emergent herbicide mixes, Ryan is cautious about applying too many residual actives at once.

“It is important to choose mixing partners carefully to get the best result without causing undue stress on the crop or restricting your ability to rotate herbicide groups through the cropping program,” he says. “The greatest risk is using Group 15 too frequently as mixing partners rather than on their own and potentially exposing the weeds to the same chemistry almost yearly.”

Winter crop weed control starts a few weeks before sowing with a double knock tactic. In the first pass, glyphosate initiates the control of a range of grass and broadleaf weeds. Approximately seven days later, the second knock of paraquat mixed with the chosen pre-emergent herbicide for the crop being sown is applied.

“If weeds are actively growing when the pre-emergent herbicide is applied, they will take up and metabolise that herbicide and prevent it from reaching the soil surface where it is needed,” says Ryan. “This is why it is better to apply the pre-emergent in the second pass after the weeds have started to shut down. We need to set up these pre-emergent products for success by maximising soil deposition and then using strategies to minimise the amount of weed seed entering the seed bank at the end of the season.”

Strong crop competition is critical to the success of pre-emergent herbicides, as these products will never provide season-long weed control. In the Central West, the most potent mechanism to maximise crop competition is to choose competitive varieties and sow them toward the front of the optimal sowing window. Row spacing of 30 cm or less also helps suppress weed growth and reduce weed seed production, particularly toward the end of the season.

The final step is to implement harvest weed seed control. Ryan says it doesn’t matter which method you choose, provided it is used consistently across the farm.

“One strategy that has worked very well for growers in this region involves using a chaff line chute in cereal crops, sometimes for a few successive years. Then switch to a narrow windrowing chute in the following broadleaf crop to place all the crop residue on the previous years’ chaff lines and burn the windrows in autumn,” he says. “This is a low-cost harvest weed seed control system that ensures full destruction of the weed seeds collected over a few years while only burning the broadleaf stubble.”

The sooner an integrated approach to weed management is implemented, the more herbicide options will remain available, and growers can enjoy the benefits of farming with low weed pressure.