Can diverse weed management push back on resistant weed numbers?

There is good evidence across Australia that farming systems using diverse weed control strategies are better placed to resist new weed incursions and loss of herbicide efficacy.

Dr Marta Monjardino, a farming systems economist at CSIRO, says that stacking practices that each provide high-level weed seed control can help protect a farming business from the economic impact of glyphosate resistance.

“Using the Mallee regions of Victoria and NSW as a case study area, we analysed the cost of gaining glyphosate resistance in annual ryegrass,” she says. “Whether the incursion of weeds with herbicide resistance comes from within the farm gate or beyond, we have shown that adding tactics to minimise additions to the weed seed bank can reduce the cost of managing these resistant weeds by 80 per cent. Without these extra tactics, losing glyphosate on a typical Mallee farm will likely cost around $18 per ha annually.”

“Implementing a diverse rotation of competitive crops, early seeding, pre-emergent and in-crop herbicides, crop topping and harvest weed seed control are all tactics that growers can employ to reduce the impact of glyphosate-resistant annual ryegrass on their bottom line.”

The analysis used the RIM model to test six realistic scenarios that growers in all regions will likely face at some time with annual ryegrass. RIM is a rigorous way to quantify the effect of different management strategies on ryegrass populations over time.

“Growers and agronomists from all regions can use models from the RIM ‘family’ to trial-run robust cropping systems that keep herbicides working effectively by keeping the weed seed bank low,” says Marta. “There is strong evidence that the rate of herbicide resistance is slowing in regions where more diverse systems have been in place for many years.”

The WeedSmart Big 6 outlines robust weed management strategies that help growers reduce the weed seed bank and grow profitable crops. Although the way tactics are deployed may vary from farm to farm and from region to region, the common thread is a focus on reducing additions to the weed seedbank throughout the crop rotation.

What scenarios were tested?

The study compared two weed management systems – 1. a system without additional weed seed management tactics and 2. a system that included additional tactics that provided an average weed seed control of 80 per cent.

Crop topping with glyphosate to reduce weed seed set and harvest weed seed control were the key tactics that differentiated the two management systems.

For both management systems, the analysis tested the impact of declining glyphosate efficacy on annual ryegrass from 95 to 40 and, finally, zero per cent efficacy. When glyphosate resistance increases for annual ryegrass, it is assumed that the grower would continue to use this herbicide to manage other weeds in their paddocks. 

What crop and weed management systems were analysed?

The analysis was based on a typical 2000 ha farm in a low-rainfall Mallee environment. The test farm had a winter cropping program based on cereals (wheat and barley) with break crops of canola and the occasional legume crop. An initial ryegrass density of 20 plants per m² was assumed. The break crops were assumed to be dry sown, while the cereals were sown after the season break. Standard seeding rates were assumed for each crop.

Selective in-crop and pre-emergent herbicides were assumed for each phase of the rotation. The difference between the two management systems was the use of glyphosate for crop topping and harvest weed seed control in 6-out-of-10 years to minimise replenishment of the weed seed bank. 

The following results rely on these selective herbicides remaining fully effective for the ten years of the analysis and beyond.

What was the impact of adding tactics to minimise weed seed recruitment?

While the annual ryegrass population remained susceptible to glyphosate, the additional weed seedbank control measures made very little difference to the ryegrass density after ten years or the average gross margin for the rotation.

Where the efficacy of glyphosate on annual ryegrass had dropped to 40 per cent and no seed bank control tactics were applied, there was a distinct increase in ryegrass density and the number of seeds entering the seedbank, and an average reduction of the gross margin by about $14/ha.

Where glyphosate efficacy on ryegrass was nil, additional weed seed control tactics kept the weed density and seedbank numbers near zero. Although the ryegrass population was fully resistant to glyphosate, using weed seed control tactics underpinned a gross margin equivalent to that achieved when glyphosate was 95 per cent effective.

In contrast, when no weed seed bank tactics were applied in a zero-efficacy scenario, a population blow-out was evident, and there was an $18/ha/yr reduction in the annual gross margin.

Marta says the pre-emptive adoption of weed seed bank-focused practices can reduce on-farm resistance selection pressure and increase the cropping system’s resilience against gaining resistance from sources outside the farm gate. These weed seed bank control practices also potentially reduce the risk of spreading weeds or resistance traits to neighbouring farms.