By CINDY BENJAMIN
The mix-and-rotate WeedSmart Big 6 tactic aims to increase diversity within a herbicide program to delay and manage herbicide resistance. While rotating modes of action is relatively simple, mixing is much more complicated.
With this in mind, Dr Chris Preston and the weeds research team at The University of Adelaide and SARDI conducted a three-year trial at two sites in South Australia and Western Australia to investigate the complexities of the ‘mix-and-rotate’ weed control tactic for managing annual ryegrass with known resistance to Group 15 [K/J] and Group 13 [Q] herbicides.
Herbicide resistance in weeds such as annual ryegrass has two phases.
First, frequent application of a particular mode of action selects for the survival of those individuals with resistance mechanisms.
Second, these individuals set seed, and the proportion of resistant individuals increases in the weed population until the herbicide is rendered ineffective.
How much you can delay the onset of herbicide resistance through seed set reduction is debatable and varies between weed species.
It depends on many things, such as how much weed seed enters the seed bank each year, what proportion of seed is susceptible to the subsequent herbicide mode of action, whether the seed is short-lived or persistent in the seed bank, or if the seed has variable dormancy traits.
“Herbicide rotation is a proven tactic that delays the onset of herbicide resistance because you use each mode of action less frequently,” Dr Preston said.
“This tactic relies on having a diverse crop rotation in place that allows the grower to use the widest array of chemistry throughout the rotation to keep weed numbers low and minimise the risk of resistant individuals surviving and setting seed.”
As a resistance management strategy, herbicide mixing is far more complex than rotating modes of action. The effectiveness of this strategy can vary markedly and requires considerable thought before a mix can be recommended.
To begin with, it is essential to know the herbicide resistance profile of the weeds present in the paddock.
For a herbicide mixture to be an effective herbicide resistance management tactic;
If the tank mix is designed solely to control a spectrum of weed species, it will not assist with herbicide resistance management for those different weeds.
For a pre-emergent herbicide mix to provide long-term efficacy, the products in the mixture need to be activated simultaneously and provide a similar length of control of ryegrass.
If one is more soluble than the other, or if one breaks down faster, there will be times when just one of the herbicides is active and applying selection pressure, potentially leading to faster evolution of resistance.
Populations of annual ryegrass commonly have a much higher percentage of individuals with mechanisms (mutations) for resistance to some herbicides (e.g. Group 2 [B]) than others (e.g. Group 9 [M]).
This is one reason why resistance seems to occur at different rates in the field, even if the two herbicides are applied at similar frequencies.
If the population's resistance allele frequency to one component of the mixture is high, this can increase the chance of selecting individuals with resistance to both modes of action in the mix.
This is why mixes that include a Group 2 herbicide often rapidly fail due to resistance.
With outcrossing species like annual ryegrass, using an inappropriate herbicide mix can result in a more rapid evolution of resistance than if the components were simply rotated within a diverse cropping rotation.
When a weed population has known resistance to some herbicides, it is important to design a strategy around maintaining low weed numbers and limiting seed set.
“In the trial at Roseworthy, we planted two populations of annual ryegrass, both with resistance to Group 15 herbicide, into an existing annual ryegrass population,” Dr Preston adds.
“We implemented a wheat, faba bean, wheat rotation for the three years of the trial and applied five different herbicide strategies,” he said.
“There was background resistance to trifluralin in the weeds already present in the trial area, so trifluralin was not used in the herbicide treatments.”
The aim was to test the efficacy of the mix-and-rotate strategy as a resistance management tool. The five treatments were:
In this trial, crop emergence was the same across all treatments, and there were significant differences in weed density, weed seed head production and crop grain yield between treatments.
Importantly, not using any herbicide (treatment 1) resulted in more weeds and less crop yield. The nil herbicide treatment resulted in the highest ryegrass density, most weed seed heads, and grain yield was about 50 per cent less than in the best herbicide treatment.
“Rotating the Group 15 herbicides (treatment 2) provided reasonable control and supported crop yield,” Dr Preston explains.
“But applying the same Group 15 mixture (treatment 3) failed within the three years with higher weed seed head numbers and lower grain yield.
“Rotating the mix of Group 15 herbicides (treatment 4) and rotating single herbicides from non-Group 15 modes of action (treatment 5) produced similar results and were the cleanest and highest-yielding treatments.”
The researchers also tested the mix-and-rotate strategy to manage a population of annual ryegrass resistant to both Group 15 and Group 13 herbicides. Three treatments were applied to this population:
Again, applying no herbicide was disastrous for weed seed production, and grain yield was 50 per cent less than the herbicide treatments. Rotating Group 13 with a mix of Group 15 herbicides and applying the same mix annually were not significantly different in weed seed production or grain yield.
These trials illustrate there is unlikely to be a perfect herbicide strategy. Appropriate mixing and rotating can be valuable strategies, and there is a place for rotating mixtures, but they can’t be relied on completely. This is why it is essential to stack as many other tactics as possible to build a robust and integrated program of herbicide and non-herbicide tactics to control weeds. This means employing crop competition and weed seed reduction practices wherever possible, in addition to herbicide use. It is also crucial to test for susceptibility to herbicides that you plan to use.
For more information about integrated weed management, visit the WeedSmart website: www.weedsmart.org.au
