We are getting new herbicides — do we need to worry about diverse weed management now?

Take home messages

  • We know that ryegrass and other weeds can be beaten by ‘driving down’ the seed bank using a diverse range of tools. Many growers who have been on this program for a decade or more are having a win. It is not easy, but we can win, however, it takes more than herbicides alone.

Introduction

Growers have been told for years that there are no new herbicides on the horizon to fix herbicide resistance problems, but alas, some new herbicides are on the way. It is possible that there will be several new herbicides on the market in the coming years, possibly even a new mode of action. Do we need to continue on the path of adopting diverse weed management practices, including herbicide and non-herbicide tools, or will the new products fix the problems?

You will probably guess that I will suggest that there is a need to use a diverse range of weed control tools, and below are a few reasons why I believe that diversity is still the answer. Do not get me wrong — I support new herbicides and am excited about the opportunity they bring. I believe we can get the best out of them by continuing to adopt some non-herbicide tools to add to the farming system.

EP162 – metabolic cross resistance

Last year, I saw the most alarming herbicide resistance data during my 25 year long career. Below is an excerpt from the Australian Herbicide Resistance Initiative (AHRI) insight, August 2018, summarising this data.

AHRI insight #105

A population of ryegrass from Eyre Peninsula, South Australia called EP162 may just be the world’s most herbicide resistant ryegrass population.  It has been confirmed resistant to all of the pre-emergent herbicides – Avadex®, Arcade®, trifluralin, propyzamide and Sakura®, as well as two lesser known herbicides, EPTC and thiobencarb. It was sampled in 2014 during the random survey led by Peter Boutsalis from the University of Adelaide with GRDC investment, just two years after the release of Sakura® in Australia. You guessed it, metabolic cross resistance is at play. Paraquat and the triazines still work on this population, but that is it!

Unpredictable pattern of cross resistance

A random survey in the South East of South Australia, also by Peter Boutsalis and team, found many more populations of ryegrass with multiple cross resistance to a range of pre-emergent herbicides, and the perplexing thing is that there is no predictable cross resistance pattern.

The only herbicide that was spared was Edge® (propyzamide), but the bad news is that EP162 may be the first ryegrass population in the world with confirmed propyzamide resistance. This is the subject of further research.

This is the research by the team of Chris Preston, Peter Boutsalis, David Brunton and Gurjeet Gill from the University of Adelaide with GRDC investment.

This is the worst herbicide resistance news that I have seen in my 25 year long career simply due to the fact that so many herbicides are failing simultaneously.

What does this mean for new herbicides?

Previously, new herbicides were tested on susceptible weeds. Now the new herbicides not only need to kill the susceptible weeds, they need to kill cross resistant monsters like those detailed above. The cross resistance pattern to the herbicides is completely unpredictable. How would a herbicide company feel about bringing out a product with a label claim that ’this product might kill your weeds, depending on its cross resistance status’? This makes for a very challenging environment for the chemical companies. On the upside, there is the ability to test new products against the ’cross resistant monsters’ before they come to market.

Test

Herbicide resistance testing has never been very popular, and at times it has been hard to see the value of the test because it often just confirmed what was already suspected. Not anymore. Testing will become a high priority in years to come. Growers will need to know if the pre-emergent herbicides they are applying will work. Given that the cross resistance pattern is completely unpredictable, the only way will be to test — a lot.

My case for the need to continue with diverse weed management

  1. Some weed populations will be resistant to the new herbicides before they are even released to market.
  2. Pre-emergent herbicide + crop competition = high level control. The best pre-emergent herbicides give approx. 90% control of ryegrass. Adding crop competition can boost this into the high 90s.
  3. Late germinating weeds. Weeds have adapted to germinate late to avoid knockdown and pre-emergent herbicides. Crop competition, stopping seed set and harvest weed seed control are needed to tackle these.
  4. Keep the cost down. New herbicides are not cheap. If the life of low cost, off patent herbicides can be extended, growers can contain herbicide costs and use new, more expensive herbicides strategically and in mixes with older products.
  5. EP162.
  6. There is no sign of a new knockdown herbicide that I am aware of. We need to keep glyphosate and paraquat working.
  7. Growers need to be confident about the future of cropping. If they feel in control of their weed seed bank, through the use of a diverse range of tools, they can be confident that they will be able to maintain a profitable cropping rotation, regardless of what resistance issues they face.

Want a little more information about diverse weed control? Check out the WeedSmart Big 6.

Conclusion

We know that ryegrass and other weeds can be beaten by ‘driving down’ the seed bank using a diverse range of tools. Many growers who have been on this program for a decade or more are having a win. It is not easy, but we can win, however, it takes more than herbicides alone.

Contact details

Peter Newman
Planfarm
Geraldton
08 9964 1170
petern@planfarm.com.au