Why the obsession with the ryegrass seed bank?

Peter Newman, Australian Herbicide Resistance Initiative (AHRI).

Keywords: weed seed bank, weed seed management, herbicide resistance, WeedSmart

Take home messages

• Herbicides are not the answer to herbicide resistance. We must manage resistant weeds through seed bank management.
• Make the commitment to use DIVERSE tactics to manage the weed seed bank now.
• “The farmer calls the shots, not the weeds” (pers. comm. Ray Harrington).

Introduction

For years we weed scientists have been banging on about controlling resistant weeds through managing the weed seed bank using a diverse range of tactics. Why are we so obsessed with the weed seed bank? Surely new herbicides will come to the rescue? The answer is not as straight forward as it seems.

Here I put to you the reasoning behind the importance of managing the weed seed bank for sustainable resistant weed management. It is a long story, so get comfortable!

Low dose research

AHRI researcher, Dr. Roberto Busi took some nasty ryegrass that was already resistant to Sulfonylurea, Fop (Diclofop), trifluralin and metolachlor herbicides. Roberto exposed 100 seeds of this ryegrass to a low dose of Sakura® (60% of label rate) and kept the survivors. He then exposed the surviving ryegrass to full rates of Sakura® for three more generations, allowing them to cross pollinate with one another. The result was that Sakura® resistant ryegrass evolved in the lab before the herbicide had even been commercially released.

Cross resistant monster

Roberto was like a mad scientist working away in his lab creating a monster! He tested this Sakura® resistant ryegrass with other herbicides and found this ryegrass to be cross-resistant to Boxer Gold® and Avadex®.

Does it work both ways?

Unfortunately it does work both ways. Roberto did more research using low doses of Boxer Gold® to select for Boxer Gold® resistance in the lab and what did he find! Cross resistance to Sakura® as well! (Insert mad scientist laughter here. Mwahahahaha!!!)

Some hangovers last a lifetime!

It seems that the herbicides we sprayed in the 70’s and 80’s have ‘primed’ the ryegrass to later evolve resistance to Sakura®, Avadex® and Boxer Gold® in the lab. Applying sulfonylurea and grass selective (Fop) herbicides in the past has created ryegrass with metabolic resistance. This means that the plant is able to degrade the herbicide with enzymes before it can reach the target site. It is highly likely that this Sakura® resistance which evolved from low doses in the lab, is due to metabolic resistance. Further evidence of this ‘priming’ effect is that when Roberto started with susceptible ryegrass, he did not evolve resistance from low doses of Sakura®.

This is not all about low dose

Yes full label rates of herbicide are a good idea but this is not what I am getting at. Australia is now covered in ryegrass with metabolic herbicide resistance. Drs Qin Yu and Heping Han from AHRI took 33 populations of ryegrass from the Western Australian wheatbelt that were Hoegrass® (diclofop) resistant. 91% of these populations were found to contain target site mutation(s), 80% showed metabolic resistance, and 70% had both. While the herbicides Sakura®, Avadex® and Boxer Gold® are from differing herbicide groups, they are prone to metabolic resistance. Herbicide group choice may not matter a great deal in the face of metabolic resistance.

No cross resistance to trifluralin

In another low dose study, Roberto started with a different population of multiple resistant ryegrass from the W.A. wheatbelt that was not trifluralin resistant. Once again he evolved cross resistance to Sakura® and Boxer Gold® but not trifluralin. Finally some good news!

The mechanism of trifluralin resistance is unknown

Surprisingly, the mechanism of resistance to one of our oldest herbicides is not known? It seems that some herbicides are easier to study than others. We do not know the mechanism of 2,4-D resistance either and that has been around since the 1950’s. What Roberto has found is that the metabolic resistance that causes Sakura® / Boxer Gold® / Avadex® resistance in the lab, does not (so far) cause trifluralin resistance. He has also found no cross resistance to propyzamide between any of these herbicides.

Drum roll please!

The discussion thus far, has been a long winded way of once again saying that herbicides are not the answer to herbicide resistance. We must manage resistant weeds through seed bank management. Why? Because it is all well and good to bring out new herbicides from novel modes of action (groups) but the herbicide group may not matter much if the weeds resist the herbicide through metabolic resistance.

The good news

So far, metabolic resistance to Sakura®, Boxer Gold® and Avadex® has only evolved in the lab under ideal (arranged marriage) conditions. The critics of this research say that it won’t happen in the field. I am not so sure. Keep in mind that Roberto only exposed 100 ryegrass to one low dose of Sakura® then selected with high doses after that. Late germinating ryegrass in a wheat crop would be exposed to a low dose of Sakura® as the herbicide degrades in the soil through time.

The bad news

Australia now has millions of populations of ryegrass with a level of metabolic resistance. Selecting this ryegrass in the field with herbicides such as Sakura®, Boxer Gold® or Avadex® is likely to select for higher level metabolic resistance. This high level metabolic resistant ryegrass will be a massive challenge for future herbicides.

What can we do?

Managing resistant ryegrass is a walk in the park compared to some of the other species. It is relatively simple to control weeds such as ryegrass and wild radish through seed bank management. Growers who put their mind to it are having a win. And that’s the key – you have to put your mind to it. Once you have made the decision to manage the seed bank and have become obsessed with stopping weed seeds entering that seed bank you are 90% of the way there. We have a range of proven tactics that work with loads of experience from pioneering growers, scientists and agronomists alike.

“Make the commitment to use DIVERSE tactics to manage the weed seed bank now”.

Herbicide rotation

Despite herbicides not being the answer to herbicide resistance, they still play an important role. Roberto’s low dose research also taught us a valuable lesson in herbicide rotation. We now know that there is a future risk of cross resistance to Sakura®, Boxer Gold® and Avadex®, but at this stage no cross resistance to trifluralin or propyzamide. There is no evidence of cross-resistance between trifluralin and propyzamide in ryegrass and other grass weeds. So we need to put these herbicides into three distinct boxes and rotate herbicides from year to year between the boxes not within the boxes.

Where do I start?

Managing the weed seed bank is easier said than done and is more complex than simply relying on herbicides. There are some great resources on-line to get you on your way.

Start with RIM

The computer model RIM (Ryegrass Integrated Management) is a decision support tool that allows you to put some of your theories about ryegrass seed bank management to the test over a ten year rotation. RIM models the ryegrass seedbank and the rotational gross margin.RIM is available for free download from the AHRI web site.

Want some practical information about managing the seed bank?

For practical information about managing the seed bank access the WeedSmart web site and check out the 10 Point Plan. Harvest weed seed control is a good place to start. WeedSmart is an industry initiative funded by a combination of chemical / seed companies and GRDC to communicate a single, consistent message about the awareness of and management of resistant weeds. WeedSmart is currently managed by AHRI.

Stay in touch with the Australian Herbicide Resistance Initiative (AHRI)

Access the AHRI web site and subscribe to AHRI insight, our regular newsletter that keeps you updated with the latest in herbicide resistance research. AHRI insight lands in your inbox about once a fortnight, and is written in simple, plain English.

Back issues of AHRI insight are available on the AHRI web site blog.

And, as Steve Jobs would say, one more thing

Ray Harrington once said and it is important for us all to remember; “Farming with a low weed seed bank is much better than farming with a high seed bank. The farmer calls the shots, not the weeds”.

Contact details

Peter Newman
petern@planfarm.com.au