When I was a young agro, I thought I knew everything.
I could mix up a cocktail of herbicides, without it turning to snot in the tank, and I could kill some weeds.
Sometimes grass and broadleaf weeds in the same pass – this was hero status
Fearless and dangerous.
And then a couple of years later, I worked out I knew nothing
And then I became useful.
Back then, we were learning about the first cases of herbicide resistance. In Australia, this was ryegrass that was resistant to fops (group 1 / A) and SU’s (Group 2 / B).
We thought we had it all worked out.
Resistance takes 4 to 6 shots of a herbicide, and there’s two main types. Metabolic and target site.
Metabolic resistance was lower level, and sometimes higher rates worked.
Target site resistance was like an on/off switch. You get target site resistance, and then it doesn’t matter how much herbicide you sprayed on the plant, it simply didn’t work.
Naturally, we thought this would apply to all other herbicides.
As with my early career confidence, we thought we knew it all, then a couple of years later, we did some more research and found out that nature bats last.
In many cases, the rules of metabolic and target site resistance hold true, but there’s more exceptions to the rule than there are examples.
Then along comes Clethodim, or Cletho as we like to call it.
Being Aussies, it was always going to be Clethy or Cletho.
Clethoooo!! Maaaaate!!!
Cletho was released as the product Select. It cost a lot more than the other products, but it worked
At half the rate.
Then, a few years later it only worked at the full rate
Then it only worked at double the rate, and so on and so on….
But it kept working. Resistance turned up, we upped the rate, and got away with it. And most of the legume crops could handle a high rate without crop damage.
Based on what we knew about Fops and SU’s, we naturally assumed that the resistance mechanism was metabolic, because these high rates overcame the resistance.
Wrong!
It’s all target site, but not the on off switch. Think dimmer switch instead.
Long story short, Cletho has kept working, albeit at higher rates, for about 20 years longer than we thought it would last.
It stayed with us long enough to get us to the point where new pre-em herbicides (or some old ones with a new lease on life) arrived to save the day.
No pre-em will ever be as good as a 99% post-em herbicide. They rely on soil moisture conditions, and they gradually wear off as the season progresses.
Compare this to waiting until all of the grass has emerged, then nuking it with a 99% post-em with nil crop effect.
Those were the days!
I feel like a boomer reflecting on the housing market.
But the pre-ems are very good, and they’re keeping the show on the road.
What’s the point of this discussion? It’s a few things.
- If Cletho is still working for you, you’re sitting on a gold mine. You might be a lower rainfall grower who hasn’t grown many legumes or canola in the past, or you may have looked after it by always using pre-ems in combination, as well as things like crop topping and harvest weed seed control. If this is you, keep it that way, and keep using all of the tools to extend the life of this valuable resource. We know that resistance level will creep up slowly over the years, so use this to your advantage. Also, be careful not to import some high level Cletho resistant ryegrass in seed or hay. This will undo all of your hard work in a heartbeat.
- If you do have significant Cletho resistance, and you need to keep putting up the rate to get a result, develop a plan. You know how this is going to play out: the resistance won’t go away, and you can’t put the rate up forever. You need a smart plan for the future.
- If Cletho is totally broken, and even the highest rate doesn’t work, it’s all about the seedbank from here on. You’ll be using the great pre-ems that we have, and you’ll need every tool in the box to keep cropping.
The other thing that we learnt along the way is that high rates of Cletho can upset the soil bugs and bugger the nodulation of legume crops and pastures. This, along with MRL limits of our export grain, are reminders that continually putting up the rate isn’t the total answer.
By all means, keep using Cletho as long as you can, but if the rate is creeping up, it may be time to take another tool out of the box.