What is the most important thing in weed control?

5 November 2023 by
Seed Terminator, Kelly Ingram

Given that this is coming to you from Seed Terminator, you’d probably think we’d say harvest weed seed control is number one.

But, weed scientists the world over will tell you that the factor that has the greatest impact on your weed spectrum and density, is crop rotation. Now is the time of year to plan your rotation for next year, and with the high costs of inputs, the stakes are pretty high.

A tight rotation of only one or two crop types is not really a rotation.

At the moment, canola : wheat or canola : barley rotations are going pretty well in terms of weeds.

The combination of pre-em grass herbicides with a competitive hybrid, glyphosate tolerant canola, is in the most part doing a good job on our problem weeds.

The cereal phase then offers us a range of new herbicides in wheat. In barley the herbicides are a little mediocre, but combined with competitive varieties and sometimes crop topping, we have some good options. This rotation is making a lot of money in medium and high rainfall areas, so growers are keen to keep riding this wave as long as they, but is it doomed to fail?

What are the risks of the canola : cereal rotation?

My short list of the major risks;

1.     Glyphosate resistant ryegrass. We have plenty of glyphosate resistant ryegrass around the country and we know that it will continue to develop. Plenty of ryegrass is hit with glyphosate alone in canola, and when crop-topping barley with glyphosate it’s the last herbicide that the ryegrass will see.

2.     Glyphosate resistant broadleaf weeds (e.g. wild radish, milk thistle, prickly lettuce). The only broadleaf weed control we get in glyphosate tolerant canola is glyphosate. We don’t really have any glyphosate resistant wild radish to speak of at the moment, but this would be a disaster if we start to see it. Keep an eye out and test if you suspect resistance.

3.     Multiple resistance to the pre-ems. We are starting to see some ryegrass resistance to pre-ems like Sakura and Boxer Gold, and this may cause cross resistance to some of the new herbicides. It’s pretty low risk at this stage, but it’s something to watch out for. On a positive note, Propyzamide is still looking very strong with no documented resistance yet.

4.     Sclerotinia. Bit of a curve ball as this is a canola disease, not a weed, but it’s a risk nonetheless.

What can we do to minimise the risks?

Of course I’m going to say, mix up the rotation a bit. Add some double breaks, or at least grow a combination of wheat and barley rather than just one or the other. But I know you’re going to say, “bugger that, we’re making too much money, we’re not changing rotation”

If you must stick with a tight rotation then do it smartly.

My short list of things you can do to keep a tight rotation alive

1. Don’t just keep rolling out the same herbicides every time because they worked well last time. We have some great new herbicides, particularly in wheat. Use them all.

2. Make sure you’re using plenty of paraquat in your knockdowns. Chances are the last herbicide your ryegrass saw was glyphosate. Don’t just throw more glyphosate at it.

3.  Instead of canola : barley : canola : barley why not canola : barley : canola : wheat? Or vice versa. This will add some herbicide choice to the rotation.

4. Mix glyphosate and clethodim post-emergent in glyphosate tolerant canola. One plus one equals three.

5. Choose the most competitive cereal varieties you can find.

6. Don’t create glyphosate resistance on your fencelines. If you’ve only got glyphosate in the tank when spraying fencelines, you’re doing it wrong.

7. Keep using harvest weed seed control. There’s no doubt about it, it would be nice to harvest with nothing but a chopper on the back of your harvester, but if you want to keep a tight rotation alive for a long time you really want to have a very low seed bank. If you must take it off for the canola then do so, but at least keep it going for cereals.

It all seems pretty easy now with glyphosate and the new pre-em herbicides working pretty well. But you’ll kick yourself down the track if these key herbicides stop working and you need to change back to a less profitable rotation. Even though running a Seed Terminator is an extra cost to your business, could it be the difference between a tight rotation succeeding or failing?

In short, if your rotation is a bit busted, then HWSC is a must.

What does a better rotation look like?

A more diverse range of crops (and pastures) in a rotation is always a good thing. Each crop type comes with different opportunities to target weeds, along with a diverse choice of herbicides. 

A diverse rotation includes;

1. At least four crop types and the more the merrier.

2. Double breaks. Lining up two break crops in a row such as legume : canola. Some growers even get a triple break e.g. hay : legume : canola

3. A range of seeding times for the various crops. Weeds get pretty accustomed to emerging at about the same time each year. Mixing it up a bit confuses them.

4. Diverse herbicides throughout the rotation.

5. Varied rotation cycles. Fast bowler Jeff Thompson once said, “I don’t know where the ball is going so the batsman has no idea”. Confuse the weeds by avoiding predictable rotations.

A rotation of just two crops can be pretty easy to manage and can make great profits in the short term, but if the farmer finds it easy, guess what, so do the weeds. If you must go with a tight rotation for a while for financial survival, then make sure you’re throwing more than just herbicides at it. Then start to think of a plan for down the track when you’re ready to change it up a bit, and add a bit of diversity.

The Terminator Agronomist
Proudly brought to you by Seed Terminator 

Please note this advice is general in nature and not based on your specific circumstances.

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