Herbicide Use and Crop Safety

A/Prof, Chris Preston, of Adelaide University, talks about the issues around crop safety and herbicide use.

Crop safety and the Issues

So to understand the issues around crop safety, we need to understand three things. One of which is the solubility of our herbicides. The second one is how they’re boundin soil, and the third one is the inherent tolerance of our crops. Solubility is about how much of the herbicide dissolves in water, and herbicides move through the profile primarily after it rains and the water moves down through the profile.

Solubility of Herbicides

Your herbicides have a wide range of solubility. Things like trifurlin have got almost no solubility which means they don’t move in the soil very far. Sakura has got more solubility, Boxer Gold’s got more again and then we get into things like Logran that are really quite water-soluble by comparison.

Binding Methods Affect Herbicide Behaviour in Soil

The other thing that influences herbicide behavior in soil is how they get bound. The sort of things that bind herbicides, they tend to be clays, and organic matter. A sandy soil with very little organic matter, it’s not going to bind herbicides very much at all, and the herbicides are going to move down further into that soil.

So the herbicides get down. Keeping that they get down to the root zone of the crop that’s when they are going to cause damage. The other thing we need of course is crop safety. Some crops are sort of inherently more tolerant to certain herbicides than others. You’re gonna have barley being more tolerable to trifurlin for example than wheat.

Barley’s more tolerant to Boxer Gold than wheat. But it’s the opposite way around for Sakura. You can manage it by seeding depth. So, if you put the crop a little bit deeper, then you’re going to reduce the likelihood of enough herbicide getting down there to damage it, or you think about it by reducing rate. One of the other soil factors that can come into play is pH.

This gets quite complicated because it actually links in with herbicide chemistry. A number of our herbicides become more soluble at high pH, particularly things like sulfonylurea herbicides. What happens in high-pH soils is because they’re more soluble, they’re gonna move further. Other herbicides imidazolinones herbicides become less soluble, and a high pH are more likely to move further than low pH soils.

So that’s just another another factor that comes into play in sort of when we get crop damage. Seeding depths is probably a really key one with those disc systems. If we don’t get that seed down away from the herbicide we’ll almost always get crop damage. Second thing is doing something to get some herbicide away from the surface above the seeds, so not having herbicide in the furrow or in the slot.

Hair pinning is a problem. We get that in disc sowing systems. It’s probably actually interesting. You’ll have more of a problem with highly water soluble herbicides that are gonna cause crop damage because you have this water soluble herbicide sitting on the stubble and then punching down the crop row and as soon as it rains, it comes off and is going to damage the crop.

Optimising Herbicide Application

To optimize the situation, I think understanding the keys about your herbicide, your soil type, and your crop tolerance. You know, pick the more tolerant herbicides that you can do. I mean sometimes you can’t because you’re chasing weeds and you might not have a lot of choices.

So, the more tolerant herbicides where they’re available, crop seeding depth, getting that right, not too deep, because you don’t want the crop to really struggle out of the ground and be, you know, sort of not growing well. So you want an ideal seeding depth, particularly important for disc seeding.

You don’t want to be too shallow, you don’t want to be too deep and keeping as much of that herbicide away from where the seed is if possible.