Herbicide Pathways – An Overview

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Video Transcript

To get a herbicide to kill a plant needs to do three things. It needs to get inside the plant. It needs to get to the tissue where the herbicide works, so that might be leaf tissue or it the neurosomatic tissue, and then it needs to inhibit the active side. For the herbicide to get into the roots, it going to be taken up just in ordinary water that moves through the plant.

Water Soluble Herbicide

So it just flows in the water, which means that you generally have to have your herbicide soluble in water in the soil to get it to taken up. That means that you need to understand the water solubility of the herbicide and you need to understand the soil type because it’s a combination between how water soluble a herbicide is and how much it binds.

So, herbicides tend to bind more to soils with high clay content.more organic matter and less to sandy soils. This also actually influences how far the herbicides move up and in the soil to very soluble herbicides will move a long way and they might get out of the root zone and not work because of that reason.

Herbicides Targeted at Leaf Entry

Looking at a herbicide that gets taken up by the leaf The environment is entirely different, because the leaf is covered in a waxy cuticle. So, here herbicide chemistry comes into play. so some herbicide is soluble in waxes and they will go through that cuticle just sort of with diffusion. Other herbicides aren’t and so they will move more slowly or they will move through little pores in the cuticle.

Charged herbicides, things like glyphosate can find it quite difficult and that’s one of the reasons why glyphosate has such a long rain fastness time compared to some other herbicides. The rooter has to take. It’s got negative charges on it, so there’s some repulsion and here things like adding ammonium sulfate with help with glyphosate just to get it into the leaf.

Of course, now we’ve got the herbicide inside plant we’ve got to get it to where it needs to go. Most herbicides, because they have some water solubility, move in the xylem. The xylem goes from ground to air, so they’re going to move up to the edges of the tips of the leaves. Some herbicides are phloem mobile, and they’ll move back down the plant neurosomatic area..

Plant Mobile Herbicides

Glyphosatee is a really good example of a plant mobile herbicide, herbicides like paraquat and atrazine are not really phloem mobile. Again that’s a lot about chemistry, but it’s also about what’s happening in the plant. So if you’ve got water stress, for example, well first of all the herbicides are not going to be taken up by the plant as readily and they’re also not going to move around because not a lot of water is flowing.

Equally, they’re not going to move in the phloem, in the photosynthate, because the plants aren’t photosynthesizing when they’re water stressed. The other thing that can happen to a herbicide inside the plant is it can be broken down. There are enzymes that degrade these. This is why a lot of our herbicides are selective in certain crops that have those enzymes, weeds, often too not to, but if you change the environment you can change the amount of herbicide that gets broken down.

Stressed Crops and Herbicides

And typically again we see this with stress, so stressed crops won’t breakdown herbicide as fast. That’s why if you put out selective herbicides under stressful conditions, you might actually get more crop damage than you would and some other conditions. Temperature, most things happen fast with temperature, so warmer conditions means your herbicides are gonna get in fast?

Are they going to move fast? Are they going to act faster? Of course if you get very high temperatures, that produces temperature stress and that tends to shut the plant down so things don’t move around as fast. Frost is quite difficult because it does a number of complex things but I think the important things are that it tends to reduce the amount of water moving in the plant, so herbicides aren’t taken up as readily and also tend to shut the plant metabolism down.

So herbicides are broken down as readily, they’re not moved and the phloem readily.

For a herbicide to effectively kill weeds it needs to get inside the plant, it needs to get to the tissue where the herbicide can work, and it needs to inhibit the active site.

Dr Chris Preston from the University of Adelaide explains how herbicides work at the Eyre Peninsula Agricultural Research Foundation (EPARF) members day held at the Minnipa Agricultural Centre.

The movement of herbicides in plants and soils depends on soil type, plant conditions and herbicide chemistry. Water solubility and soil properties are important for the activity of soil-applied herbicides whilst herbicide chemistry is important for leaf-applied herbicides.

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