For Release September 17, 2006
How Herbicides Work, Or Why They Don't
by Chuck Otte, Geary County Extension Agent
I'm going to try to condense an entire college semester's class on herbicides down into one brief article, so hang on! Herbicides are either pre-emerge or post-emerge. Pre-emerge products are applied, like crabgrass preventers, to kill the weed seedling at germination. Post emerge products are applied to growing weeds. You use a pre-emerge to stop what might happen, you use a post emerge to stop what has happened.
Herbicides work in many different ways, or what we call modes of action. Some are growth regulators that cause plants to grow themselves to death. The classic dandelion sprays containing 2,4-D are in this category. Some herbicides cause the cells to burst and lose all their water, other herbicides attack very specific chemical pathways in the plant. One of my favorites, are the triazine herbicides like atrazine. It interrupts the photosynthetic pathway. Photosynthesis is the key process that makes plants grow. It takes simple products and produces energy for plant growth. Atrazine is a chemical sponge that absorbs all the electrons half the way through the photosynthesis pathway. The plant dies, or is damaged because of a loss of energy for other plant growth processes..
Not all herbicides work on all plants. A chemical that stops a certain critical enzyme or plant process in sunflowers, will probably not stop a similar enzyme in crabgrass. In general, it is harder to fight a weedy grass in another grass crop (think of your lawn), and it's harder to stop a broadleaf weed in another broadleaf crop (like sunflowers in soybeans). Grass plants and broadleaf plants have many differences in how they function. So the herbicides have to be formulated differently.
Let's throw a twist in the system. A plant is not going to be equally susceptible to a herbicide at all stages of growth and under different environmental conditions. Small weeds are much easier to control than large weeds. Henbit, in the fall, is a small plant with just a few leaves. Most people will look right past it. It is easily controlled with herbicides. But wait until next March, when it is blooming bright purple, and it is very hard to kill. The amount of herbicide that you have to apply becomes so high, that you risk damage to the crop or nearby plants. And while it's dying it'll go ahead and produce seed anyway!
When the weather is good and plants are growing rapidly, they are often easiest to kill. But wait until the middle of a hot dry summer and you've got problems. The plant is under stress and most normal biological processes are not functioning like normal. A herbicide applied then, may not be taken up by the plant at all and so will not be effective.
Some plants have very small leaves, and it may have a lot of stems and roots. Spurge and puncturevine come to mind. The herbicide is actively taken up by leaves. If you have a lot of plant, but not a lot of leaves, it will take more herbicide applied to the leaves to affect the plant. Some herbicides are absorbed by plants quickly, others may take hours. A rain, too soon after a herbicide application, will render a control attempt a waste of time.
So why doesn't a herbicide work? Take your pick of possible problems. It could be that the crabgrass preventer has simply been deactivated by time, high temperatures or excessive rainfall. It starts with proper identification of the problem. Then you apply the right product, at the right time in the right dosage. It's simple in theory, but tricky in practice!
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