This week I'm discussing the effect moisture has on postemergence herbicides. The relationship here is somewhat different than with soil-applied herbicides but equally important.

With a pre-emergence application of Command, Facet or Newpath, moisture is required after the application and you have no herbicide activity until you get moisture, because the herbicide first has to move into the weed seed zone in the soil, and it also has to become available to the germinating weeds in the water phase of the soil.

With a postemergence herbicide, the herbicide is available to the plant by being sprayed on it.

If things were that simple, soil moisture would not be a factor with postemergence herbicides. However, how actively a weed is growing has a big effect on how much of the herbicide is absorbed into the plant after it is sprayed.

Temperature and humidity can also affect absorption, but the biggest factor is usually soil moisture.

If the plant is drought-stressed, it closes itself up to conserve moisture and survive. When this happens, spray absorption into the plant is also greatly reduced.

Once the herbicide is absorbed into the plant, the killing activity occurs due to some type of effect on certain metabolic processes in the plant.

The specifics here vary by herbicide, but the principle is the same. More active plant growth results in more active metabolic processes and increased herbicide activity.

By the same token, when plant growth reduces or stops due to limited soil moisture, herbicide activity within the plant reduces or stops.

With pre-emergence herbicides, it is the moisture that occurs after the application that is important. In contrast, with postemergence herbicides, especially those that have no residual activity, it is the soil moisture at the time of the application that is important.

How much moisture you need depends somewhat upon the herbicide you have chosen. Keep in mind, however, that most weeds, especially grasses, germinate and root very shallowly.

A lot of grass seeds germinate right on the soil surface and most germinate in the top 1/8 to ¼ inch.

That means you can easily have drought-stressed grass even though you consider the soil moisture in general to be good and the rice crop is growing well. This is especially true on the clay soils. They will “check up” on the surface where the weeds are, and still be gummy wet down where the rice is.

I still remember the first time in my university years we were doing research with Clincher and Ricestar. We had excellent control from both in our research plots at Lonoke, Ark., and I was thinking, “These things are foolproof!” Ken Smith at Monticello, Ark., had both a cotton stop and rice stop at the Rohwer Field Day and talked me into doing the rice stop for him. He set my stop up in front of the same Ricestar and Clincher test that I had in Lonoke. The only difference was his test had been bush-hogged down because every treatment had failed miserably (which makes for an interesting talk).

This is not a criticism of the herbicides, but rather something we have learned about moisture relationships with these herbicides. Ken has learned that by the time the clay soils get firm enough to walk on with a backpack sprayer, he can have drought-stressed weeds on the soil surface.

How quickly the clay soils dry on the surface no doubt is a big reason why weed control in general is more difficult than on the silt loam soils.

Next week I will get into individual herbicides and some practical things you can do. I will also get into some of the herbicides that have both postemergence and residual activity.

I have tried to give you just a little herbicide physiology in simple terms — which is all I am capable of. I know it is easier to manage herbicide/moisture relationships from my computer than from the field. However, if I can convince you how important this relationship really is, you will find ways to implement the principles.


Ford Baldwin, Practical Weed Consultants. e-mail: ford@weedconsultants.com.