Much attention has been given recently to the herbicides with soil residual properties as our best tools to control glyphosate-resistant weeds.

Some are learning how to use these tools for the first time while those of us with a little gray in our hair may be re-learning some things we have forgotten about these products.

We all understand that rainfall is needed to activate soil residual herbicides, but how much rainfall is needed and long will the herbicide lie on the soil surface waiting for rain are questions most often asked. A good university answer to these questions is “it depends.”

Although this is true — factors such as temperature, amount of sunlight, and soil type do influence herbicide behavior — we do have some guidelines that may aid in utilizing the soil-applied herbicides.

Dr. Jason Norsworthy has just completed some studies that showed Reflex and Dual still being active after lying on dry soil 21 days prior to receiving activating moisture. Other commonly used herbicides, including Direx, Cotoran, Valor, and Prowl H2O, were still active after lying on dry soil for 14 days.

These types of comparisons are extremely difficult to conduct in the field in Arkansas because it is near impossible to predict a rain-free period of sufficient length to get good evaluations. Even though the studies mentioned above were conducted in the greenhouse, they give us some assurance that if used with a little planning, these herbicides will perform when the rainfall does occur.

There is seldom a year with a 14-day rain free period during planting time in the Mid-South.

The figures in the accompanying charts show 20-year summaries of rainfall data in Keiser, Ark.

Over the past 20 years, at least 0.25 inch of rainfall occurred within 10 days of any possible herbicide application in April 86 percent of the time and 80 percent of the time in any given period in May. It is easy to see that 1992 and 2002 were drier in April. There was a greater than 60 percent chance of the herbicide lying on the soil longer than 10 days before getting rainfall if it was applied at random. Some planning and good weather forecasting may have improved these probabilities.

Our greatest problem is not the herbicide breaking down prior to getting rainfall, but instead sufficient moisture for weeds to germinate, but not enough for herbicide activation.

Weeds germinating prior to herbicide activation must be controlled with postemergence herbicides. Unfortunately, we do not have many postemergence options for pigweed in soybean and cotton.

I mentioned in an earlier article the pros and cons of applying some of these herbicides preplant to increase our chances of having an activated herbicide when the crop is planted. This is an option, but it shortens the length of control in crop.

Note that I used 0.25 inch of rainfall in the data charts. This is usually sufficient to get some activation of the herbicide if it occurs in a single rainfall event instead of over a two- or three-day period. In Dr. Norsworthy’s data, Direx required slightly more moisture for activation than did Cotoran and Dual. Complete activation of all herbicides was achieved with 0.5 inch rainfall.

No doubt, herbicides with soil residual properties will be required to effectively manage glyphosate-resistant weeds in cotton and soybean. With some planning, these herbicides can be used effectively. Environmental factors such as temperature and rainfall have a profound effect on the length of residual control provided by the various herbicides.

We have studies under way to help make comparisons of the commonly used products and I will discuss this issue in another article.

e-mail: smithken@uamont.edu