Incorporating S-metolachlor

Follow these recommendations to properly activate this herbicide with irrigation.

By J. Ferrell and R. Leon, University of Florida Weed Scientists

Herbicide-resistant weeds have changed how we design our weed management programs. To combat these weeds, we are increasingly returning to soil-applied herbicides. However, a soil-applied herbicide has to be mixed in the soil, or activated, to be effective. If that herbicide is simply left on the soil surface, it will often breakdown in sunlight within seven to 14 days of the application.

An Essential Process

S-metolachlor, or Dual Magnum, is an example of a soil-applied herbicide that provides effective control of a wide range of weed species, including herbicide-resistant species such as Palmer amaranth. Other advantages are that this herbicide has a wide window of application timings, can be tankmixed with multiple preemergence and postemergence herbicides, and it has few rotational restrictions. Therefore, S-metolachlor is an excellent tool component of weed management herbicide programs for peanut production. However, incorporation after application is essential.

A fast and cost-effective way to incorporate herbicides, particularly in minimum-tillage systems, is with irrigation. But, how much irrigation is required to properly activate these herbicides?

Irrigation Rate Needed

SmetolachlorChartResearch was conducted at the University of Florida to determine how much irrigation S-metolachlor required to work at maximum effectiveness. The herbicide was applied in a location with a sandy soil and heavy Palmer amaranth pressure. The site was tilled to ensure all seedlings were destroyed, and the herbicide was applied to the freshly tilled area. We then applied irrigation at rates of 0.5 inches, 0.25 inches, 0.12 inches, 0.6 inches or zero inches. No additional rainfall or irrigation was received within the next seven days.

After seven days, the whole study received 0.5 inches of irrigation to stimulate weed germination. SMetolachlorThe number of Palmer amaranth seedlings emerging in these plots were then counted for the next 35 days to determine effectiveness.

Balancing Effectiveness, Cost

As expected, we found that S-metolachlor performed better as irrigation volume increased. We also found that the minimum amount of irrigation to gain effective control was 0.12 or 1/8 inch. Few differences between 0.12 inches and 0.5 inches were observed. We feel confident that one-half inch of irrigation is not required to adequately incorporate S-metolachlor, but we would not recommend using less than 0.12 inches. So somewhere between these two, such as one-quarter inch, should work well on most sites.

We would also expect that applying even a low amount of irrigation immediately after S-metolachlor application is better than waiting a few extra days or applying none at all.

S-metolachlor offers many benefits to a weed management program. However, a full one-half inch of irrigation is not always required for effective weed control. Using less irrigation can result in good herbicide activity while also reducing irrigation cost.