Irrigation to improve herbicide performance

Irrigation can greatly improve herbicide performance in dry soil conditions.

If you have irrigation available consider applying water to improve herbicide performance, when the lack of rainfall threatens to cause poor herbicide performance. Most farmers appreciate a nice rainfall the night after they finish applying soil-applied (pre-emergence) herbicides. Producers with irrigation can take advantage of their investment by making sure every irrigated field has optimum herbicide performance.

Commonly irrigated sandy loam soils of Indiana and Michigan need about a half inch of irrigation to wet the soil down to 5-6 inches. This amount most often is enough to incorporate soil applied herbicides. Heavier loam soils may need 0.7 inch to 1.0 inch of water to wet the top 6 inches of soil to activate herbicides.

Many herbicide labels have recommendations to use irrigation to improve performance or warnings to expect weed escapes if lack of rain leaves the herbicide incorporated. An example is the label from the commonly used Bicep II Magnum Herbicide. “Dry weather following an application of Bicep II Magnum or a tank mixture may reduce effectiveness. Cultivate if weeds develop.” Followed by, “If available, sprinkler irrigate within two days after application. Apply a half inch- 1 inch of water. Use lower water volumes (half inch) on coarse-textured soils, higher volumes heavier soils (1 inch) on fine-textured soils.”

The economics of using irrigation to help incorporate soil applied herbicides are potentially good. Most Indiana and Michigan farmer’s cost to irrigate with one inch of water is between $1.50 and $8.00 per acre. A post emergence rescue weed control program will often cost toward $10 per acre and the additional application cost is often greater than the cost of applying an inch of water.

In drought conditions irrigating fields prior to post emergence weed control applications can often improve performance. Glyphosate based herbicide labels often include wordage “apply to only actively growing weeds.” Drought stressed weeds have difficulty absorbing and translocating the active ingredient where it is needed to kill the plant.

One of the excuses most often cited for not taking advantage of the option to irrigate to improve herbicides performance in dry conditions is that the irrigation system was not ready to run. Having the irrigation system ready to run when you plant can makes the difference between looking at a weed free field that is primed for maximum yield potential or paying for post emergence control and hoping it will work.

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