Cotton growers in the region often rotate cotton with peanuts and either conventionally-till the soil or use a common conservation practice, strip-tillage. When they strip-till, they typically use rye as a cover crop, spraying it with a herbicide in March or April to kill it. The dead ryegrass provides a mulch cover for the fields.

When herbicides like fomesafen are sprayed, the mulch can intercept the chemical and prevent it from reaching the soil where it will be most effective. Herbicides intercepted by mulch can also damage cotton crops if they wash off after the cotton germinates.

Potter and colleagues evaluated how well a conservation practice known as “irrigation incorporation” would wash the herbicide off the mulch and move it into the soil, reducing the potential for crop injury and excessive runoff. Irrigation incorporation involves irrigating a few days after applying a herbicide. The practice greatly enhances weed control by improving herbicide contact with germinating weeds.

They divided a field of cotton equally between strip-tillage and conventional-tillage. In the strip-tillage section, they planted rye as a cover crop. Fomesafen was applied to the whole field, and irrigation incorporation was used on half of it. They then applied simulated rainfall and diverted runoff into troughs at the lower end of the fields for analysis. The rye crop residue was also collected and analyzed.

The results, published in the Journal of Agricultural and Food Chemistry (2011), showed that fomesafen is more likely than other herbicides to wash off surface residue and penetrate into the soil as desired. Fomesafen’s “wash-off rate” was much higher than that of other herbicides studied.

The results demonstrated the benefits of using irrigation incorporation with fomesafen, particularly when conservation-tillage is practiced. The product’s high wash-off rate means that by applying a small amount of irrigation after the herbicide is applied, most of it will move into the soil, where it will not damage the cotton and will be most effective at controlling weeds.

The results also showed that irrigation incorporation substantially reduces runoff of fomesafen and minimizes the potential for adverse water-quality impacts.

The results will help growers concerned about glyphosate resistance make better-informed decisions about herbicide alternatives.

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