Initially, the most important use pattern for Sharpen application in Mississippi rice will be targeting Palmer amaranth. In Mississippi research, Sharpen at 1 ounce per acre and RiceBeaux at 3 quarts per acre provided similar control of small (one to three inches) Palmer amaranth. Furthermore, Sharpen at 1 ounce per acre controlled Palmer amaranth better than Aim at 2 ounces per acre following early- and late-postemergence applications. Increasing the postemergence Sharpen rate above 1 ounce per acre did not improve control of Palmer amaranth or other weeds.

As a preemergence application, Sharpen at 2 ounces per acre has consistently controlled Palmer amaranth better than Sharpen at 1 ounce per acre. Soil texture is an important point to consider when applying Sharpen preemergence targeting Palmer amaranth. Most rice in Mississippi is grown on fine-textured, clay soils. These soils are aptly referred to as “buckshot” due to their tendency to produce small clods after tillage, particularly if worked before the soil is completely dry.

Palmer amaranth only emerges from the top one-quarter to one-half inch of the soil profile. Residual control of Palmer amaranth with Sharpen has been reduced when applications were made to fields that were somewhat rough (cloddy) at application. This was likely due to the fact that the rainfall or surface irrigation needed for incorporation moved the herbicide below the germination zone for Palmer amaranth.

Rates and timing

Supplemental labeling for rice in 2014 will allow Sharpen applications at 1 to 2 ounces per acre as a preplant and/or preemergence application up to three days after planting. Preplant applications should include the previously labeled Sharpen adjuvant system of methylated seed oil plus urea-ammonium nitrate. Postemergence applications of Sharpen at 1 ounce per acre should be made from the two-leaf rice stage up to internode elongation. Crop oil concentrate is the only adjuvant allowed during postemergence applications of Sharpen to rice. Sequential applications are labeled but should be made at least 14 days apart.

Sharpen should contribute to the rice herbicide arsenal for broadleaf weeds. However, like all other herbicides, it is just a tool. It is not perfect. Sharpen is primarily a contact herbicide when applied postemergence, so application timing is critical. Although it has not reduced yield in Mississippi research, Sharpen can potentially cause rice injury if application conditions are poor. Finally, Sharpen is not labeled for postemergence application in any other crop grown in the rice-producing area of the Mid-South, so all necessary precautions should be taken to avoid off-target movement to adjacent crops.