Broadleaf weeds have become an increasing problem in Mid-South rice production over the last decade. The reasons for this are debatable. However, two contributing factors in Mississippi are the prevalence of the Clearfield rice system and the spread of herbicide-resistant Palmer amaranth.

Newpath and Beyond do not control hemp sesbania and volunteer soybean, which are both among the top 10 most common weeds of Mississippi rice. Most populations of Palmer amaranth in the rice-producing area of Mississippi are resistant to glyphosate and ALS-inhibiting herbicides like Newpath and Beyond. Herbicide resistance in Palmer amaranth populations in Mississippi have allowed this species to increase in importance. Consequently, Palmer amaranth is second in importance only to barnyardgrass as a weed of rice in Mississippi.

Currently, few broadleaf herbicides that exhibit soil residual activity are available for rice. Sharpen is a Group 14, or protoporphyrinogen oxidase (PPO) inhibiting herbicide, manufactured by BASF Corporation. It is labeled for fallow, preplant and preemergence application in a variety of grain crops. A preplant label for rice was approved for Sharpen in 2011; however, applications were restricted to 15 days prior to planting. Current labeling will allow in-season applications to rice in 2014.

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Sharpen efficacy in rice has been evaluated at the Delta Research and Extension Center in Stoneville since 2010. From 2012 to 2013, this research was coordinated by Garret Montgomery, a master’s student at Mississippi State University housed at the Delta Research and Extension Center. This research demonstrated that postemergence applications of Sharpen at 1 ounce per acre controlled hemp sesbania and ivyleaf morningglory as well as Aim, which is another PPO-inhibiting herbicide often applied postemergence for broadleaf weed control in rice.

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Sharpen efficacy as a postemergence treatment in rice was not impacted by the type of adjuvant in the Mississippi research. However, crop oil concentrate will be the recommended adjuvant to include with Sharpen to minimize early-season rice injury. Rice injury was observed following applications of Sharpen at two ounces per acre (twice the labeled rate), but this did not translate to delays in rice maturity or reductions in rough or milled rice yield. Rice injury following Sharpen applications was more severe when applications were made during cool and/or wet conditions.

Palmer amaranth

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.