Soybean Injury from Sulfentrazone

May 26, 2000
Soybean injury caused by the active ingredient (sulfentrazone) of Authority (sulfentrazone is also a component of Canopy XL and Command Xtra) has been recently reported from several sources. This season is not the first this type of injury has occurred, and an article in the last issue of the Bulletin ("FMC Receives Label for Command Xtra") briefly mentioned some concerns. While it is not yet clear how widespread this injury is occurring, the issue does warrant some discussion. What are the soybean injury symptoms caused by sulfentrazone? What are some factors that can contribute to soybean injury from sulfentrazone?

Sulfentrazone injury symptoms: Soybean injury symptoms are often noticeable shortly after soybean emergence. In severe cases, plants may actually die following emergence of the cotyledons, leaving necrotic cotyledons on the soil surface. Other injury symptoms include necrotic lesions on the soybean hypocotyl very near the soil surface, as well as reddish-colored spots or lesions on the cotyledons. Lesions on the hypocotyl may not always kill the young soybean plants but often create an area of weakened tissue that may lead to plants' snapping off during rain or high wind. Plants that survive early season sulfentrazone injury may remain stunted for a significant portion of the growing season. Even though soybean stands can be reduced by sulfentra-zone injury, this may not always translate into significant yield loss, as surviving plants can often compensate for reduced stands.

Factors Contributing to Soybean Injury from Sulfentrazone

Varietal differences: Similar to soybean varietal responses to other herbicides (metribuzin, for example), some soybean varieties are more sensitive to sulfentrazone than others. Previous research by weed scientists at the University of Illinois and other universities has indicated the range of soybean varietal response to sulfentrazone can be quite broad. Following the commercialization of sulfentrazone-containing herbicides, several soybean seed suppliers began screening their varieties for sensitivity to the herbicide, and many seed companies now make these data (sensitivity or tolerance ratings) available. If the soybean variety you will be planting is known to be sensitive to sulfentrazone, alternative herbicide selection would be suggested.

Application timing: The Authority label allows applications to be made up to 30 days prior to soybean planting, at planting, or after planting but before soybean emergence. Applications made after soybeans have begun to emerge will result in severe crop injury or death. Our experience has suggested that applications made at planting or after planting but before soybean emergence have resulted in cases of injury much more frequently than applications made 2 weeks or more before planting. Applications made 2 weeks or more prior to planting allow time for the herbicide to be evenly distributed into the soil by precipitation. Applications made at or after planting can place a greater amount of herbicide very near the emerging soybean, and precipitation following application (but before soybean emergence) would result in a high concentration of herbicide in the soil solution available for soybean uptake.

Soil characteristics: Soil characteristics, such as texture and organic- matter content, can influence soybean injury from sulfentrazone by influencing the amount of herbicide available in the soil solution for plant uptake. Soils with a high clay and organic-matter content tend to adsorb more sulfentrazone than soils with lower clay and organic-matter content. Adsorption to soil colloids reduces the amount of herbicide in solution and available for plant uptake. Thus, soil-applied herbicide rates are often determined by soil texture and organic-matter content, with higher rates often recommended for "black" soils. The Authority label suggests the higher end of the application rate range be used on soils with high organic-matter content (greater than 4%). While sulfentrazone injury has been observed to occur on "black" soils, the likelihood of injury would be greater on coarse-textured soils with 2% or less organic matter.

Environmental considerations: Environmental conditions that reduce soybean vigor can result in enhanced injury, compared to more favorable growing conditions. Sulfentrazone selectivity in soybeans is achieved through rapid herbicide metabolism by the soybean, and environmental conditions that slow soybean metabolism (cool temperatures, wet soils, etc.) may contribute to enhanced herbicide injury.

In our research trials, we have typically noted soybean injury from sul-fentrazone more frequently when planting no-till soybean, applying the herbicide after planting, and receiving significant precipitation after application but prior to soybean emergence. While several factors previously described can influence soybean response to sulfentrazone, special attention should be paid to variety selection if a sulfentrazone-containing herbicide is a part of your soybean weed- management program.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague