With the widespread problem of glyphosate resistant weeds, many soybean producers are relying on residual herbicides to control problematic weeds. Metribuzin (Sencor, Canopy, etc.) is a PSII inhibitor (Group 5) herbicide that provides residual control of an assortment of annual grasses and broadleaf weeds in soybean, including Palmer amaranth. With the extensive use of PPO inhibitors (Group 14) and chloroacetamide (Group 15) herbicides in soybean and rotational crops such as corn and cotton, use of an additional mode of action (MOA) is a sound strategy to reduce the risk of resistance to these other herbicide MOAs.
The handicap to metribuzin use is the sensitivity of soybean varieties to this herbicide. Other environmental factors, including: soil texture, organic matter, rainfall, soil pH, and product use rate, may also play a part in sensitivity. The University of Arkansas Division of Agriculture ran a screen of soybean varieties testing their tolerance to metribuzin. In this screening, soybean varieties were tested using 0.5 lb ai/A of metribuzin on a silt loam soil. The injury scale used for this screening is as follows: tolerant (<1), slight (1-3), moderate (4-6), severe (7-9), and death (10). Please note that most varieties show an acceptable tolerance to metribuzin. Choose a variety with a high level of tolerance.
Many combination products (Authority MTZ, Boundary, etc.) have use rates that contain lower amounts of metribuzin than what was used for this screening. With these reduced rates, soybean varieties may display a lesser amount of damage than what is reported here. Therefore, extreme caution or alternative weed control options should be explored for varieties rating moderate and more severe injury.
The data from this year’s screen are reported in the fact sheet below and will remain available on Arkansas Row Crops under the Publications tab.