Optimizing Smutgrass (Sporobolus spp.) Control in Bahiagrass Pastures.

Smutgrass (Sporobolus spp.), a native of tropical Asia, is a serious perennial weed that affects improved pastures in Florida.  There are two species of smutgrass currently in Florida: small smutgrass (Sporobolus indicus) and giant smutgrass (Sporobolus indicus var. pyramidalis).  Small smutgrass was first observed in Florida in the 1950s and by the 1970s it was estimated that it infested greater than 70% of the improved pastures of central Florida.  Giant smutgrass was first detected in south Florida in the early 1990s and rapidly displaced small smutgrass in central and south Florida.  Regardless of species, smutgrass is a major problem in grazed pastures due to its low palatably among cattle and rapid spread.  The only herbicide labeled for use on pastures to control smutgrass is hexazinone. However, giant smutgrass is clearly more aggressive than small smutgrass, but it is unknown if a higher hexazinone rate than is used for small smutgrass control is required.   A rate titration experiment was conducted to determine if a higher rate of hexazinone was needed than currently recommended for small smutgrass.  There were not any significant differences found in the hexazinone rate between the two species and both required approximately 1.12 kg/ha hexazinone for optimum control.  A second set of experiments examined the use of adjuvants to improve hexazinone efficacy on giant smutgrass control.  It was determined that an adjuvant is not needed to optimize smutgrass control with hexazinone as long as rainfall was sufficient to incorporate hexazinone into the soil solution for uptake by the plants.  In conclusion, both smutgrass species require the same hexazinone rate for control and an adjuvant is not needed.