published in


April 2018

Impacts of Rainfall on Smutgrass Control with Hexazinone

Ona Report - Dr. Brent Sellers

Brent Sellers and Jose Luiz C.S. Dias

For questions or comments regarding this publication contact: Brent Sellers, University of Florida, IFAS

Smutgrass species have been problematic in Florida pastures for the last 60-70 years. It is a perennial bunch-type grass that is capable of producing at least 45,000 seeds per plant. Our recent work with seed germination shows that seed can germinate nearly year-round, but germination will most likely occur during the rainy season when soil moisture is relatively high. Although the hot and rainy conditions of summer are optimum for seed germination, it is common to see smutgrass seedlings in the spring and fall if soil moisture is adequate. Therefore, prevention of seed production is necessary to limit the amount of smutgrass spread. Preventing seed production, however, is extremely difficult considering that seeds are produced as early as March in south Florida, and mowing tends to stimulate seed-head production. 

Currently, the only viable option for smutgrass control is applying an equivalent rate of 1.0 lb/acre hexazinone (2 qt/A Velpar/Tide Hexazinone or 1.67 qt/A Velossa) during the rainy season (July through September). This amount of hexazinone is quite expensive relative to other weed control products and optimizing control with this herbicide must be taken into consideration. Rainfall after application is essential since hexazinone has relatively no leaf activity on smutgrass and must be absorbed by the roots. This being the case, no additional surfactant is needed when applying hexazinone. However, the amount of rainfall after application that results in control failures is not well understood, and this may be the main reason behind the observed variability in smutgrass control with hexazinone across the state. 

Greenhouse and field studies were conducted in 2016 and 2017 to evaluate the effects of rainfall after hexazinone application on smutgrass control. Smutgrass plants were established in gallon-sized pots in the greenhouse for at least four months prior to hexazinone application at 1 lb/acre. After allowing the hexazinone to dry for four hours, rainfall was simulated at 0, 0.25, 0.5, 1, 2, 4, and 8 inches. To evaluate the effects of rainfall under field conditions, a smutgrass infested bahiagrass pasture was treated weekly with hexazinone at 1 lb/acre beginning the last week of April and ending the last week in August. Rainfall was collected weekly to evaluate the effects of rainfall on smutgrass control. 

Data from the greenhouse study indicated that the amount of rainfall after hexazinone application significantly impacts smutgrass control. Rainfall amounts equal to 0.25, 0.50 and 1.0 inches provided good to excellent control of smutgrass at 30 days after treatment, however, rainfall greater than 1 inch provided insufficient control or complete failure (Figure 1). While this greenhouse data provides some beneficial information for the effects of rainfall on smutgrass control with hexazinone, we expect that soil micro- and macro-pores will be substantially different under field conditions, resulting in different levels of rainfall required for optimum control of smutgrass. 

Rainfall occurred throughout the spring in 2016 resulting in early season growth of smutgrass. Nearly 1 inch of rainfall fell within 7 days after our initial application of hexazinone on April 22, which resulted in approximately 65% control 30 days after treatment (Figure 2). However, when no rainfall was recorded the week following the April 29 application, only 20% smutgrass control was observed. Only 50% control was observed following application on June 3 when over 5 inches of rainfall were recorded the week following application, and after July 1 when no rainfall was recorded the week following application. In general, we observed acceptable levels of smutgrass control when rainfall was above 0.25 inches and below 3.0 inches under field conditions. Results appear to be similar from the 2017 study, with the exception of the very dry spring that resulted in nearly no control prior to the beginning of rainfall in late May and early June. However, we have yet to record the year after treatment data for the 2017 applications. 

From these data, it appears that a minimum of 0.25 inches of rainfall is necessary to incorporate the herbicide within the root zone and rainfall in excess of 3.0 inches typically resulted in reduced control. We also observed that smutgrass control is reduced even when rainfall is recorded the second week after application, indicating that we may have a short window for rainfall to occur following hexazinone application for optimum smutgrass control. A portion of this work was funded through the Florida Cattlemen Enhancement Board. For our current recommendations on smutgrass management in pastures, please see our factsheet on EDIS (http://edis.ifas.ufl.edu/aa261) or contact your local county extension agent. 

Response of smutgrass at 30 days after treatment with 2 qt/A hexazinone following simulated rainfall.
Figure 1. Response of smutgrass at 30 days after treatment with 2 qt/A hexazinone following simulated rainfall. Rainfall was simulated from 0 to 8 inches approximately 4 hours following hexazinone application. 

Response of giant smutgrass to 2 qt/acre hexazinone (1 lb/acre) 30 days after treatment in 2016.
Figure 2. Response of giant smutgrass to 2 qt/acre hexazinone (1 lb/acre) under field conditions 30 days after treatment in 2016. Rainfall was collected weekly; rainfall amounts indicated on each day of application represents the total rainfall recorded for 7 days following hexazinone application.