Smutgrass Control in Perennial Grass Pastures1
Brent Sellers, J. A. Ferrell, and J. J. Mullahey2
Introduction
Smutgrass (Figure 1)–an invasive bunch grass, native to tropical Asia–is a serious weed of improved perennial grass pastures, roadsides, natural areas, and waste areas in Florida. Results of a survey conducted by The South Florida Beef Forage Program in 2003 indicated that smutgrass ranks as the second-most-problematic weed species in Florida pastures, behind tropical soda apple, which is the most problematic weed. However, because practices to control tropical soda apple have been widely adopted in Florida since that survey was conducted, it is likely that smutgrass has by now become the most problematic weed species in Florida pastures.
Two smutgrass species are found in Florida–small smutgrass (Sporobolus indicus; Figure 2) and giant smutgrass, which is also known as West Indian dropseed (Sporobolus indicus var. pyramidalis; Figure 3). Small smutgrass was once the predominant smutgrass species throughout Florida. By the 1990s, however, giant smutgrass had become the most common smutgrass species throughout Central and South Florida. Giant smutgrass continues to move northward in Florida.
Small smutgrass, once the most troublesome smutgrass species in Florida, is still found throughout the state.
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Mature smutgrass plants are unpalatable to livestock, but some grazing of mature smutgrass does occur. New regrowth of smutgrass, which is similar in quality to that of bahiagrass, is grazed for two to three weeks after burning or mowing. However, it is difficult to graze cattle on smutgrass due to the need to rotate cattle among smutgrass-infested paddocks, so that growth of the smutgrass does not reach a stage where cattle will not graze the plants.
Biology
Both smutgrass species – small and giant – are perennial bunch grasses. Average bunch size of small smutgrass is approximately 8 - 10 inches in diameter while giant smutgrass diameter is approximately 12 - 18 inches.
Small smutgrass has a compact seedhead (Figure 4) with the panicle branches touching the panicle. The small smutgrass seedhead is almost always infected with a black fungus. Small smutgrass plants produce approximately 1,400 seeds per seedhead and 45,000 seeds per plant.
In contrast, giant smutgrass has an open seedhead with panicle branches directed somewhat upward (Figure 5). The seedhead of giant smutgrass is usually not infected with the black fungus, but giant smutgrass plants are sometimes infected with this fungus. Little information exists concerning seed production of giant smutgrass, but some indications suggest this plant may be a more prolific seed producer than small smutgrass.
Seed production of both species occurs throughout the growing season, and new seedheads are produced shortly after mowing or burning. The seeds, which are red to orange in color, remain attached to seedheads for some time after maturing and are spread by adhering to livestock and machinery or by movement via water and wind. Natural seed germination has been shown to average less than 9%, and seed are thought to remain viable in the soil for at least two years.
Small smutgrass seedheads are appressed near the raceme, giving the seedhead a cylindrical appearance.
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Giant smutgrass seedheads are open and generally not infested with the black smut fungus that typically infests small smutgrass, but sometimes smut fungus is also found on giant smutgrass.
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Control
Cultural practices to control smutgrass species have not been successful to date. Mowing decreases the diameter of the clumps, but often results in increased density. Burning is thought to increase the germination of seeds in the soil seed bank. However, both burning and mowing allow for approximately two to three weeks of grazing. Smutgrass forage quality during this two to three week window is often equal to or slightly greater than bahiagrass.
Chemical control of smutgrass includes applying Velpar at 2 qt/acre (1.0 lb hexazinone/acre) to small and giant smutgrass. A surfactant may be added to the spray mixture, but recent research has indicated that a surfactant is not necessary. Mowing smutgrass prior to Velpar application does not increase control. Velpar should be applied from June through September, when rainfall is typically sufficient to move the herbicide into the root zone for uptake. There is little foliar activity from Velpar on smutgrass. If rainfall does not occur within a two-week period after application, the herbicide treatment will likely fail. There are no grazing restrictions for Velpar if the application rate is below 4.5 pt/acre (1.13 lb hexazinone/acre). However, there is a 38-day haying restriction.
Velpar is a highly effective herbicide, but is also expensive. Experiments were recently conducted to determine when Velpar should be applied to maximize smutgrass control, especially in light of the best timing for application to realize return on the herbicide investment. An economic analysis indicated that Velpar should not be applied until smutgrass density is approximately 50 percent of the area of a pasture. Applications of this herbicide prior to this level of infestation will not result in enough additional bahiagrass biomass (i.e., ability to increase stocking rate) to justify the cost of Velpar application. However, in terms of preventing smutgrass infestation, it may be economically justifiable to spray highly infested areas of a pasture, even before 50% of the entire pasture is infested.
Oak trees are extremely sensitive to Velpar, and care should be taken to stay at least 100 ft away from oak trees when applying Velpar. If smutgrass is present under or near oak trees, spot applications of 3% glyphosate are effective.
Forage Grass Tolerance
Bahiagrass will turn slightly yellow about 15 - 20 days after spraying with Velpar at the recommended rates. However, bahiagrass will recover and turn dark green within about 40 days. This green color will be darker than the non-treated pastures. Bermudagrass will turn yellow with some necrosis for approximately 30 days before new green growth occurs.
Recommendations
General
Do not apply Velpar within 100 feet of oak trees because application within this range may cause death of the tree.
Read the Velpar label for complete instructions on reapplication interval, safety and grazing restrictions.
Cattle may graze treated pastures if applications are less than 4.5 pt/acre.
To realize economic gains from Velpar application, smutgrass infestation should be approximately 50 percent of pasture.
If the initial smutgrass density covers more than 80 percent of the pasture area (if 8 out of 10 regular steps touch the base of smutgrass plants), complete renovation of the pasture should be considered.
Bahiagrass/Bermudagrass Pastures
Graze pasture in the spring until the beginning of the rainy season.
Apply 2.0 qt/acre Velpar during the summer rainy season, but not later than the end of September. Apply when plants are actively growing and rainfall is dependable and consistent.
Fertilization after Velpar application will increase forage production and allow bahiagrass to quickly fill the open areas created by dying smutgrass.
Floralta Limpograss
Velpar is not currently labeled for smutgrass control in limpograss.
Stargrass
Velpar is not currently labeled for smutgrass control in stargrass.
Mulato
Velpar is not currently labeled for smutgrass control in Mulato as it will be severely injured – DO NOT USE.
Footnotes
This document is SS-AGR-18, one of a series of the Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original date of publication: February 2000. Original authors included M. B. Adjei, associate professor, and P. Mislevy, professor, both of the Agronomy Department and formerly based at the Range Cattle Research and Education Center--Ona, FL. This publication was revised in February 2006, November 2009, and April 2011. Visit the EDIS website at http://edis.ifas.ufl.edu.
Brent Sellers, assistant professor, Agronomy Department, Range Cattle Research and Education Center--Ona, FL; J. A. Ferrell, associate professor, Department of Agronomy, Gainesville, FL; J. J. Mullahey, professor, Department of Wildlife Ecology and Conservation, West Florida Research and Education Center--Milton FL; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition. Use herbicides safely. Read and follow directions on the manufacturer's label.
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