THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Tolerance of Limpograss to Herbicide Applications
For questions or comments regarding this publication contact: Dr. Brent Sellers, University of Florida, IFAS
Limpograss (Hemarthria) is a common forage grown on many ranches in central and south Florida. Weed control in limpograss has been considerably challenging since many believe that only a few products can safely be applied without reducing the density of the stand. For example, the statement “do not apply 2,4-D or products containing 2,4-D to limpograss” is commonly found in many limpograss publications. Since early 2005, we have investigated the effects of various herbicides on established limpograss.
In April 2005, 2,4-D amine at 32, 64, and 128 oz/A, and Remedy at 16, 32, and 64 oz/A were applied to both mowed (10-inch regrowth) and non-mowed limpograss. Much to our surprise no injury was observed from either herbicide at any application rate to either mowed or non-mowed limpograss. Biomass at 6 months after treatment was not impacted by either herbicide when not mowed prior to application, but limpograss biomass was reduced by 17 to 23% when treated with 64 and 128 oz of 2,4-D amine as well as 32 and 64 oz of Remedy.
The lack of injury or reduction in limpograss biomass was surprising and strips of limpograss (10-inch regrowth) were treated with Pasturegard HL at 16, 24, and 32 oz/A, WeedMaster at 16, 32, and 64 oz/A, and 2,4-D at 24 and 48 oz/A in September, 2005. Pasturegard HL at 16 and 24 oz, WeedMaster at 16 and 32 oz, and 2,4-D at 24 oz resulted in less than 15% injury (chlorosis and necrosis) 1 month after treatment. By 2 months after treatment, no chlorosis or necrosis was present from any herbicide application. However, stand height was reduced by at least 15% from applications of Pasturegard HL at 32 oz, 2,4-D at 48 oz, and WeedMaster at 64 oz. Unfortunately, biomass was not recorded in these demonstration plots.
Since 2005, we have observed little to no injury in limpograss when treated with various products containing 2,4-D when applied from November through April. Oftentimes, however, substantial injury was observed when these products were applied during the hot and humid months, but especially when applied to limpograss that was rapidly growing after cutting or grazing.
In 2013 and 2014 we investigated the effects of Banvel (dicamba; 32 oz/A), 2,4-D (64 oz/A), WeedMaster (48 oz/A), GrazonNext HL (24 oz/A), metsulfuron (0.3 oz/A), PastureGard HL (24 oz/A), Chaparral (3 oz/A), and Velpar (32 and 64 oz/A) in established limpograss hayfields. All herbicide treatments were applied during July in 2013 and August in 2014 when limpograss was at least 6 inches in height. In this particular instance we wanted to determine the effect of these herbicides under rapid limpograss growth with the thought that the herbicides would be the most damaging during the summer as compared to the winter months when growth is often slower. Limpograss biomass was harvested from each plot 90 days after treatment to determine the long-term effects on limpograss. Biomass data were converted to percent of the Banvel-treated plots, since this herbicide is currently the most widely used for weed control in limpograss.
Chaparral and metsulfuron, as expected, resulted in no yield loss compared to Banvel-treated limpograss (Figure 1). WeedMaster and GrazonNext resulted in approximately 20% less yield than Banvel-treated limpograss. Of the auxin-type herbicides, Pasturegard HL and 2,4-D resulted in at least a 25% yield reduction. Velpar was the most damaging to the stand causing 50% yield loss at 32 oz/A and nearly 80% yield loss at 64 oz/A, which is the standard application rate for smutgrass control in perennial grass pastures.
Chaparral (aminopyralid + metsulfuron) and metsulfuron are very good herbicides that can be used to control many weeds in limpograss pastures, with the exception of dogfennel. While Banvel has been used for many years to control dogfennel in limpograss, control of dogfennel greater than 36 inches tall becomes quite variable. Once dogfennel become greater than 36 inches tall, the most optimum herbicide of choice is Pasturegard HL. Our data show that a 25% yield reduction will likely be observed when applied to limpograss during the summer rainy season. However, this yield loss will likely be offset by controlling dogfennel that could result in more yield reduction than when treated with Pasturegard HL.
In August, 2014, Chaparral was applied at 2.5 oz/A plus 8, 10, or 12 oz/A Pasturegard HL to determine the effect of this combination on limpograss yield. Limpograss was approximately 20 inches tall at the time of application. While some transient chlorosis was observed in plots treated with this combination, especially at the highest rate of Pasturegard HL, there was no effect on limpograss biomass 90 days after treatment. This indicates that this combination is fairly safe to apply to limpograss pastures and hayfields to consistently control larger dogfennel.
Overall, we believe that limpograss tolerance to herbicides is highly related to the maturity of the stand. A newly established limpograss pasture will not likely have the energy reserves to overcome the stress of a herbicide application. This is likely the reason for establishment failures when using 2,4-D in the past. However, established limpograss stands appear to be more resilient, and herbicides like 2,4-D, WeedMaster, GrazonNext HL, and Pasturegard HL can be applied when warranted. For example, if dogfennel is greater than 48 inches tall in August and need to be controlled prior to cutting for hay, Pasturegard HL at 24 oz/A would provide the best control of dogfennel. Although this rate of Pasturegard HL has been shown to result in a 25% reduction in yield, the yield recovered from removal of dogfennel will likely outweigh this loss.
We will continue to investigate the effects of herbicides on limpograss injury and biomass production over the coming years. Much of our effort will be to understand the effects of commonly used tank-mixes including Chaparral plus Pasturegard HL as well as GrazonNext HL + Pasturegard HL. This is especially important as two new cultivars have been released, which will probably increase the amount of limpograss planted throughout the state.
Figure 1. Influence of various herbicides on limpograss biomass production at Ona, FL in 2013 and 2014. Herbicides were applied during the summer growing season and biomass harvested 90 days after treatment. Data are expressed as % of Banvel (dicamba) – treated limpograss; therefore, the higher the number, the less biomass reduction.