INTEGRATED PEST MANAGEMENT Yield and Botanical Composition of Rhizoma Peanut-Grass Swards Treated with Herbicides

between the presence of grass and weedy forbs in RP– grass swards (Harrelson et al., 1993). Weeds are an increasing problem in rhizoma peanut (RP) (Arachis In Florida, high summer rainfall and mild winter temglabrata Benth.), a warm-season perennial forage legume. The objective of this field study was to measure the effect of glyphosate [Nperatures provide a favorable environment for peren(phosphonomethyl)glycine] at 1.12, 2.24, or 3.36 kg a.i. ha21 and triclonating Mexican-tea plants which can reach a height of pyr {[(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid} at 0.56, 1.12, or 1.68 1.5-m each summer and dominate RP–grass swards. Cokg a.i. ha21 applied in the summer on dry matter (DM) yield and gongrass, widely regarded as an invasive weed, is also botanical composition of weed-infested RP–grass swards at 2 and favored by these growing conditions (Dozier et al., 4 mo after application. Mexican-tea (Chenopodium ambrosioides L.) 1998). This perennial grass has invaded large areas of and cogongrass [Imperata cylindrica (L.) P. Beauv.] were the most pasture and forestland in the state and is widespread common weeds. Glyphosate, at all rates, reduced Mexican-tea DM along roadsides. Encroachment of these weeds on RP– 2 mo after application in both years. However, substantial recovery grass swards represent a problem for hay producers and of existing MT plants was observed 4 mo after application at all but graziers. Weeds compete directly with desirable forage the high rate. Glyphosate had no effect on cogongrass or other grasses in 1995 or 1996. In both years, rhizoma peanut DM declined as the species for water, light, and nutrients and reduce both rate of glyphosate increased. Some recovery of RP was noted at the the nutritional and dollar value of the hay. The vigorous low (1.12 kg ha21) rate of glyphosate by 4 mo after application. Edible growth and unpalatable nature of these weeds makes DM (RP 1 other grasses) was reduced due to glyphosate treatment them difficult to control with grazing management alone only at the high glyphosate rate. In both years, triclopyr was effective (Valencia et al., 1999). in reducing Mexican-tea DM 2 mo after application with limited Strategic use of herbicides to minimize weed impact recovery of treated plants 4 mo after application. Cogongrass and and increase edible forage has not been assessed in other grasses increased in the triclopyr treatments in both years, possiRP–grass swards. Glyphosate [N-(phosphonomethyl) bly due to reduced competition from Mexican-tea. Rhizoma peanut glycine] and triclopyr (3,5,6-trichloro-2-pyridinyloxyaDM decreased as the rate of triclopyr increased in 1995 at 2 and 4 mo cetic acid) are both herbicides registered for use in after application, but this effect was observed only at 2 mo after application in 1996. Triclopyr application had little effect on edible pastures. Glyphosate is a systemic broad-spectrum herDM, but this was a consequence of the substitution effect of other bicide that is foliarly absorbed but inactive in soil. grasses for RP. Both triclopyr and glyphosate can be useful in weedTriclopyr is a systemic broadleaf herbicide that is foliarly infested RP stands, but glyphosate at the rates tested was not as absorbed and degrades in soil in 20 to 40 d by microbial effective as triclopyr in controlling Mexican-tea. activity (Ware, 1989). Fully established stoloniferous perennial peanut (A. pintoi Krapov & W.C. Greg.) was found to be tolerant of glyphosate (Dwyer et al., 1989). R peanut is a warm-season perennial forage Glyphosate has been reported to substantially reduce legume with exceptional forage quality, high biocogongrass infestations, but required multiple applicamass production, long-term persistence, and multiple tions during the growing season (Dozier et al., 1998). uses (Williams et al., 1991; Ortega-S. et al., 1992; French Triclopyr at a 0.56 kg ha21 controlled sand blackberry et al., 1994). In subtropical Florida, RP provides herbage (Rubus cuneifolius Pursh) in bahiagrass (Paspalum nofor grazing from April to October or two to three hay tatum Flügge) pastures, reducing its cover by 58 percentharvests per year. There are more than 8000 ha in full age units (Kalmbacher and Eger, 1994). production in Florida (Quesenberry, 1999). The reduction or response of weeds and rhizoma peaInvasion of weeds into RP–grass swards has recently nut DM to glyphosate and triclopyr are unknown. The been documented (Williams, 1994). Rhizoma peanut– objective of this study was to determine the effect of grass swards have not shown any apparent decline in RP glyphosate or triclopyr applied at three rates during contribution (Williams et al., 1991); however, pasture the summer on DM yield and botanical composition of botanical surveys have shown a negative correlation weed-infested RP–grass swards. E. Valencia and L.E. Sollenberger, Agronomy Dep. Univ. Florida, MATERIALS AND METHODS Gainesville, 32611; M.J. Williams, USDA-ARS, Subtropical Agric. Res. Stn., Brooksville, FL 34601-4672. Joint contribution of the This experiment was conducted in 1995 and 1996 at separate USDA-ARS and the Florida Agric. Exp. Stn. Journal Series no. Rsites at the USDA-ARS Subtropical Agric. Res. Stn. in 06495. Received 5 Oct. 1998. *Corresponding author (mjwi@icon. Brooksville, FL (288379 N, 828229 W), on a Nobleton fine kv.ufl.edu). Abbreviations: DM, dry matter; RP, rhizoma peanut. Published in Agron. J. 91:956–961 (1999).