Switchgrass and Big Bluestem Hay, Biomass, and Seed Yield Response to Fire and Glyphosate Treatment

just below the soil surface, such as crown buds or rhizomes, aids plant tolerance to fire. Timing of spring fire and glyphosate [N-(phosphonomethyl)glyFire may not be an option for managing warm-season cine] treatments is critical to control weeds without compromising grasses in densely populated areas of the northeastern production of warm-season grasses. To determine the forage and seed USA where rural and urban land uses intermingle. Thus, yield response we burned or applied glyphosate to switchgrass (Panherbicides may be the only tool available to control weeds. icum virgatum L.) and big bluestem (Andropogon gerardii Vitman) in mid-April, early May, and late May in 1998 and 1999 at Rock Broadleaf weeds in perennial warm-season grasses can Springs, PA. Compared with fire in mid-April or early May, a late be controlled with selective herbicides such as 2,4-D May burn in 1998 reduced July dry matter yields (simulating a hay [(2,4-dichlorophenoxy)acetic acid] and dicamba (3,6harvest) by 40 to 48% but did not affect dry matter yields in September dichloro-2-methoxybenzoic acid) (Mitchell and Britton, (simulating a biomass feedstock harvest) of either grass. In switch2000). Imazapic (2-[4,5-dihydro-4-methyl-4-(1-methylgrass, glyphosate applied in late May reduced July yields by 70% ethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarand September yields by 30%. In bluestem, late May application of boxylic acid) can be used postemergence to control many glyphosate reduced July yields by 90% and September yields by 40%. weeds in big bluestem; however, imazapic severely injures In both grasses, fire in late May reduced yields less than a late May switchgrass. Glyphosate, a nonselective postemergence application of glyphosate. Late May glyphosate treatments reduced herbicide, can be used to control invasive cool-season seed yield compared with the mid-April date. We conclude that in grasses in stands of dormant perennial warm-season central Pennsylvania switchgrass or bluestem can be burned through grasses (Waller and Schmidt, 1983); however, applicathe first week of May (10–15 cm growth) with little effect on hay or biomass yield. Glyphosate should be applied (i) before midto late tion timing will influence crop safety. April (just before green-up) if switchgrass or big bluestem is to be Timing of spring fire and herbicide treatments is critiharvested as hay in July or for seed in September, or (ii) by the first cal to both control weeds and prevent damage to the week of May if the cumulative growth is to be harvested once in autumn. warm-season grasses. Sometimes weeds may not have reached the proper developmental stage for effective herbicide control before the warm-season grass has P warm-season grasses, such as switchgrass greened up in the spring. Thus, waiting to apply a nonand big bluestem, have multiple uses including soil selective herbicide, such as glyphosate, or burning too conservation, wildlife habitat, and livestock forage in late may damage the warm-season grass and reduce the northeastern USA (Clubine, 1986). In the future, production. Applying herbicide or fire too early may miss the initial flush or allow a second flush of weeds switchgrass may also be useful as a biofuels crop (Sanderto emerge and compete with the warm-season grasses. son et al., 2004). Cool-season weeds often invade warmMoreover, use of fire too early may reduce the availabilseason grasses during early spring before the warmity of soil moisture for warm-season grass growth by season grasses are competitive. Options for early season exposing the soil to sunlight and encouraging evaporaweed control on dormant warm-season grass sods in the tion (Mitchell and Britton, 2000). northeastern USA include fire and herbicides. Generally, the use of fire in late April or early May Fire in spring has been used to control cool-season has increased production, reproductive stem density, weeds in tallgrass prairie of the Great Plains (Towne and seed yields of some warm-season grasses in native and Kemp, 2003). Developmental stages at the time of tallgrass prairie in the central Great Plains (Masters et burning and morphological characteristics of the plant al., 1992, 1993). Because fire may not be an acceptable are key factors in determining how fire affects plants option in densely populated areas of the northeast, (Steuter and McPherson, 1995). Burning when the coolglyphosate may be the only option for controlling some season weeds are actively growing and the growing invasive cool-season grasses. However, data are lacking points exposed is most effective for their control. Plant to indicate how switchgrass and big bluestem respond morphology that maintains a pool of active meristems to glyphosate applied at various spring dates. The purpose of this study was to determine how fire or glyphosate application at various times in the spring affected M.A. Sanderson, R.R. Schnabel, W.L. Stout, and D. Genito, USDAARS Pasture Systems and Watershed Management Research Unit, forage and seed yields in switchgrass and big bluestem. Bldg. 3702, Curtin Road, University Park, PA 16802; W.S. Curran, Dep. Crop and Soil Sciences, 116 ASI Bldg., Pennsylvania State Univ., MATERIALS AND METHODS University Park, PA 16802; and B.F. Tracy, Dep. Crop Sciences, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL. Received 17 Mar. The field experiment was conducted in 1998 and 1999 at 2004. *Corresponding author ([email protected]). the Russell E. Larson Agricultural Research Center near Rock Springs, PA. Soil at the site is a Hagerstown silt loam (fine, Published in Agron. J. 96:1688–1692 (2004). mixed, semiactive, mesic Typic Hapludalfs). Soil tests indi© American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA cated no need for lime, P, or K.