Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2004
Publication Date: 11/20/2004
Citation: Sanderson, M.A., Schnabel, R.R., Curran, W., Stout, W.L., Genito, D., Tracy, B.F. 2004. Switchgrass and big bluestem yields differ with fire and herbicide management. Agronomy Journal. 96:1688-1692. Interpretive Summary: Fire and herbicides are the main options for early season weed control on dormant warm-season grasses. The purpose of this study was to determine how burning or glyphosate (Round Up herbicide) application at various times in the spring affected forage yields in switchgrass and big bluestem. Generally, the timing of burning treatments did not have a large effect on hay or biomass yields of switchgrass and big bluestem. When yields were affected, the late burning treatment did not reduce yields as much as a late application of glyphosate. Delaying glyphosate application into May frequently reduced warm-season grass yields. Thus, mid to late April (just before grass green-up) is the cutoff date for glyphosate application to switchgrass and big bluestem sods in climates similar to central Pennsylvania. Frequently, the herbicide effect on hay yields persisted into the autumn harvest; however, burning affected only summer yields. Thus, burning or glyphosate application may be more flexible for warm-season grasses managed for a single late-season harvest, such as for biomass energy feedstock production.
Technical Abstract: Timing of spring burning and glyphosate [N-(phosphonomethyl)glycine] treatment is critical to controlling weeds without damaging newly emerging warm-season grasses. To determine the forage and seed yield response we burned or applied glyphosate to switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii Vitman) in mid-April, early-May, and late-May in 1998 and 1999 at Rock Springs, PA. Dry matter yield was measured in July (hay harvest) and September (biomass feedstock harvest). Seed (switchgrass) and caryopsis (bluestem) yield were measured in September. Compared with burning in April or early May, a late-May burn in 1998 reduced July dry matter yields by 40 to 44% but did not affect September yields of either grass. In switchgrass, delaying glyphosate application until late May reduced July yields by 80% and September yields by 30%. In bluestem, late application of glyphosate reduced July yields by 90% and September yields by 40%. In both grasses, a late burn reduced yields less than a late application of glyphosate. Late burning did not affect seed or caryopsis yield in 1998 and increased it in 1999. Late glyphosate treatments, however, reduced seed and caryopsis yield compared with the mid-April date. We conclude that in central Pennsylvania switchgrass or bluestem can be burned through the first week of May (10 to 15 cm growth) with little or no effect on yield. Glyphosate should be applied (i) before mid to late April (just before green-up) if switchgrass or big bluestem is to be harvested in July or (ii) by the first week of May if the grasses are to be harvested in autumn.