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

Timing of spring fire and glyphosate [N‐(phosphonomethyl)glycine] treatments is critical to control weeds without compromising production of 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. Compared with fire in mid‐April or early May, a late May burn in 1998 reduced July dry matter yields (simulating a hay harvest) by 40 to 48% but did not affect dry matter yields in September (simulating a biomass feedstock harvest) of either grass. In switchgrass, glyphosate applied in late May reduced July yields by 70% and September yields by 30%. In bluestem, late May application of glyphosate reduced July yields by 90% and September yields by 40%. In both grasses, fire in late May reduced yields less than a late May application of glyphosate. Late May glyphosate treatments reduced seed 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 (10–15 cm growth) with little effect on hay or biomass yield. Glyphosate should be applied (i) before mid‐ to late April (just before green‐up) if switchgrass or big bluestem is to be harvested as hay in July or for seed in September, or (ii) by the first week of May if the cumulative growth is to be harvested once in autumn.