Harvest Regimen and Nitrogen Fertilization Affect Cellulosic Biomass Production in Native Warm-Season Perennial Grasses.

Native warm-season perennial grasses, which commonly serve as forage for cattle on native rangelands in south-central North America, also have the potential to be used for cellulosic bioenergy production due to their potentially high yields. These grasses do not always respond to nitrogen (N) fertilizer by increasing biomass production. The first question we addressed was: what is the trade-off in biomass yield when these grasses are harvested for forage early in the forage year, followed by a harvest for cellulosic biomass later in the forage year? The second question we asked was: how does native warm-season perennial grass biomass change with minimal spring N application? We worked with shortspike windmillgrass (WMG; Chloris subdolichostachya Muell.), plains bristlegrass (PBG; Setaria vulpiseta Scribn. & Merr.), multiflower false rhodesgrass (MFR; Chloris pluriflora Fourn.), and pink pappusgrass (PPG; Pappophorum bicolor Fourn.), all native to south-central North America, grown under two fertilizer treatments (fertilized or not with 67 kg N ha^-1 each spring) and harvested in July (forage harvest, not included in yield totals) and again in October or harvested once per year in October (cellulosic biomass harvest) in 2009 and 2010 at Stephenville TX. There was a species by harvest by fertilizer interaction (P<0.05) for DM and N yields pooled over years. Total DM and N yields of native warm-season perennial grasses were greater in fertilized treatments than in unfertilized treatments. Dry matter yields of both PPG and MFR were greater when harvested only for cellulosic biomass in October than when harvested for July forage followed by October cellulosic biomass. There was a year by harvest by fertilizer interaction (P<0.05) for DM and N yields pooled over species. With the exception of the unfertilized treatment, there were no differences in DM and N yields between harvest regimens in 2009, a year with 47% greater rainfall than 2010. In 2010 both DM and N yields were greater when harvested only for accumulated season-long cellulosic biomass than when harvested for forage followed by cellulosic biomass for fertilized and unfertilized treatments. Results indicate that, when soil moisture is limiting, there is a trade-off in cellulosic biomass production of PPG and MFR when harvesting for forage in July followed by cellulosic biomass in October, rather than eliminating a July forage harvest and harvesting season-long accumulated cellulosic biomass in October. Native warm-season perennial grasses also responded favorably to fertilization by increasing cellulosic biomass yields. In summary, harvest regimen, N fertilization and precipitation have the potential to influence cellulosic biomass production of native warm-season perennial grasses in south-central North America.