Effectiveness of weed management methods in establishment of switchgrass and a native species mixture for biofuels in Wisconsin

Establishment of native grassland species can be impaired by weed populations, presenting considerable economic risk for growers of perennial bioenergy crops. We established switchgrass (Panicum virgatum) in southwestern Wisconsin using four weed management strategies, and established a mixture of diverse grassland species to evaluate productivity of diverse mixtures relative to monocultures. Weed management for switchgrass included pre-emergent glyphosate (1.12 kg ae/ha; GLY), pre-emergent glyphosate (1.12 kg ae/ha) + post-emergent 2,4-D (1.06 kg ae/ha; GLY + 2,4-D), pre-emergent glyphosate (0.14 kg ae/ha) and imazapic (0.07 kg ae/ha; S-IMAZ + GLY), and oats planted as a companion crop + pre-emergent glyphosate (1.12 kg ae/ha; GLY + OATS). The diverse species mixture was established with pre-emergent glyphosate (0.14 kg ae/ha) and imazapic (0.07 kg ae/ha; D-IMAZ + GLY). We estimated plant species composition in 2008 and 2009, and biomass yield in autumn of each year. Imazapic decreased grass weed abundance and 2,4-D decreased broadleaf weed abundance in switchgrass in 2008, whereas there were no differences in weed abundances in 2009. Abundance of grass weeds was negatively correlated with biomass yield, and S-IMAZ + GLY produced greater yield in 2009 (7.31 ± 0.75 Mg ha−1) than GLY and GLY + OATS. Of the four switchgrass establishment treatments we tested, only S-IMAZ + GLY yielded significantly greater biomass than the diverse species mixture. Total costs of establishment were greatest for D-IMAZ + GLY and equivalent among the four switchgrass treatments. These results contribute to current understanding of the agronomic potential and economic risks of native grassland species for bioenergy crops in the first two years of establishment.