Biomass sorghum [Sorghum bicolor (L.) Moench.] is a candidate bioenergy feedstock in the Midwest, US. Research suggests that biomass sorghum is more drought tolerant and has higher water-use-efficiency (WUE; the ratio of cumulative biomass production to total evapotranspiration; g kg-1) than Zea mays (maize) in water-limiting environments. However, comparisons of the seasonal evapotranspiration (total ET) and WUE of biomass sorghum and maize have focused on irrigated systems and are scarce for the rain-fed, Midwest. We conducted a side-by-side comparison of the total ET and WUE of maize and biomass sorghum at a site within the US Corn Belt. Total ET was estimated using a micrometeorological method and aboveground plant biomass was determined using destructive hand harvests. Theoretical ethanol yield (EY; l m-2) and ethanol water requirement (EWR; l water l ethanol-1) were also determined for each species.

Over two non-drought growing seasons, we found similar mean WUE for maize (3.51 ± 0.26 g kg-1) and biomass sorghum (3.47 ± 0.22 g kg-1). Total ET was 567 ± 26 mm and 600 ± 20 mm, for maize and biomass sorghum, respectively. The total ET and WUE of maize and biomass sorghum were not significantly different in this study (p > 0.1). Maize had significantly greater EY and less EWR, relative to biomass sorghum (p < 0.1). Since drought was not encountered during this experiment, our results do not capture the response of total ET and WUE to the full range of climate variability in the Midwest, US.