As irrigation water becomes scarce, brown midrib (bmr) sorghum [Sorghum bicolor (L) Moench] varieties have emerged as a viable forage alternative to corn (Zea mays L.). Bmr forages, although of a higher nutritional quality, have been shown to produce less biomass than non bmr varieties. It was hypothesized the biomass differences observed between bmr and non bmr varieties are associated with differences in the transpiration efficiency (TE) of the different genotypes. A greenhouse study was conducted using the near-isogenic wildtype, bmr-6 and bmr-12 varieties of three parental lines: Atlas, Rox Orange and Early Hegari Start. Fifty-four covered plastic boxes containing 45kg of dried soil with two water treatments of 75 to 100% and 30 to 75% plant available water (PAW) were used. TE was calculated as the ratio of dried above ground biomass to water transpired (T). A significant difference was found for T (P < 0.0001) and biomass produced (P < 0.0001) for the three parental lines and the two water treatments. A significant difference was observed for TE (P = 0.0073) between the two water treatment levels. A significant difference was found in TE between the wildtype Atlas variety (75 to 100% PAW) and its near isogenic bmr-6 and bmr-12 varieties. No other significant difference was detected between the TE of the near-isogenic wildtype, bmr-6 and bmr-12 for all three lines.

Key Words: limited irrigation, transpiration efficiency, brown midrib