Irrigation frequency effects on growth and ethanol yield in sweet sorghum

With the increasing costs of fossil fuels, new methods of generating renewable fuels need to be researched and developed. This study was undertaken to assess the effects of irrigation frequency on crop growth and ethanol yield of sweet sorghum [Sorghum bicolor (L.) Moench]. Nine combinations of pre- and postanthesis irrigation frequencies triggered by 35, 50, or 65% depletion of plant-available soil water were established in Tucson, AZ. Soil type was a Gila very fine sandy loam. Water application plus pre- cipitation ranged from 1075 to 1394 mm. Crop water use was greatest with the most frequent irrigation treatment (35% depletion all season) (1067 mm), while the 50 and 65% depletion treatments (all season) were not different (882 and 844 mm, respectively). Despite increased biomass at the V-17 and anthesis stages, no differences due to irrigation frequency were found at harvest for yield of biomass or ethanol. The 65% and 50% depletion treatments (all season) returned more biomass per unit of water applied than the 35% depletion treatment (all season) (21.9, 21.6, and 18.8 kg ha¹ mm¹, respectively). A difference in total sugar con- centration was not found at harvest, implying that water stress does not increase the sugar concentration in sweet sorghum, or the stress imposed in our study (24% reduction in postanthesis water use) was not sufficient to do so. Irrigating sweet sorghum more frequently than at 50% depletion does not increase ethanol yield and reduces biomass produced per unit of water.