The Water Use of Sweet Sorghum and Development of Crop Coefficients

Reliable estimates of crop water use are essential to economic production of sweet sorghum under irrigated conditions. In Tucson, Arizona, USA, two planting dates, May 5th and June 15th, and four sweet sorghum varieties (M81E, ST, SM and A4) were evaluated to estimate crop coefficients (kc) based on the heat unit accumulation concept (GDD). One irrigation level at 50% depletion of available water capacity prior to irrigation and its effect on juice, biomass and ethanol yields was evaluated. Results show that it was possible to develop heat unit based kćs for two day-neutral varieties (ST and SM) and that they required 25% less water than the photo-sensitive varieties. Peak crop coefficients were determined as 1.13, 1.05, 1.16 and 1.22 for M81E, ST, SM and A4. Results also showed that growing degree days accumulated to flowering were 2226 and 1759 GDD for ST and SM respectively. At a P-value of 5% there were differences in crop water use, juice yields and bagasse production among treatments. For the May planting there was a slight effect on ethanol yields but no effect for the June planting, showing that regardless of the planting date and variety they are more likely to be similar.