Studies using the time-temperature threshold (TTT) method for irrigation scheduling have been documented for cotton, corn, and soybean. However, there are limited studies of irrigation management of grain sorghum ( Sorghum bicolor, L.) with this plant-feedback system. In this two-year study, the TTT method was investigated as an automatic irrigation control algorithm for a late-maturing grain sorghum hybrid (Pioneer 84G62) grown in 2009 and an early maturing hybrid (Moench, NC+ 5C35) grown in 2010. The method was evaluated by comparing grain sorghum responses of biomass and dry grain yields, crop evapotranspiration (ET c), water use efficiency (WUE), and irrigation water use efficiency (IWUE) between automatic and manual control methods of irrigation scheduling at different deficit irrigation treatments (i.e., 80%, 55%, 30%, and 0% of full replenishment of soil water depletion to 1.5 m depth). Irrigation scheduling using the TTT method produced mean response variables of yield (biomass and grain), WUE, and IWUE that were similar or better than those from the manually scheduled method in both years. Water use efficiency was highest at the 80% and 55% levels in 2009 and 2010, respectively. Average IWUE was highest at the 55% level in 2009 and at the 30% level in 2010. For both of these responses, differences among irrigation treatment levels were not always significant. Crop production functions were curvilinear in both years as dry grain yields began to plateau between water application amounts delivered from irrigation treatments at the 55% and 80% levels. Results from this study indicate that both late and early maturing hybrids of grain sorghum are responsive to the TTT method of irrigation scheduling. Irrigation management with this algorithm can produce biomass and dry grain yields, ET c, WUE, and IWUE levels that are similar to those achieved with an accurate irrigation scheduling method based on direct soil water measurement.
