Transactions of the ASABE Vol. 55(2): 451-461 2012 American Society of Agricultural and Biological Engineers ISSN 2151-0032 451 GRAIN SORGHUM RESPONSE TO IRRIGATION SCHEDULING WITH THE TIME-TEMPERATURE THRESHOLD METHOD AND DEFICIT IRRIGATION LEVELS S. A. O’Shaughnessy, S. R. Evett, P. D. Colaizzi, T. A. Howell ABSTRACT. 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 irri- gation 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 bio- mass 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 func- tions 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 al- gorithm 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. Keywords. Automated irrigation scheduling, Grain sorghum, Infrared thermometry, Time-temperature threshold. rain sorghum is a relatively reliable crop for producers in the Texas High Plains Region be- cause it is known to tolerate drought well and is in demand as feedstock for confined animal feeding operations and ethanol production. Both early and late-maturing sorghum hybrids are used in this region, and producers typically irrigate this crop at deficit levels (Sweeten and Jordan, 1987). Grain sorghum is thus a good candidate for farms with limited well capacity, and it plays a beneficial role in crop rotations with soybeans, cotton, and wheat (Baumhardt et al., 2007). With declining water available for irrigated agriculture, deficit irrigation schemes are unavoidable. The low-energy precision application (LEPA) irrigation concept was de- signed to utilize limited water supplies with some degree of deficit irrigation (Lyle and Bordovsky, 1983). Managing crops with deficit irrigation levels requires vigilance to pre- vent significant yield losses, reduction in quality, and early crop lodging (Carmi et al., 2006). Bordovsky and Lyle (1996) demonstrated that grain sorghum responds to high- er-frequency irrigations and that yields were significantly better when irrigations were applied using LEPA socks at a 3.5 day interval, as compared to intervals that were 2×, 3×, and 4× greater. Applying irrigation to meet crop water de- mands is critical to limiting yield losses and improving wa- ter use efficiency. Manual scientific irrigation management methods can determine when and how much to irrigate, but they are labor-intensive and require specialized instru- ments. Therefore, producers have traditionally relied on visual observation of crop water stress for irrigation man- agement. Symptoms of plant water stress in grain sorghum can be visually observed. For example, at the pre-flowering stage, leaf rolling, uncharacteristic leaf erectness, leaf tip and margin burn, delayed flowering (Rosenow et al., 1983), and decreased peduncle extension (Eck and Musick, 1979) may occur. At the post-flowering stage, water stress symp- toms can be manifested as premature plant death, stalk lodging, stalk rot, and significant reduction in seed size Submitted for review in September 2011 as manuscript number SW 9373; approved for publication by the Soil & Water Division of ASABE in February 2012. The use of trade, firm, or corporation names in this article is for the in- formation and convenience of the reader. Such use does not constitute an official endorsement or approval by the USDA of any product or service to the exclusion of others that may be suitable. The USDA is an equal oppor- tunity provider and employer. The authors are Susan A. O’Shaughnessy, ASABE Member, Re- search Agricultural Engineer, Steve R. Evett, ASABE Member, Research Soil Scientist, Paul D. Colaizzi, ASABE Member, Research Agricultural Engineer, and Terry A. Howell, ASABE Fellow, Laboratory Director, USDA-ARS Conservation and Production Research Laboratory, Bushland, Texas. Corresponding author: Susan A. O’Shaughnessy, USDA-ARS Conservation and Production Research Laboratory, 2300 Experiment Sta- tion Road, P.O. Drawer 10, Bushland, TX 79012; phone: 806-356-5770; e- mail: Susan.oshaughnessy@ars.usda.gov. G