Responses of Grain Sorghum to Profile and Temporal Dynamics of Soil Water in a Semi-arid Environment

Development of efficient irrigation strategies is a priority for producers faced with water shortages. Managed deficit irrigation attempts to optimize water use efficiency (WUE) by synchronizing crop water use with reproductive stages. Soil water use and yield of grain sorghum [Sorghum bicolor (L.) Moench], on a Torrertic Paleustoll in the Texas High Plains, USA, were evaluated during the 2010 to 2012 growing seasons under three sprinkler irrigation strategies: full (FI), deficit (DI), and managed deficit irrigation (MDI). Soil water contents were measured weekly at 0.20-m intervals from 0.10 to 2.30 m depth using a neutron moisture gage. Irrigation for the FI treatment was scheduled when root zone water (0 to 1.6 m) was depleted to 50% of the potential plant available water (PPAW). The DI treatment was irrigated at 50% of FI. The MDI treatment was irrigated at 75% of FI between growing point differentiation and half-bloom, 50% of FI after half-bloom, and less than DI prior to growing point differentiation.

Fully irrigated sorghum grain yields averaged 3.7 Mg ha^(-1) greater (p < 0.001) than deficit irrigated sorghum in all years. Seasonal crop water use under MDI averaged 29 mm greater than DI. Concomitant with increased water use principally during the reproductive period, MDI yields averaged 1.6 Mg ha^(-1) greater than DI, which was significant in 2010 and 2012 (p ? 0.006). The WUE of FI sorghum was significantly greater than MDI in 2012 (p = 0.003) and DI in 2010 and 2012 (p ? 0.001). In 2011, crop water uptake was restricted to above 0.6 m when water contents deeper in the profile were less than 42% PPAW. In 2010 and 2012, seasonal crop water uptake in the profile below 1.0 m was small (<14 mm) and did not appreciably increase in response to imposed soil water deficits. The rooting zone for evaluating plant water status and hence irrigation scheduling depended on initial profile water contents and possibly root density deeper in the profile. Results suggest that WUE?s of grain sorghum are not compromised under MDI compared with FI in most cropping seasons.