Qingwu Xue, Wenzhao, Liu and B. A. Stewart
Texas AgriLife Research-Amarillo

Winter wheat is a major crop grown in the US Southern Great Plains. The area has a semi-arid climate and the seasonal precipitation only can meet one-third of the evapotranspiration (ET) required for high yield. As a result, wheat yield and water-use efficiency (WUE) are primarily limited by drought stress from late spring to early summer. In the area, irrigation from Ogallala Aquifer provides a mean to maintain high yields and productivity in wheat. However, with the declining irrigation water supplies and increasing energy costs, deficit irrigation will be the primary practice in the future. Deficit irrigation is defined as the application of less water than it is required for full ET and maximum yield, resulting in conservation of limited irrigation water and increased WUE. Deficit irrigation has shown to be a viable management practice for improving wheat yield and WUE in the Southern Great Plains. In this paper, our objective is to better understand the ecological and physiological basis for improving yield and WUE under deficit irrigation. Based on long-term field studies in Bushland, TX, seasonal ET requirement ranged from about 200 mm to 540 mm for dryland wheat, and from 400 mm to as high as 950 mm for irrigated wheat. Dryland wheat yield and WUE ranged from 0 to 3 Mg/ha and from 0 to 0.8 kg/m3, respectively. The irrigated wheat yield was mostly between 3-7 Mg/ha and WUE between 0.4-1.2 kg/m3. Comparing to dryland, deficit irrigation can significantly increase yield and WUE. For example, spring irrigation of 200 mm (50% ET requirement) increased yield up to 6.8 Mg/ha (2.5 Mg/ha in dryland) and WUE up to 1.14 kg/m3 (0.57 kg/m3 in dryland). Comparing to full irrigation, irrigation at 50% ET requirement resulted in 86-95% of the yield at full irrigation (100% ET), which saved irrigation water by 50%. Field studies showed that there are several ecological and physiological mechanisms for practicing deficit irrigation. First, maintaining high available soil water (ASW) at planting is important for successful practice of deficit irrigation for efficient use of both precipitation and irrigation water. High ASW at planting not only ensured the vigorous early shoot growth but also resulted in deep root system. With the deep root system, irrigation can be delayed until booting stage or anthesis if only one-irrigation is allowed. Second, irrigation must be applied at critical stages for wheat yield determinations. Critical growth stages for irrigating winter wheat generally occur from early spring growth to early grain development. Third, increased yield and WUE under deficit irrigation is related to increased harvest index (HI). The HI is determined during grain filling by both current photosynthesis and remobilization of pre-anthesis carbon reserve from stems. The increased HI under appropriate deficit irrigation is due to maintaining current photosynthesis and the remobilization of pre-anthesis carbon reserves. Like many other semi-arid areas, the sustainable wheat production in the Southern Great Plains will face more challenges due to more demands for increasing food and biofuel production, limited natural resources and future climate change. Nevertheless, deficit irrigation will be an important management practice for long time. The future challenge for the area will be integration of new irrigation technologies and better managing crops under drought stress. The improvement of irrigation technologies will increase the irrigation efficiency. While adoption of cultivars with more drought resistance will lead to more water savings and maintain yields. Improving drought resistance through breeding will provide benefits but is a long-term task. The current challenges are identifying important physiological traits and looking for high throughput field phenotyping tools. Some of our new field studies will be able to address these challenges.