Improving water management in sorghum cultivation Jourdan Bell, Texas A&M AgriLife Research and Extension, USA; Robert C. Schwartz, USDA-ARS Conservation and Production Research Laboratory, USA; Kevin McInnes, Texas A&M University, USA; Qingwu Xue and Dana Porter, Texas A&M AgriLife Research and Extension, USA

1 Introduction

2 Dryland production

3 Irrigation

4 Deficit irrigation

5 Soils and irrigation management

6 Conclusion

7 Where to look for further information

8 References

Grain sorghum (Sorghum bicolor [L.] Moench) is a drought-tolerant crop that has been grown across the American Great Plains since the early twentieth century, primarily as a feed crop. Physiologically, grain sorghum is well adapted to the semi-arid conditions of the American Great Plains because it can withstand periods of water stress. Under ideal conditions and management practices, modern hybrids can yield over 11 000 kg ha−1, but in semi-arid regions, plant available water is a major factor limiting crop production (Abunyewa et al., 2011). While early cultivation of grain sorghum was under dryland production, it was recognized that the efficient use of water, including precipitation and stored soil moisture, was critical for improved performance. Evaluation of sorghum water use efficiencies (WUEs; biomass production including grain yield per unit of water used) was reported as early as 1914 by Briggs and Shantz (1914), who noted improved WUEs of sorghum in comparison to other grain crops. Numerous authors have since defined the improvements in WUEs as a result of sorghum being a C4 plant with a high transpiration efficiency (Xin, 2009; Rooney, 2004; Mortlock and Hammer, 1999; Boyer, 1996). As a C4 plant, sorghum produces more biomass per unit of water transpired than cool-season or C3 plants because C4 plants have a more efficient photosynthetic rate. This enables C4 plants such as sorghum to lose less CO2 during photorespiration, resulting in greater conversion of CO2 to biomass. In addition to

Improving water management in sorghum cultivation Improving water management in sorghum cultivation

being a C4 plant, sorghum is more drought tolerant than other C4 plants such as corn due to a deep root system and epicuticular wax to minimize transpirational water losses. Sorghum’s deep, fibrous root system provides greater root volume and therefore potential access to a greater soil water volume, enabling sorghum to withstand greater periods of water stress if there is ample stored soil water. Additionally, sorghum is able to maintain turgor pressure in cells through the regulated accumulation of solutes (osmotic adjustment) (Ferres et al., 1978; Assefa, 2010). Under water deficits, the low osmotic potential in the roots reduces the water movement through the plant, resulting in less water loss through the stomatal openings (Fekade and Daniel, 1992). Furthermore, Sanchez-Diaz and Kramer (1971) concluded that after periods of water stress, the plant water potential and saturation potential recovered more quickly than corn. While sorghum is physiologically adapted to withstand prolonged periods of water stress, such tolerances can come at the expense of reduced yield (Assefa, 2010; Peacock, 1982; Fereres et al., 1978).