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The extensive and fibrous root system of a grain sorghum plant (which can extend to depths of 4 to 6 feet) allows it to withstand drought conditions better than corn. When drought stress occurs in grain sorghum, the growth stage of the plant will have a strong impact on the potential for yield loss (Table 1).
Figure 1. Sorghum plant with poor exertion because of minor drought stress.
The total water usage (soil evaporation and plant transpiration) to produce a sorghum yield of 7,000 lb/acre has been estimated to be about 28 inches of water/acre.1 However, total water usage can vary by product maturity, planting date, environmental conditions, and final yield. Daily water usage varies by growth stage (Table 2, page 2).
Planting to early growth.During early growth (germination to 6 mature leaves), water use is relatively low; however, stress at this time can affect future growth, plant size, and yield potential. After seedling establishment, water use by the plant increases sharply. While in the rapid growth phase, the seed panicle and the number of ovules that can potentially develop into seeds are forming within the stalk. Any water shortage during this developmental stage can be detrimental to yield potential and overall plant health.
Early reproductive stage.The most critical period to avoid water stress is from about one week prior to the emergence of the grain head from the boot (exertion) through two weeks past flowering. During this time, plants suffering from severe drought may not be able to push the grain head out of the boot (Figure 1). The plant needs about 0.2 to 0.3 inch of water/day during this phase. About 8 to 10 inches of water (if available) will be consumed by bloom.
Reproductive stage through maturity. The reproductive phase begins with bloom and lasts about 45 days until physiological maturity or black layer. About 0.3 inch of water/day will be used by the plant from just before bloom until early grain fill. Additional water after maturity will not add weight to the seeds, but may help with stalk integrity if conditions are extremely dry.
In general, soil moisture levels should be used to dictate when irrigation should be applied. If irrigation water is limited, farmers should consider the growth stage of the crop and reserve the water for the most critical stages.
Tools available to help determine soil water range from simple soil probes to soil moisture sensors and sophisticated web-based applications that have access to soil databases and weather networks.
The use of conservation tillage and residue management can help preserve soil moisture during early crop development when the soil is exposed.
Available moisture is an important factor in determining sorghum row spacing and planting population. A soil-shading canopy can develop quicker with narrower rows, which can help decrease the loss of soil water through evaporation.
Water use increases with higher populations because more plants are drawing upon soil water reserves; therefore, lower populations may be needed when water is limited.
A general full-irrigation strategy from Texas A&M University is outlined below:
Several rules-of-thumb for determining the final irrigation include:
Arkansas information suggests terminating furrow irrigation when > 50% of the heads are at hard dough and soil moisture is adequate and terminating pivot irrigation when > 75% of the heads are at hard dough and soil moisture is adequate.2
Sources: 1 Stichler, C. and Fipps, G. 2003. Irrigating sorghum in South and South Central Texas. L-5434. Texas A&M University. 2 Klocke, N.L., Eisenhauer, D.E., and Bockstadter, T.L. 1991. Predicting the last irrigation for corn, grain sorghum and soybeans. NebGuide. G82-602-A. University of Nebraska. 3 Kelley, J. and Lawson, K. 2013 Arkansas grain sorghum quick facts. University of Arkansas. 4 New, L. 2004. Grain sorghum irrigation. PROFIT. B-6152. Texas Cooperative Extension. The Texas A&M University System. Other source: Stichler, C., McFarland, M., and Coffman, C. 1997. Irrigated and dryland grain sorghum production, South and Southwest Texas. Texas A&M University. 150410115543