Grain sorghum, despite its reputation as a drought tolerant crop, requires ample water to help maximize each product’s yield potential (Table 1). Factors influencing water usage include product maturity, planting date, environmental conditions, plant growth stage, and final yield. An estimated 28 inches of water/acre (soil evaporation and plant transpiration) are needed to produce a sorghum yield of 7,000 lb/acre.1
Impact on CropGrain sorghum’s drought tolerance is a function of its fibrous root system that can extend to depths of 4 to 6 feet. However, more than 75% of the water and nutrients absorbed by the root system are from the top 3 feet.1
Water usage during the sorghum plant’s life varies by product maturity and progress through different growth stages (Table 2). Usage is relatively low during seedling development (germination to 6 mature leaves); however stress during this time can affect future growth, plant size, and yield potential.
About 40 days after planting, the plant enters a rapid growth phase where its water use greatly increases. The seed panicle and the number of potential ovules (seeds) form within the stalk during this phase.
Though not as important as the next growth stage, any water shortage during this developmental stage can reduce yield potential and compromise plant health.
The most critical period for water availability and yield maximization is from about one week prior to the grain head being pushed out (exertion) of the flag leaf through two weeks past flowering. Individual sorghum products vary in their ability for head exertion under stress. Depending on the stress level, full, partial (Figure 1), or no exertion is possible. About 0.2 to 0.3 inch of water/acre/day is required for adequate plant growth during this phase. About 8 to 10 inches of water, if available, may have been utilized up to bloom.
Flowering or bloom signals the initiation of the reproductive growth stage which concludes with physiological maturity or black layer. The stage lasts around 45 days. About 0.35 inch of water/day may be used by the plant from just before bloom until early grain fill. Irrigating after maturity is not recommended because the seed becomes “sealed” at maturity. However, any available soil moisture can help sustain stalk health. Stalks that die prematurely from drought stress or suffer from a hot-weather disease, such as charcoal rot, can become weak and prone to lodging. Severely lodged sorghum is difficult to harvest and the exposed seeds can sprout when in contact with the soil.
|Table 1. Potential reduction in yield from moisture stress.*|
|Growth Stage||Percent Yield Reduction|
|Emergence to V8||10-15|
|Boot to Flowering||30-50|
|Soft Dough to Maturity||10-20|
|* 2-18 Arkansas grain sorghum quick facts, University of Arkansas|
|Table 2. Estimated grain sorghum water use by growth stage.*|
|Days after Planting||Inches/day|
|0-30(early plant growth)||0.05-0.10|
|30-60 (rapid plant growth)||0.10-0.20|
|60-80(boot and flowering)||0.25-0.30|
|80-120 (grain fill boot to maturity)||0.25-0.10|
|* 2018 Arkansas grain sorghum quick facts, University of Arkansas|
he final irrigation should carry the crop to maturity. When moisture is adequate, the University of Arkansas suggests terminating furrow irrigation when more than 50% of the heads are at hard dough and pivot irrigation when more than 75% of the heads are at hard dough.3 University of Nebraska Extension information suggests basing final irrigation timing on soil moisture deficits, soil type, and growth stage. If soil moisture deficit information is not available, Table 3 provides irrigation timing recommendations based on existing growth stage and irrigating to a fully refilled root zone, which is hard to achieve with irrigation and is not a recommended practice.2
Tools for determining available water in the soil include a standard rain gauge, moisture sensors, and probes. A rain gauge placed at the edge of a field can provide information on natural rainfall and local radio, TV, and internet websites may provide local rainfall data. Depending on the state, web-based evapotranspiration (ET) calculators may be available to help determine water usage, needs, and availability.
Moisture sensors can be installed at 1-, 2-, and 3-foot depths to measure moisture levels. The sensors should be located within the crop row and not placed into low areas. Information from the sensors should be read and recorded at least twice a week.
Soil probes are another means of measuring soil water and compaction. There is no subsurface moisture if the probe cannot penetrate the surface. Prior to the boot stage, the probe should be able to be pushed to a depth of 3 to 4 feet, which indicates there is about 4 to 6 inches of stored water depending on soil type. During heading, flowering, and grain fill, the probe should go to a minimum depth of 8 to 10 inches. After black layer, a probe depth of at least 6 to 8 inches should be sufficient to help maintain stalk integrity to harvest.4
|Table 3. Last irrigation determination guidelines* (assumes root zone is completely refilled)|
|Soil Type||Stage of Growth for Last Irrigation|
|Silty Clay Loam||4 days after soft dough|
|Upland Silt Loam||1 day after soft dough|
|Bottomland Silt Loam||10 days after half bloom|
|Very Fine Sandy Loam||3 days after soft dough|
|Sandy Loam||6 days after soft dough|
|Fine Sands||10 days after soft dough|
|* 1991. Predicting the last irrigation for corn, grain sorghum and soybean. NebGuide. G82-602-A. University of Nebraska|
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. 2018 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