Irrigation Scheduling for Grain Sorghum

  • To maximize sorghum yield potential under stress conditions, water is a key component.
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  • The fibrous root of a grain sorghum plant can extend to depths of 4 to 6 feet.
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  • Sorghum yield potential can be greatly reduced depending on drought stress timing relative to growth stage.
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  • About 1 week prior to the grain head being pushed out of the boot through 2 weeks past flowering is the most critical period for water availability.
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Grain Sorghum Plant Characteristics

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The extensive and fibrous root system of a grain sorghum plant allows it to withstand drought conditions better than corn and as a result, is often planted into soils and conditions that are less than ideal for crop production. The roots can extend to depths of 4 to 6 feet; however, more than 75% of the water and nutrients acquired by the root system are from the top 3 feet.1

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All grain crops require water to survive and grain sorghum is no exception. Under stress conditions, water is required to maximize yield potential. Sorghum yield potential can be greatly reduced by the timing of drought stress relative to growth stage (Table 1).

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Table 1. Potential Reduction in Yield from Moisture Stress*

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Growth Stage % Yield Reduction
Emergence to V8 10-15
Boot to Flowering 30-50
Soft Dough to Maturity 10-20
*2013 Arkansas grain sorghum quick facts, University of Arkansas *2013 Arkansas grain sorghum quick facts, University of Arkansas
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Row Spacing and Seeding Rate

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Available moisture is an important factor in determining sorghum row spacing and planting population. A soil-shading canopy can develop quicker with narrower rows. Rows that are canopy covered can help decrease the loss of soil water through evaporation.

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According to Texas A&M AgriLife Extension Service, 70,000 to 80,000 established plants/acre should be a goal for irrigated acreage and 50,000 to 60,000 established plants/acre for dryland acreage.1 Dryland populations are likely to approach the higher irrigated populations in regions where rainfall increases. Water use increases with higher populations because more plants are drawing upon soil water reserves and any water added through irrigation or rainfall. If stressful conditions exist, high populations can result in depleted water reserves prior to reproductive stages.

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Growth Stages and Water Use

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A sorghum plant’s total water usage varies by product maturity, planting date, environmental conditions, and final yield. The total water usage (soil and plant evaporation) to produce a sorghum yield of 7000 lb/acre has been estimated to be about 28 inches of water/acre.1

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During a sorghum plant’s life, water usage varies depending on its growth stage (Table 2). 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. Since soil water can evaporate quickly when soil is exposed, conservation tillage, residue management, row spacing, weed control, and planting date can help preserve available soil moisture.

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Table 2. Estimated Grain Sorghum Water Use by Growth Stage*

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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 to maturity) 0.25-0.10
*2013 Arkansas grain sorghum quick facts, University of Arkansas
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After seedling establishment and about 40 days after planting, water use by the plant increases significantly and is very important for maintaining genetic yield potential. While in the rapid growth phase, the seed panicle and the number of ovules that can potentially develop into seed are forming within the stalk. Any water shortage during this developmental stage can be detrimental to yield maximization and overall plant health.

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The most critical period for water availability is from about one week prior to the sorghum grain head being pushed out of the boot (exsertion) 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. Individual sorghum products vary on their ability for head exsertion under stress. Some may not be able to push the head out at all and others may have partial exsertion (Figure 1). The plant needs about 0.2 to 0.3 inch of water/acre/day during this phase. About 8 to 10 inches of water (if available) will be consumed by bloom.

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Figure 1. Sorghum plant with poor head exsertion because of minor drought stress.  

 

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The reproductive phase begins with bloom and lasts about 45 days until physiological maturity or black layer. About 0.35 inch of water/day will be used by the plant from just before bloom until early grain fill. The final irrigation should carry the crop to maturity. Additional water after maturity will not add weight to the seeds. Arkansas information suggests terminating furrow irrigation when > 50% of the heads are at hard dough and moisture is adequate and terminating pivot irrigation when > 75% of the heads are at hard dough and moisture is adequate.3 University of Nebraska Extension information suggests basing final irrigation timing on soil moisture deficits, soil type, and growth stage. If soil deficit information is not available, Table 3 provides timing recommendations based on growth stage and a fully refilled root zone, which is hard to achieve with the previous irrigation and is not a recommended practice.2

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Table 3. Last Irrigation Determination Guidelines* (assumes root zone is completely refilled)

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Soil Type Soil Type
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 10 days after half bloom
Sandy Loam 6 days after soft dough
Fine Sands 10 days after soft bloom
*Predicting the last irrigation for corn, grain sorghum and soybeans. NebGuide. G82-602-A. University of Nebraska.
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Tools to Help Determine Soil Water

13;10;Tools available to help determine the soil’s ability to supply water include rain gauges, moisture sensors, and soil probes.

A standard rain gauge can be placed at the edge of a field to provide information on natural rainfall. Local radio, TV, and internet websites may also provide rainfall information, but will not be specific to each field. Depending on the state, web-based evapotranspiration (ET) calculators may be available to help determine water usage, needs, and availability.

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Moisture sensors can be installed at 1-, 2-, and 3-foot depths to measure moisture levels at various depths. 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.

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Soil probes are another means of measuring soil water and compaction. If the probe cannot penetrate the surface, there is no subsurface moisture. Prior to sorghum 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 depth of 8 to 10 inches. After black layer, a probe depth of 6 to 8 inches is sufficient to nourish the plants to harvest.4

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Irrigation Timing

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  • Assuming the soil water profile is adequate to full at planting, additional water should not be necessary until the reproductive stage begins.
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  • At the beginning of the reproductive stage, a 4-inch irrigation can last until the flag leaf appears.
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  • Two 3-inch irrigations about two weeks apart at flag leaf or boot stage should carry the plants to the soft dough stage of kernel fill.
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  • A 3- to 4-inch irrigation at soft dough stage allows for the completion of grain fill.1