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Irrigating Grain Sorghum
DR. BRENT BEAN
KANSAS CITY, MO.
Sorghum is a reliable dryland crop under many environments, but it also can respond well to irrigation. This versatility allows sorghum to fit into many cropping systems where the availability of irrigation water may be limited. The amount of water necessary to maximize the yield of any crop depends on the specific environment where it is grown. For example, it takes a lot more water to successfully grow sorghum in Arizona than in south central Kansas or North Carolina. Daily and seasonal water demand is dependent on climate factors such as daily maximum temperature, humidity, wind speed and solar radiation.
Sorghum yield is at its best when rainfall and/or irrigation water sufficiently meets the crop’s daily water requirement. Ideally, soil water should be maintained at or greater than 50 percent available water to a soil depth of 3 feet. Yield of grain sorghum normally is not reduced as long as soil-available water stays above 30 percent - 40 percent. The actual amount of water needed depends on soil type, with sandy soils holding less water than a silty clay loam soil.
However, growers often plant sorghum where the supply of irrigation water does not fully meet daily and seasonal water demand. When this occurs, the goal is to supplement rainfall and any stored soil water to maximize the amount of grain produced for every inch of water applied.
When irrigation water is limited, growers need to know the three critical periods in the growth and development of sorghum that most influence yield.
• Period 1. This critical period occurs approximately 30 days after emergence and often is referred to as the growing point differential. At this time, the plant moves from the vegetative stage to the reproductive stage, the immature grain head (panicle) begins to form, and the plant determines the number of potential grains that each head will have. If moisture stress occurs, grain heads will be small and fewer grains will be present in each head.
• Period 2. This critical period occurs in the boot stage, which happens just before the head emerges at the top of the plant and as blooming and grain set begin. Research shows that when soil lacks water at this stage, water added through irrigation can greatly enhance yield.
• Period 3. This critical period occurs when the grain is forming in the soft dough stage. Growers know when this stage has been reached because the grain has turned from green to its final color and can still be crushed between the thumb and forefinger. Avoiding water stress at this time results in better grain fill and test weight.
Once the grain has reached the hard dough stage, irrigation water added to dry fields will not contribute much to the final yield, though it can improve stalk strength, which reduces the potential for lodging and increases harvest efficiency.
For a more detailed discussion on sorghum growth stages, along with pictures, visit Growth and Development on the Crop Production page of the United Sorghum Checkoff Program website at https://www.sorghumcheckoff.com/. ∆
DR. BRENT BEAN: Sorghum Checkoff Agronomist
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