Posted on Jan 09, 2017
By Brent Bean, Sorghum Checkoff Agronomist
A question that many sorghum growers struggle with is when to quit irrigating. Maximum water use by sorghum is between boot and early soft dough stage. During this time, the average daily water use will approach 0.25 inches. On hot, windy days, water demand can reach 0.35 inches or higher.
As the crop begins to mature, the daily water use gradually decreases until the grain reaches physiological maturity. It takes approximately 40 to 45 days for the plant to go from flowering to mature grain. To reach the potential yield set at flowering, it is important adequate water be supplied through the soft dough stage of development.
Irrigation cutoff can be considered once the sorghum grain has reached the hard dough stage or when the grain color has changed throughout most of the sorghum head. At this time, the decision to terminate irrigation should be made based on stored soil moisture and anticipated rainfall.
Grain sorghum can still use more than 5 inches of water from late soft dough to harvest. During this time, available plant soil moisture also can be important in maintaining stalk strength in high-yielding environments. As long as the plant remains green, some water is required to maintain stalk integrity. Lack of adequate soil moisture to maintain the stalk can lead to an increase in lodging.
Grain physiological maturity can be difficult to recognize but typically takes about 14 days to reach once the grain is in the hard dough stage and can no longer be crushed by pinching between the index finger and thumb. A dark spot at the base of the kernel indicates the grain has reached maturity. At maturity, the moisture content of the grain is usually between 25 and 30 percent, and starch accumulation has reached its maximum.