Late Season Grain Sorghum Management
By Brent Bean, Ph.D., Sorghum Checkoff Agronomist
As a continuation to the article on early season growth management of grain sorghum, this article will focus on management of sorghum from heading to physiological maturity and harvest.
Once the sorghum head begins to emerge from the flag leaf sheath, the sorghum plant takes approximately 40 to 50 days to reach physiological maturity, depending largely on accumulated heat units.
Sugarcane aphids can cause major yield loss if thresholds are exceeded from heading through the hard dough stage. Growers are strongly encouraged to scout fields and treat for aphids as soon as threshold levels are reached.
Research has shown both heat and drought stress just prior to heading and through flowering can reduce yields. For growers who have the ability to irrigate the sorghum should not be allowed to stress from lack of water during this critical time. In addition, applying 30 to 40 pounds of the recommended nitrogen for the targeted yield goal in the irrigation water will increase overall nitrogen use efficiency. Growers should also consider a fungicide application during this time if they expect foliar diseases to be a problem.
Sorghum begins flowering from the top of the head and moves downward over a four- to seven-day period, depending on weather conditions and the hybrid. Sorghum has perfect flowers, meaning that each flower contains both anthers and the female stamen. Unlike corn, where the pollen must travel from the tassel to the ear, sorghum pollen has to move only a very short distance for successful pollination. For this reason, sorghum can withstand heat stress better than other crops during this critical stage. The main risk to sorghum during flowering comes from midge. Late-planted sorghum is particularly vulnerable to midge infestation. In the mid-South, many growers elect to apply an insecticide for midge control at flowering without scouting, assuming midge will be an issue. However, midge will not always be present, so scouting every other day during flowering is the recommended practice. Growers are encouraged to see state extension service midge recommendations for specific guidelines.
Once sorghum moves into the milk and soft dough stages, it becomes vulnerable to headworms, which include corn earworm, fall armyworm and sorghum webworm. For corn earworm and fall armyworm, an insecticide application usually is warranted when one to two worms that are at least 0.25 to 0.5 inches in length are present per head. Since webworms are smaller, a higher threshold of five or more worms per head are needed to justify an insecticide application. Growers also should scout for various stinkbugs during this time and make an insecticide application if five stinkbugs are found per head. The grain is very susceptible to bird feeding at this stage.
The hard dough stage is reached when the majority of grain in the head can no longer be mashed between the thumb and index finger. At this stage, approximately 75 percent of the grain’s final dry weight has now accumulated, and most of the grain’s seed coat color has turned from green to its final color. The grain is now too hard for insects to cause major yield loss. During this time, stress on the plant from drought, diseases or insects can potentially lower test weight and lead to an increase in lodging. For this reason, growers are discouraged from terminating irrigation too soon. The grain is considered to have reached physiological maturity once hard starch has formed at the base of the kernel. Similar to flowering, maturity progresses from the top to the bottom of the head and generally takes about 14 days.
Growers can harvest once grain is mature and grain moisture has dropped to a suitable level for storage or mechanical drying, and when the plant itself has dried sufficiently. Drydown time is highly dependent on weather conditions and the hybrid. Some hybrids have a stay-green characteristic that is desirable during late-season drought but can slow down the drying of the plant significantly. For these hybrids, a harvest-aid treatment may be required. For more information, growers can visit the Agronomy Library and read the Sorghum and the Use of a Harvest-Aid Product article.