Tillering is the most important factor in grain sorghum's tremendous ability to compensate for environmental changes and management inputs. Most grain sorghum hybrids possess the ability to tiller. Environmental conditions and management factors have a limited effect on the plant's ability to initiate tiller, but they do influence tiller development. While sorghum usually initiates tillers, environmental conditions determine how many of the tillers actually produce a head. Hot dry conditions suppress tiller survival while cool temperatures promote tiller survival. Cool fall conditions can result in extremely high tillering rates and delayed sorghum maturity.

Plants in the 4 to 6-leaf stage will tiller when the average daily temperature (as opposed to the daily high) is below 65 F. The cooler the average daily temperature is during the 4 to 6-leaf stage, the more plants will tiller (Figure 1). Figure 1 illustrates how earlier planting dates (cooler average daily temperatures) will produce more tillers per plant. Normal planting dates will allow the plant to tiller, thus compensating for extreme differences in plant population. Frequently, tillering ability is not taken into consideration at planting time and the result is many grain sorghum fields are planted at populations that are too high.

Figure 1. Contribution of main heads and tillers to total yield of population at specified planting dates at Manhattan, KS.

As planting date is delayed (Figure 1), plant populations need to be increased in order to compensate for reduced tillering to maintain maximum yields. The increase in planting rate will be gradual until the planting date becomes extremely late, such as is the case in a double crop or late replant situation. Under extremely late planting situations, where only main heads and no tillers are produced, it may be necessary to double the normal seeding rate.

Planting rates also need to be increased when the planting date is earlier than desired. It is desirable to plant sorghum when the soil temperature at the two inch depth at 10:00 a.m., has been 65 F for three consecutive days. When planting at lower soil temperatures, consider increasing plant population 10 percent or more due to an increase in mortality of the seedlings.

Hybrid maturity can influence planting rates since shorter season hybrids tend to have shorter plant types. For shorter hybrids, planting rates can be increased slightly to maintain canopy without increasing overall water demand. Hybrid maturity and planting date usually go together. If planting later in the growing season, short-season hybrid maturity is used. Under these circumstances planting rate is increased because of date of planting and hybrid maturity.

Soil moisture availability at planting can affect plant populations. Under dry to extremely dry conditions it may be necessary to increase planting depth to reach moisture. Sometimes it is impossible to plant in a desirable or optimum seedbed. If poor stand establishment is expected because of a shallow drought prone soil, a wet crusting soil, insect problems, disease problems, or early no-till planting, increase your rate of planting to account for a low survival rate.

In arid environments, producers have the ability to "fit" their plant populations to available stored soil moisture at planting. Assessing available moisture is easily done with a soil moisture penetrometer. For best results, plant 5,000 to 6,000 seeds for every foot of available stored soil moisture (Table 2) in the arid environments of the high plains region (western Texas, western Kansas, western Oklahoma, eastern New Mexico, and eastern Colorado).

Table 2. Adjusting planting rate to available stored soil moisture in arid environments.

Verle Amthauer holds a Master of Science degree from Kansas State University, with a research thesis on grain sorghum management systems. He currently is a District Sales Manager in Kansas and previous worked eight years as a Field Sales Agronomist. He has been with Pioneer Hi-Bred since 1986.