Introduction
The right grain sorghum plant population is important in producing top yields. Plant population planning should vary depending upon seasonal rainfall, evaporation potential, hybrid tillering potential, planting date, hybrid maturity, soil moisture at planting, soil type/depth and potential disease or insect problems.
Planting According to Available Moisture
Seasonal rainfall and evaporation potential are two of the primary reasons grain sorghum is an important crop in arid environments. Many grain sorghum-growing regions have low rainfall and periods of high soil water evaporation potential, creating a moisture stress environment. Plant populations are dependent on moisture availability.
Table 1. Plant population and seed spacing guidelines for grain sorghum.
Grain sorghum seed size is typically 14,000 to 16,000 seeds per pound, but can range from 9,000 to 22,000 seeds per pound. It is important to read the information on the bag tag to accurately calculate the number of acres each 50-pound bag will plant, based on projected seeds per acre. Germination percentage is listed at 85 percent on seed bags as mandated by seed labeling laws. Pioneer® brand grain sorghum typically germinates well above 85 percent. To determine the actual germination percentage, provide your local Pioneer sales representative with a bag tag or the lot number on the tag. The seeding rate can be calculated by using the following formula.
Desired planting rate divided by percent actual germination equals number of seeds to plant to for the desired population. Example: 30,000/95 percent =31,579 seeds per acre planting rate to get 30,000 germinating seeds per acre. An additional 10 percent should be added to the seeding rate to account for field mortality unrelated to germination percentages.
Tillering
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.
Source: Jaiyesimi and Vanderlip. 1979. Kansas State
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
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.
Replanting
Once the seed is planted, various factors such as insects, soil crusting, wet/dry seedbeds, soil temperature, disease, and planter problems, can contribute to stand reduction in grain sorghum. Such a stand reduction is characterized by within-row skips in stand, resulting in non-uniform plant spacing. A study was conducted by E. J. Larson and R. L. Vanderlip, at Kansas State University, to determine the magnitude of yield reduction from non-uniform stands. The design of the experiment was as follows.
Plots were planted 10.7 meters long and consisted of 5-by-30 inch rows. Different stand reductions and skip patterns were simulated through thinning and yield was measured. Selected results are shown in Figure 2. Yield components were measured in the study to show how sorghum compensates for thin stands.
Yield from the thinned rows was reduced in all cases. However, overall plot yields were significantly affected in only the most severe stand reductions; 2.7 m skip, and 3 different 0.9 m skips in 3 rows. Yield compensation was mainly due to changes in the number of heads per plant (tillering) and the number of seeds per head.
Figure 2. Replanting options and respective yield reduction
From the above study, thin stands may result in yield losses lower than expected and therefore, it is rarely economically feasible to replant.
Soil Moisture Availability
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.
References
Jaiyesimi, S.T., and R.L. Vanderlip,1979. Yield and tillering response of grain sorghum [Sorghum bicolor (L.) Moench] hybrids to planting date and density. Ph.D. Dissertation. Kansas State University. 50 p.