Posted on Mar 20, 2020
By Brent Bean, Ph.D., Sorghum Checkoff Director of Agronomy
Of all the major crops grown in the U.S., grain sorghum clearly has the widest range of seeding rates. Depending on the region of the country, and to a lesser extent within a region, seeding rates can vary from 20,000 to 120,000 seeds per acre. Two experienced sorghum agronomists, even from the same region, are likely to offer two different recommendations for any given set of conditions, largely due to the unique ability of the sorghum plant to adapt to its environment.
The sorghum plant has the ability to tiller, as well as adjust the size of its panicle (head), in response to growing conditions. Tillers are secondary stalks that grow from the basal nodes of a plant at or just below the soil surface, usually within two weeks of plant emergence. Sorghum’s ability to tiller gives growers a lot of flexibility in seeding rates. A minor change in a seeding rate of a few thousand seeds per acre often results in no significant difference in yield.
Yield is determined by the interaction of three components: number of grain kernels per panicle, weight of the individual grain kernels and number of panicles per acre. The typical number of grain kernels per panicle can vary from 1,000 to 3,000, for an average of approximately 2,000 kernels. Weight of the individual grain kernels is normally expressed as weight per 1,000 kernels (TKW). The most common TKW is approximately 21 grams. Using these numbers the average size panicle will have 0.1 pounds of grain. Using 0.1 pounds of grain per panicle, growers can easily calculate the number of panicles per acre needed for any given yield goal.
The number of harvestable panicles is determined by the seeding rate, emergence rate and tillering. The interaction between the seeding rate and environmental conditions soon after emergence plays a large role in the number of viable tillers per plant in any given field. For a more detailed discussion on tillering, visit the Agronomy Insights page of the Sorghum Checkoff website.
Growers should select their seeding rate based on yield goal and current and expected short-term environmental conditions at planting. An optimum seeding rate should be low enough that the established plants can withstand short periods of drought yet high enough to achieve a reasonable yield goal with or without significant tillering. The table provides general guidelines for seeding rates based on yield goal.
These seeding rates can be adjusted to fit local conditions, but are appropriate under most environments. Some agronomists recommend increasing the seeding rate on narrow rows, for example when switching from a 30-inch row spacing to a 15-inch row spacing. Typically, the seeding rate can remain the same regardless of row spacing as long as a row planter is used. If using a drill, growers should increase the seeding rate by 20 percent to account for a decrease in emergence.