While grain sorghum’s water-efficient nature, soil enhancement capabilities, and nutritional virtues increase in recognition, an often overlooked feature that adds a vivid stroke to the crop is the captivating array of colors of its grain kernels.
There is nothing more visually appealing in agriculture than driving by sorghum fields or visiting a variety trial just prior to harvest and observing the spectrum of grain colors. From pearly whites to lemon yellows, deep browns to fiery reds, and even obsidian blacks, the range is astonishing. But what might astonish you further is the intricate web of traits that determine the final color of each sorghum hybrid, not just the altering of the color of the outer layer of the kernel.
Color, the defining factor, is influenced by three genetically controlled variables: the pericarp’s color and thickness (the outer seed coat or bran), the presence or absence of the testa (a sub-layer beneath the pericarp), and the color and texture of the endosperm.
The pericarp is made up of multiple layers and can be red, yellow or white (colorless) and its thickness can vary greatly. A thin pericarp will be more transparent, allowing the testa and endosperm to have a greater influence on the final color. A thick pericarp will be opaque and the grain will range from white to brown with a dull appearance.
The testa will be yellow or brown but is not present in most grain sorghum hybrids planted in the U.S. This is because the testa contains tannin which makes the grain less desirable for the animal feed industry. However, tannins are high in antioxidants and are increasingly favored in human and pet food products. Therefore, a few testa containing hybrids are planted under contract for these markets. These hybrids are normally black or burgundy in color, but there are exceptions to this rule. There are a few countries, such as Argentina, that plant high tannin containing grain sorghum hybrids for their bird resistant properties.
The endosperm of the sorghum seed is either white or yellow and influences the seed color when the pericarp is thin and transparent. For example, grain with a red thin colored pericarp with the absence of the testa or tannin layer and yellow endosperm will be bronze in color. If the pericarp is red but thin, and there is not a testa layer, then the color of the underlying endosperm will influence the final grain color. A yellow endosperm color when mixed with the red pericarp would result in a bronze-colored grain.
As end-users often have specific preferences, such as the Chinese’s inclination for red grain in baijiu production, the sorghum industry adapts to cater to varying needs. In the U.S., the poultry and food industries have historically preferred white grain devoid of plant pigment stains. The stains come from colored (red or purple) glumes which are the two dry, leaf-like structures that surround the kernel. To meet this need, the sorghum industry has developed high yielding grain sorghums with white grain and tan glumes that are generally considered food-grade sorghum.
While there’s a common belief among producers that lighter-colored seeds might germinate less efficiently than their darker counterparts, recent studies have brought surprising insights. In germination and seed vigor trials conducted by the University of Nebraska, grain color exhibited no significant influence on warm germination or seedling vigor. However, under field conditions, a slight advantage was observed for red seed over white seed. Intriguingly, in laboratory tests conducted under cool conditions, hybrids with purple glume colors exhibited higher germination and seedling vigor. Notably, substantial variations among lines suggest that other factors are at play.
Sorghum has exceptional attributes and a mesmerizing diversity of colors, which are a testament to the crop’s intricate genetics and versatility. As we embark on another harvest season, let’s celebrate the unique charm that sorghum brings to agriculture, reminding us that its beauty extends far beyond what meets the eye.
This story originally appeared in the Fall 2023 Issue of Sorghum Grower magazine.