Sorghum Grain Color - Relationship to Grain Marketability or Feed Value
By Kay Porter
Early sorghum varieties came in a broad range of colors. Some of the earliest varieties, such as hegari, were chalky white. Others like Martin and Wheatland had light red grain and while other varieties had a dark brown seed color. With the development of grain sorghum, hybrids an even broader range of colors became possible as parents of different seed colors were crossed to make hybrids. Today, sorghum grain color is described as white, yellow, cream, hetero-yellow, hetero-white, bronze, orange, dark-red, reddish-brown, white-brown, lemon yellow, chalky-white, intensified red, pearly-white, etc. Does grain color have any relationship to the performance of a hybrid? Does grain color influence the feeding value or marketability of the sorghum crop? This article will clarify some of the issues often confused when grain sorghum color is discussed.
Chemical Composition of Sorghum
The sorghum kernel is made up of three distinct parts, the pericarp (outer layer), the germ (embryo) and the endosperm (storage tissue). The relative proportion of each varies by hybrid and environment (e.g. larger proportion of embryo to endosperm if the seed develops under stress). Table 1 represents typical sorghum kernel composition data.
|COMPONENT||WHOLE GRAIN %||ENDOSPERM %||GERM %||PERICARP %|
|Whole Kernel Range||81.7 - 86.5||8.0 - 10.9||4.3 - 8.7|
|Protein Range||11.5 - 12.3||8.7 - 13.0||17.8 - 19.2||5.2 - 7.6|
|Fat Range||--||0.4 - 0.8||26.9 - 30.6||3.7 - 6.0|
|Starch Range||72.3 - 75.1||81.3 - 83.0||--||--|
|Ash Range||1.6 - 1.7||0.3 - 0.4||--||--|
†Source: Haikerwal and Mathieson (1971), Hubbard et all (1950), Jambunathan and Mertz (1973) and Taylor and Schussler (1986)
Among the factors influencing the visual color of grain sorghum are the genetics of pericarp color, pericarp thickness, the presence or absence of a testa, color and thickness of the testa, and the endosperm color (Rooney and Miller, 1981). (The testa is a thin layer of cells beneath the pericarp that may contain tannins). Pericarp thickness ranges from 8 to 160 um. Pericarp thickness influences seed color to range from shades of white to various shades of pink, orange, red and even brown. In hybrids with very thin pericarp, the seed coat is almost transparent and seed will have a translucent appearance.
If the pericarp is thicker, the seed coat is opaque and the seed will have a dull appearance although the seed color may range from white to brown. A few of the hybrids with thick pericarp have a testa layer containing tannin. High tannin hybrids (less than 2 percent of total production in the United States) are often called bird resistant hybrids because birds prefer not to eat high tannin grain.
Even in bird resistant hybrids, the tannin content will vary depending upon the thickness of the testa layer (range 8-40 um) within the individual kernels. If the testa layer is present, the hybrid will have tannin. If the testa layer is absent, the hybrid will have no tannin. Most red and white seeded hybrids have no tannin.
In hybrids with a transparent pericarp, one is able to see underlying endosperm. Endosperm color is either white or yellow. The grain color of these hybrids comes from the combination of the pericarp and endosperm colors. For example, grain from a hybrid with a red pericarp will be red in color if the underlying endosperm is white.
The seed color is called bronze when a hybrid has a red pericarp with an underlying yellow endosperm. The bronze or orange color comes from the ability to see the underlying yellow endosperm through a red, transparent seed coat. In such hybrids with an opaque pericarp, grain color is determined almost entirely by the pericarp color except in the case of some bird resistant hybrids.
Two other single genes, the "intensifier gene" and the "spreader gene" are also involved in determining grain color. The intensifier gene affects the intensity of the pericarp color. Hybrids with bright red or orange grain carry the intensifier gene. The presence of the spreader gene causes hybrids with a testa layer to have brown grain. In the absence of the spreader gene, some high tannin hybrids with a thick white pericarp appear to be white because the testa layer is masked. (Rooney et al 1981, Sema-Saldivar et al 1995). Table 2. shows the kernel characteristics resulting in the various grain colors in sorghum.
Table 2. Kernel Characteristics and Resulting Grain Color in Sorghum
|Pericarp Color||Pericarp Thickness||Endosperm Color||Tannin Layer||Intensifier Gene||Spreader Gene||Grain Color|
Classes of Sorghum Grain and Sorghum Grading Standards
Today, sorghum grain is placed into one of four classifications based on the presence of a test and/or the pericarp color as defined by the Federal Grain Inspection Service (FGIS). The classifications are as follows:
- Sorghum - Sorghum low in tannin due to absence of testa and contains less than 98 percent white sorghum and not more than 3 percent tannin sorghum. The pericarp color of this class may appear white, yellow, pink, orange, red or bronze.
- Tannin Sorghum - Sorghum high in tannin content due to the presence of a testa and contains no more than 10 percent non-tannin sorghum. Pericarp color of this class is usually brown but may also be white, yellow-pink, orange, red or bronze.
- White Sorghum - Sorghum low in tannin due to absence of testa and contains no more than 2 percent sorghum of other classes. Pericarp color is white or translucent and includes sorghum containing spots that cover less than 25 percent of the kernel
- Mixed Sorghum - Sorghum not meeting the requirements for any of the classes already described.
Sorghum in all classifications must meet U.S. grading or marketing standards established by FGIS if it enters commercial trading channels. The major factors considered when grading sorghum grain are: test weight, percentage of damaged kernels, the amount of broken kernels or foreign material present and the number of non-sorghum seeds (usually weed seed) or other foreign particles. (See Table 3.)
Table 3. Grain and Grade Requirements for Sorghum^
|Grading Factors||Grades, U.S. Numbers|
Minimum Test Weight (Lbs/bu)
|Maximum Percent Limits
for Damaged Kernels
|Broken Kernels/Foreign Matter (%)
|Maximum Count Limits
†Source: Federal Grain Inspection Service, U.S. Department of Agriculture, 1993.
Numerous studies over the last 30 years have shown the nutritional value of sorghum grain is 95 to 98 percent that of corn. The objective in using sorghum varies with the type of animal to be fed. In ruminant animals such as cattle, sheep and goats, sorghum is used primarily as a carbohydrate (energy) source and protein is supplemented in the form of alfalfa, soybean meal, etc. For non-ruminants such as swine, poultry and fish, sorghum is viewed as an energy source but the quality and quantity of the protein is also important. Sorghum based diets may supply up to one-third of the dietary crude protein needed by chickens and up to one-half of the protein needed by growing pigs. Vitamin content of corn and sorghum are similar but sorghum has a slightly higher concentration of most minerals. A large number of feeding trials have been conducted to identify factors that influence the feeding value of sorghum for both ruminants and non-ruminants. (Bramel-Cox et al 1995).
Factors That DO NOT Influence Feeding Value
A number of early trials were conducted to compare the feeding value of hybrids with different grain color. The results were inconsistent. Later studies showed that the performance differences were probably associated with differences in endosperm type and texture more than differences in pericarp color. Sullivan and Douglas (1989) concluded that, "the color of the cuticle or outer layer of the sorghum berry (red, bronze, yellow, cream or white) has little or no correlation with nutritional value."
Referring back to Table 1, it can be noted that the pericarp alone could not impact the feeding value of the grain to any large extent since it is such a small component (6.5 percent) of the sorghum kernel. The pericarp also represents a very small percentage of the available nutrients: total protein (4.0 percent), fat (10.6 percent), starch (3.8 percent) and ash (10.8 percent). Feeding value is only affected when the pericarp has a testa layer with high tannin content.
Factors That DO Influence Feeding Value
Several factors have been shown to affect the feeding value of sorghum. Among these are endosperm type and texture, starch and protein digestibility, presence of tannin, test weight, the growing environment and processing method. (Bramel-Cox et al 1995).
Endosperm Type and Texture
Hybrids with a waxy endosperm have been shown to be more digestible and improve rate of gain and be more feed efficient than normal sorghum. Very few waxy hybrids are available commercially and hetero-waxy hybrids (one or two recessive waxy genes) have shown mixed results in cattle feeding trials. Among hybrids with normal endosperm (25 percent amylose, 75 percent amylopectin), studies have shown hybrids with intermediate texture outperform soft-floury or hard-corneous endosperm hybrids when compared on the basis of dry matter and energy digestibility, or on the basis of swine performance. (Hancock et al 1992, Hibberd et al 1978, Noland et al 1977).
Starch and Protein Digestibility
The starch in the endosperm of the sorghum kernel is surrounded by a dense, hard peripheral endosperm layer that resists both physical and enzymatic digestion. In addition, the starch granules of the sorghum endosperm are embedded in a dense protein matrix. Together, these factors contribute to the lower protein and starch digestibility of sorghum. Processing methods such as grinding or steam flaking that expose the starch granules and protein matrix to digestion help overcome this problem.
Presence of Tannins
Almost all feeding trials with non-ruminants (poultry and swine) clearly demonstrate tannins have a negative impact on performance and weight gain. The effects of tannin in ruminant diets are not as detrimental. Research has shown tannins bind a portion of the protein in the grain to make it indigestible, further limiting protein availability from high tannin sorghum. Tannins also inhibit the normal metabolism of digested and absorbed nutrients, especially protein (Ejeta et al 1989). The negative effects of tannin can be overcome through supplementing the diet with additional protein. High tannin sorghum grain can be detoxified by passing gaseous ammonia through the grain or by treating with aqueous alkaline solutions. The aqueous alkali treatment is especially effective on ground grain (Butler 1978).
Swine fed low test weight sorghum in trials at Kansas State University showed inferior daily gain and feed efficiency than those fed higher test weight grain. Broiler chicks were even more sensitive to low-test weight grain. Test weight did not, however, affect the feeding value of low-test weight grain fed to ruminants (sheep and cattle) (Hancock et al 1992).
There is little information available on the effects of specific growing conditions on the quality of nutrients in sorghum grain. It is known, however, the growing environment can change the relative proportions of the kernel parts. Stressful environments may result in shrunken endosperm and kernels with a higher proportion of embryo. Grain from stressed areas will often be higher in protein than grain grown in optimum environments. Luce et al (1988) sampled sorghum grown in a number of different parts of Oklahoma where crude protein ranged from 10.9 percent to 16.5 percent. The interaction of hybrid genotype and the growing conditions to which it is exposed, may have a major impact on the nutritional value of the grain. Sorghum, perhaps due to the wide range of environments in which it is grown, is more variable for feeding value than corn. Additional research is needed to document the relationship between crop growing conditions, management practices and nutritional value of the grain.
Processing sorghum grain can enhance the nutritional value of the feed. Most methods break up the sorghum kernel, either by grinding, rolling, steam flaking or extruding. These methods expose greater surface area of the starch and protein to digestive enzymes and bacteria. In general, as particle size decreases, the feeding value increases. Poultry and swine utilize feed more efficiently if it is ground instead of rolled. Beef and dairy cattle perform best with steam flaked sorghum. Steam flaking improved sorghum utilization in beef cattle 12 to 15 percent. In dairy cattle, steam flaked sorghum increased both total milk production and milk protein levels. (Hancock et al 1992).
Pericarp color and thickness, endosperm color and the presence or absence of a testa layer determine grain color in sorghum. Sorghum is separated by the Federal Grain Inspection Service (FGIS) into four classes based on pericarp color and the presence or absence of tannin. Grain color is not considered when sorghum is graded for commercial use or trading.
Grain color (white, cream, yellow, bronze or red) is not associated with feeding value in 98 percent of the sorghum grown in the United States (less than 2 percent high tannin sorghum produced). Several other factors such as endosperm type and texture, starch and protein digestibility, test weight and growing environment are important and have been shown to influence animal performance and feed efficiency in research trials.
Although sorghum is considered to have 95 percent of the feeding value of corn, it is possible to improve sorghum's performance through using one of several feed processing methods such as grinding, rolling or steam flaking. Additional research is needed on newer processing methods and to get better information on the effect of growing conditions and crop management on the nutritional value of specific hybrids.
Kay Porter earned a doctoral degree in plant breeding and genetics from Purdue University. He currently serves as Worldwide Director of sorghum research for Pioneer. He has been with Pioneer Hi-Bred since 1977.
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