Cereal grains are processed into products for human consumption or industrial application and the resulting co-products are used in livestock feeding. Cereal co-products tend to be more variable in nutrient concentration and digestibility and, therefore, their inclusion in swine diets may be limited. Distillers dried grains with solubles (DDGS) is commonly used in swine diets and is the primary co-product of ethanol production from corn, milo, or wheat. Corn co-products from the corn milling industry include corn gluten feed, corn gluten meal, and corn hominy feed. Wheat co-products are also included in swine diets and derived from the wheat milling industry.
Corn DDGS is extensively used to partially replace corn in swine diets in the United States. Distillers dried grains with solubles is a co-product of fermentation during ethanol production, which results in removal of most of the starch in corn. As corn composition is approximately 2/3 starch and 1/3 other nutrients, it is generally assumed that the other nutrients are concentrated by approximately 3 times in DDGS compared to corn (Stein et al., 2016). The fermentation process also releases a large proportion of phosphorus bound to phytate, which greatly increases the digestibility of phosphorus in DDGS (Almeida and Stein, 2012). In contrast, the fermentation and drying processes negatively affect digestibility of most amino acids, particularly lysine (Kim et al., 2012). Also, if originally present in corn, mycotoxins are unaffected by the fermentation and drying processes and are concentrated in DDGS.
Corn DDGS contain similar concentration of metabolizable energy than corn, but net energy is variable. The concentration of oil may vary from less than 5% to more than 10% depending on the degree of oil recovery during processing. Production tools were developed to aid in determining the energy value of DDGS sources (KSU Energy Value of DDGS, Figure 1) and the economic dietary DDGS level in the grow-finish phase (DDGS Calculator).
Corn DDGS is rich in polyunsaturated fatty acids, which negatively affects pork fat quality and increases carcass iodine value. Carcass yield is also negatively influenced by the addition of DDGS in diets because of the high fiber content that increases gut fill and visceral weight. Adoption of a withdrawal strategy for DDGS prior to marketing is important to reduce the impact on iodine value and restore carcass yield (Asmus et al., 2014; Lerner et al., 2018).
Figure 1. Equations to predict digestible (DE) and net energy (NE) values of DDGS varying in oil content. Every ±1% in oil content represents ±115 kcal/kg or ±52 kcal/lb of NE (KSU Energy Value of DDGS, Graham et al., 2014 doi:10.2527/jas.2014-7678).
Corn co-products derived from the wet milling industry include corn gluten feed and corn gluten meal, whereas corn hominy feed is a corn co-product from the dry milling industry. The milling processes fractionate corn into its components to produce a variety of food products while the product streams are utilized in feeds.
Corn gluten feed is derived from the wet milling industry and contains many product streams from the milling process. Because of that, the composition of corn gluten feed is typically more variable than that of other corn co-products (Stein et al., 2016). Corn gluten feed contains very low concentration of starch and high fiber content, which results in lower energy value compared to corn (NRC, 2012).
Corn gluten meal is also derived from the wet milling industry and is considered a high protein co-product, containing around 60% crude protein (NRC, 2012). However, the amino acid profile in any corn co-product is similar to that of corn, which is not the ideal profile for swine (Almeida et al., 2011). Corn gluten meal is considered as a partial corn replacement due to its lower fiber content compared to other corn co-products (NRC, 2012).
Corn hominy feed is derived from the dry milling industry and contains a combination of corn bran, germ, and starch. Corn hominy feed is the corn co-product with the most similar composition to that of corn (NRC, 2012). The concentration of starch and oil is greater than any other corn co-product (NRC, 2012). Corn hominy feed is, therefore, considered suitable for use as a partial corn replacement in swine diets (Stein et al., 2016).
Co-products from the wheat flour industry are collectively known as wheat middlings, but sometimes are divided according to protein and fiber concentrations into wheat bran, wheat shorts, wheat red dog, and wheat mill run. The wheat milling process removes the starch from wheat, which results in co-products with lower energy density and higher protein and fiber content (Rosenfelder et al., 2013). The concentration of total fiber is usually between 25 and 35%, which may reduce growth performance and carcass yield with high inclusion of wheat middlings. Wheat middlings is often used as a fiber source in diets for gestating sows.
The low bulk density of wheat middlings increases the volume of the feed unless is in a pelleted form. Capacity of mixers, trucks, feed bins, and feeders must be considered when adding unpelleted wheat middlings or other ingredients with low bulk density to the diet, particularly at relatively high inclusion rates.