Wheat middlings and red dog are two coproducts of the wheat milling process that are used as sources of energy and protein in animal feed. Wheat middlings are granular particles of the wheat endosperm, bran and germ. Red dog consists mainly of the aleurone layer that lies between the bran and the endosperm, along with small particles of bran, germ, and flour. It is often used in extrusion mixtures and as a pellet binder. The composition of wheat middlings varies from mill to mill because of the different conditions under which they are produced.

Wheat and wheat co-products contain more non-starch polysaccharides (NSP) than other grains commonly fed to pigs. Because pigs lack the enzymes needed to digest NSPs, this may affect the digestibility of energy and nutrients. An experiment was conducted to determine the apparent total tract digestibility (ATTD) of gross energy (GE), dry matter (DM), and organic matter (OM), and the concentration of digestible energy (DE) and metabolizable energy (ME) in ten sources of wheat middlings as well as one source of red dog.

Twelve growing pigs with an average initial body weight of 31.0 kg were used in this experiment. The experimental ingredients were ten sources of wheat middlings from Colorado, Iowa, Illinois, Kansas, Michigan, Minnesota, Ohio, and Pennsylvania, and one source of red dog from Iowa. The analyzed nutrient and GE composition of the wheat middlings and red dog are shown in Table 1. Twelve diets were formulated. The basal diet contained corn and soybean meal, and the eleven experimental diets each contained 39.5% of one of the test ingredients. The 12 pigs were placed in metabolism crates and fed the 12 diets in an incomplete 12 x 8 Latin square design with the 12 diets and 8 14-day periods. Feces and urine were collected for five days following a 7-day adaptation period to the diets. The ATTD of GE, ash, and DM were calculated for all diets, and energy and nutrient digestibility in wheat middlings and red dog were calculated via the difference procedure. Concentrations of DE and ME in all sources of wheat middlings and in red dog were also calculated using the difference procedure.

The mean ATTD values for GE, DM, and OM in the ten sources of wheat middlings were 67.2, 71.2, and 72.9%, respectively, with minor variations among sources (Table 2).  The mean ATTD values of GE, DM, and OM in wheat middlings were less (P < 0.005) than the values for red dog. Red dog contained 3,058 kcal/kg DE and 2,952 kcal/kg ME on an as fed basis, and 3,418 kcal/kg DE and 3,300 kcal/kg ME on a dry matter basis. All values for DE and ME were greater (P < 0.001) in red dog than the mean values for wheat middlings.

Digestible energy in wheat middlings ranged from 2,352 to 2,844 kcal/kg on an as fed basis (mean: 2,653 kcal/kg) and from 2,637 to 3,185 kcal/kg on a dry matter basis (mean: 2,990 kcal/kg). Metabolizable energy ranged from 2,272 to 2,751 kcal/kg as fed (mean: 2,568 kcal/kg) and from 2,546 to 3,125 kcal/kg dry matter (mean: 2,893 kcal/kg).

• The digestibility of energy and nutrients varied among the ten sources of wheat middlings from the U.S.
• GE, DM, and OM were more digestible in red dog than in wheat middlings.
• Red dog contained more DE and ME than wheat middlings.