Wheat middlings and red dog are coproducts of the wheat milling process that are used as a source 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 digestibility of crude protein and amino acids in wheat coproducts produced in China has been reported, but there is limited information about the nutritional value of wheat middlings and red dog produced in the United States. In addition, because wheat coproducts vary in terms of the conditions under which they are produced, their nutritional value may vary as well. Therefore, the objective of this experiment was to determine the concentration and digestibility of crude protein and amino acids in ten sources of wheat middlings and in one source of red dog.

Experimental design

Twelve cannulated pigs with an average initial body weight of 29.23 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. Twelve diets were formulated based on cornstarch, sucrose, and soybean oil. Eleven of the diets contained 45% of one of the test ingredients as the only source of amino acids. The twelfth was an N-free diet that was used to determine basal endogenous losses of crude protein and amino acids. Because all diets contained amino acids in quantities less than the requirements for growing pigs, a supplemental amino acid mixture was fed during the first five days of each seven-day period but not on the last two days of each period when ileal digesta were collected.

Amino acid composition and digestibility in wheat middlings and red dog

The concentration of crude protein in the ten sources of wheat middlings varied between 17.0 and 18.8%, with a mean of 17.67% (Table 1). Red dog contained 17% crude protein. The concentration of indispensable amino acids was consistent among the ten sources of wheat middlings. Wheat middlings contained an average of 0.72% lysine and 0.25% methionine, compared with 0.63 and 0.37% in red dog. The mean concentration of Lys relative to CP in wheat middlings was 4.05%, and in red dog it was 3.7%.

The mean SID of crude protein in wheat middlings was 58.5% (Table 2). There were no differences in SID of crude protein, histidine, or methionine among the ten sources of wheat middlings, but there were variations in the SID of other indispensable amino acids.

The SID of crude protein and all amino acids except arginine and histidine was greater (P < 0.05) in red dog than in wheat middlings. This result is probably due to the high concentrations of NDF and ADF in wheat middlings. In particular, the SID of lysine was 72.3% in red dog but averaged only 46.2% in wheat middlings, suggesting that the wheat middlings may also have been damaged by excessive heat during production.

Key points

• The concentration and digestibility of crude protein and amino acids were generally consistent among the ten sources of wheat middlings from the U.S.
• Crude protein and most amino acids were more digestible in red dog than in wheat middlings, probably due to the latter's high fiber content.
• Lysine digestibility in wheat middlings was relatively low, which suggests possible heat damage.

Table 1. Analyzed protein and amino acid composition of the 10 sources of wheat middlings and red dog (as-fed basis)

Table 2. Standardized ileal digestibility of crude protein and amino acids in wheat middlings and red dog

This report is based on unpublished research by Gloria Casas and Hans H. Stein.