Animal Fat in Low-Roughage Diets for Ruminants: The Effects of Nitrogen Source and an Amino Acid Supplement

A 2 × 2 × 2 factorial procedure was used to formulate eight different diets such that each diet received a different combination of 0 or 5% animal fat, soybean meal or urea and zero or added amino acids (MHA and L-Lysine HC1). Ground shelled corn was the principal constituent of these diets, which were balanced to maintain a constant gross energy: crude protein ratio (0.30 to 0.31 Mcal/kg GE/% CP) between each. The diets were fed ad libitumto 40 steers during the finishing period. Inclusion of animal fat in either urea or soybean meal supplemented diets did not significantly (P<.05) affect average daily gain or feed gain ratios. An animal fat × nitrogen source interaction (P<.05) on feed intake was caused by fat increasing intake of soybean meal diets by 10% but depressing intake of urea diets by 5%. There was an interaction between all main effects for average daily gain (P<.05) since an improvement through feeding soybean meal compared to urea was greater for diets in which the fat and amino acid supplements were provided separately rather than in combination or omitted. Diets supplemented with animal fat caused greater carcass backfat and liver fat concentration (P<.05) than unsupplemented diets. The diets were fed to 16 lambs at isocaloric intakes in a digestion and balance study (replicated three times). All digestion coefficients except acid detergent fiber were greater (P<.05) for diets containing urea rather than soybean meal. An interaction (P<.05) between the animal fat and amino acid supplements on all digestion coefficients was due to higher values obtained when the supplements were fed separately rather than in combination or when both were omitted.