LAUSR

Five cannulated Holstein steers fed a steam-flaked corn-based growing diet containing 40% of alfalfa hay were used in a 5 × 5 Latin square design to examine the effect of ‘solid’ supplemental fats as a substitute for yellow grease (YG) on the extent and site of digestion. Treatments were (% of diet DM) as follows: (1) no supplemental fat; (2) 5% YG; (3) 5.88% calcium soaps (ML); (4) 5% flaked palmitic acid (RP10); and (5) 5% hydrogenated palm fatty acid distillate (HPFAD). Supplemental fats replaced the corn in the control diet. Supplemental fat decreased (P < 0.01) ruminal and total-tract digestion of organic matter and tended to decrease (P = 0.06) ruminal digestion of neutral detergent fibre, with no effects on ruminal digestion of feed N, microbial N efficiency, or total-tract digestion of N and neutral detergent fibre. With the exception of RP10, fat supplementation decreased (P < 0.03) postruminal digestion of C18:0. Compared with the original C16:0:C18:0 ratio of solid fats, the C16:0:C18:0 ratio of fatty acids (FAs) entering the small intestine markedly decreased for all solid-fat treatments. Ruminal biohydrogenation of YG and ML were 73% and 49% respectively. On the basis of FA intake, postruminal FA digestion of YG, ML, RP10 and HPFAD was 0.97, 0.94, 0.92 and 0.80 of expected respectively. This experiment confirmed that postruminal digestion of total FAs of conventional supplemental yellow grease is a predictable function of total FA intake per unit of bodyweight. However, in the case sources of the solids fats, this relationship (FA intake and postruminal digestion of FAs) was less consistent. This may be due to their physical and chemical nature (saturatedFA:unsaturated FA ratio). On the basis of the nutrient digestion and postruminal FA digestibility observed in the present experiment, solid supplemental fats do not afford appreciable advantages over conventional YG when supplemented in growing diets (forage level ~400 g/kg diet DM) for feedlot steers.