lnfIuences of Saturation Ratio of Supplemental Dietary Fat on Digestion and Milk Yield in Dairy Cows

摘要

Four multiparous, ruminally and duodenally cannu- lated Holstein cows in midlactation were utilized in a 4 x 4 Latin square to evaluate the effects of supplemen- tal fat from sources varying in proportions of unsatu- rated and saturated fatty acids on nutrient digestion and lactation performance. All diets (45/100 alfalfa hay) contained 12/100 whole cottonseed (as-fed); treatments were no supplemental fat (control, 3/100 total fatty acids, dry matter basis) or additional 2/100 tallow, 2/100 yellow grease, or 2/100 blend (60/100 tallow: 40/100 yellow grease). The unsaturated to saturated fatty acid ratios were 1:1 for tallow and 2.5:1 for yellow grease. Dry matter in- take, apparent ruminal and total tract digestibilities of organic matter, neutral detergent fiber, acid detergent fiber, N, and fatty acids, and microbial efficiency were similar across treatments. Microbial N flow to the duo- denum was increased by yellow grease. Supplemental fat reduced the postruminal digestibility of fatty acids, primarily the saturated fatty acids; increasing satura- tion of the fat source magnified the reduction. Total volatile fatty acid concentrations and ruminal fluid pH were unaffected by fat supplementation or saturation level. Blend decreased ruminal pH and acetate to propi- onate ratio. Yields of milk and milk fat increased with fat supplementation. Concentrations and yields of trans vaccenic acid in milk increased linearly with the unsat- urated fatty acid content of the fat supplement. Modest supplementation using highly unsaturated fats to diets containing whole cottonseed can increase milk produc- tion without disturbing rumen function, evident by the similar VFA concentrations, nutrient digestibilities, and milk composition.