Influences of saturation ratio of supplemental dietary fat on digestion and milk yield in dairy cows

Four multiparous, ruminally and duodenally cannulated Holstein cows in midl actation were utilized in a 4 x 4 Latin square to evaluate the effects of s upplemental fat from sources varying in proportions of unsaturated and satu rated fatty acids on nutrient digestion and lactation performance. All diet s (45% alfalfa hay) contained 12% whole cottonseed (as-fed); treatments wer e no supplemental fat (control, 3% total fatty acids, dry matter basis) or additional 2% tallow, 2% yellow grease, or 2% blend (60% tallow: 40% yellow grease). The unsaturated to saturated fatty acid ratios were 1:1 for tallo w and 2.5:1 for yellow grease. Dry matter intake, apparent ruminal and tota l tract digestibilities of organic matter, neutral detergent fiber, acid de tergent fiber, N, and fatty acids, and microbial efficiency were similar ac ross treatments. Microbial N flow to the duodenum was increased by yellow g rease. Supplemental fat reduced the postruminal digestibility of fatty acid s, primarily the saturated fatty acids; increasing saturation of the fat so urce 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 propionate ratio. Yields of mil k and milk fat increased with fat supplementation. Concentrations and yield s of trans vaccenic acid in milk increased linearly with the unsaturated fa tty acid content of the fat supplement. Modest supplementation using highly unsaturated fats to diets containing whole cottonseed can increase milk pr oduction without disturbing rumen function, evident by the similar VFA conc entrations, nutrient digestibilities, and milk composition.