Effects of dietary fat and protein on fatty acid flow to the duodenum and in milk produced by dairy cows.

Four Holstein cows fitted with ruminal and duodenal cannulas were used in a 4 x 4 Latin square with treatments in a 2 x 2 factorial arrangement. Treatments were 1) soybean meal, no fat; 2) fish meal, no fat; 3) soybean meal, fat; and 4) fish meal, fat. Cows were fed for ad libitum intake a diet of alfalfa haylage, corn silage, and concentrate (30:20:50) on a DM basis. Intake of gross energy (105 Mcal/d) was not altered by treatment comparisons. However, feeding fat decreased energy digested in the rumen (15 vs. 24%) and increased energy digested postruminally (55 vs. 43%) but resulted in similar amounts of energy (72 Mcal/d) digested in the total tract. The flow of C14:0, C16:0, C18:0, C18:1, C18:2, C18:3, and total fatty acids to the duodenum was increased by feeding fat. The average flow of C14:0, total C18, and total fatty acids to the duodenum was greater than their intake for all treatments, suggesting de novo synthesis of fatty acids by ruminal microbes. Biohydrogenation of unsaturated C18 was decreased 70, 67, 59, and 51% for treatments 1 to 4, respectively, by feeding fat and fish meal. Digestibility of total fatty acids entering the small intestine (78%) was not altered by treatment comparisons; however, feeding fat altered digestibility of individual fatty acids. The proportion of C16:0 and C18:1 was increased, and the proportion of C6:0, C8:0, C10:0, C12:0, and C14:0 was decreased in milk fat produced by cows fed fat.