Dietary fat and nitrogen composition of milk from lactating cows

Effects of dietary fat on milk composition, particularly milk N, were evaluated in lactating dairy cows at two stages of lactation. Complete mixed diets containing 0, 3.5, or 7% of the diet DM as animal fat were fed to 12 cows in a 3 X 3 Latin square. Cows were divided into two status categories based on stage of lactation resulting in two squares of early and two of late lactation cows. Percentages of fat, solids, lactose, and protein were decreased and ash increased in the milk of cows fed the 7% fat diet. Percent of casein N was elevated while nonprotein N was depressed by fat fed in diet. Percentage of solids and protein was higher and lactose lower for cows in late lactation than for those in early lactation. Dietary fat reduced the proportion of short-chain fatty acids in milk fat and increased C18:0 and C18:1 fatty acids. Digestibility of DM, energy, and fiber was not significantly affected by dietary fat, but estimates decreased with increased dietary fat. Cows in late lactation had a higher digestibility of dietary fractions than early lactation cows even though intakes of DM were similar.