Influence of amount and degradability of dietary protein on nitrogen utilization by dairy cows.

Four Holstein cows fitted with ruminal and T-type duodenal cannulas were utilized in a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. The TMR contained 25% alfalfa haylage, 25% corn silage, and 50% concentrate and provided either 16.4 or 19.6% CP, with ruminal degradability calculated to be 30 or 45%. Intakes of DM, OM, ADF, NDF, and N were not altered by either amount or degradability of CP. Intake and ruminal and postruminal digestibility of starch were greater when cows were fed diets high in undegradable CP but was not altered by amount of CP. Apparent total tract digestibilities for DM, OM, starch, ADF, and NDF were similar among treatments. Apparent total tract digestibility of N was 4.7 percentage units greater for diets low in ruminally degradable CP. Apparent digestibility of OM, ADF, and NDF and true digestibility of OM in the rumen were not altered by amount of CP or undegradable CP. Increasing the CP content of the diet and the proportion of undegradable CP in the diet increased NAN flow to the duodenum. Except for Met, flows of all AA to the duodenum were increased when CP was increased. Flow of Met to the duodenum was not altered by undegradable CP content of the diet. Production of milk, 4% FCM, and milk CP was not altered by amount of CP or undegradable CP. Milk fat content and yield were increased when diets high in undegradable CP were fed. Results suggest that all diets supplied adequate amounts of AA for these cows or that Met was deficient for all cows.