Nitrogen utilization, whole-body urea-nitrogen kinetics, omasal nutrient flow, and production performance in dairy...

The objective of this study was to determine whether the partial replacement of barley starch with lactose (fed as dried whey permeate; DWP) affects N utilization, whole-body urea kinetics, and production in dairy cows. Eight lactating Holstein cows were used in a replicated 4 × 4 Latin square design with 28-d periods. Four cows in one Latin square were ruminally cannulated and used to determine dietary effects on whole-body urea kinetics and N utilization. Cows were fed a barley-based diet that contained 3.6% (dry matter basis) total sugar (TSG; designated control), or diets that contained 6.6, 9.6, or 12.6% TSG. Dietary TSG content was increased by the replacement of barley grain with DWP (83% lactose). Diets were isonitrogenous (∼17.3% crude protein), and starch contents of the control, 6.6, 9.6, and 12.6% TSG diets were 24.3, 22.2, 21.2, and 19.1%, respectively. Whole-body urea kinetics were measured using 4-d infusions of [¹⁵N¹⁵N]-urea with concurrent total collections of feces and urine. Dry matter intake (mean = 26.7 kg/d), milk yield (mean = 34.9 kg/d), and milk protein and fat contents were unaffected by diet. Ruminal ammonia-N concentration decreased linearly as TSG content increased, whereas ruminal butyrate concentration increased linearly as TSG content increased. Urinary excretion of total N and urea-N changed quadratically, whereas urinary excretion of total N (% of N intake) tended to change quadratically as TSG content increased. Fecal N excretion linearly increased as TSG content increased. A quadratic response was observed for total N excretion as TSG content increased. Milk N and retained N were not affected by diet. As TSG content increased, we observed quadratic responses in the omasal flow of fluid-associated and total bacterial nonammonia N, endogenous production of urea-N, urea-N recycled to the gastrointestinal tract, and urea-N returned to the ornithine cycle. Dietary TSG content did not affect the anabolic utilization of recycled urea-N or the proportion of recycled urea-N that was used for bacterial growth. Our results indicate that feeding DWP did not influence dry matter intake, milk yield, or milk composition. Feeding DWP decreased ruminal ammonia-N concentration, but this did not result in positive responses in milk protein secretion or N balance. The quadratic response in omasal flow of total bacterial nonammonia N indicated that including TSG beyond 9.6% of diet dry matter might depress ruminal microbial protein synthesis.