title: Effects of dietary crude protein and rumen-degradable protein concentrations on urea recycling, nitrogen balance, omasal nutrient flow, and milk production in dairy cows.
abstract: The objective of this study was to determine how interactions between dietary crude protein (CP) and rumen-degradable protein (RDP) concentrations alter urea-nitrogen recycling, nitrogen (N) balance, omasal nutrient flow, and milk production in lactating Holstein cows. Eight multiparous Holstein cows (711±21kg of body weight; 91±17d in milk at the start of the experiment) were used in a replicated 4×4 Latin square design with a 2×2 factorial arrangement of dietary treatments and 29-d experimental periods. Four cows in one Latin square were fitted with ruminal cannulas to allow ruminal and omasal sampling. The dietary treatment factors were CP (14.9 vs. 17.5%; dry matter basis) and RDP (63 vs. 69% of CP) contents. Dietary RDP concentration was manipulated by including unprocessed or micronized canola meal. Diet adaptation (d 1-20) was followed by 8d (d 21-29) of sample and data collection. Continuous intrajugular infusions of [(15)N(15)N]-urea (220mg/d) were conducted for 4d (d 25-29) with concurrent total collections of urine and feces to estimate N balance and whole-body urea kinetics. Proportions of [(15)N(15)N]- and [(14)N(15)N]-urea in urinary urea, and (15)N enrichment in feces were used to calculate urea kinetics. For the low-CP diets, cows fed the high-RDP diet had a greater DM intake compared with those fed the low-RDP diet, but the opposite trend was observed for cows fed the high-CP diets. Dietary treatment had no effect on milk yield. Milk composition and milk component yields were largely unaffected by dietary treatment; however, on the low-CP diets, milk fat yield was greater for cows fed the low-RDP diet compared with those fed the high-RDP diet, but it was unaffected by RDP concentration on the high-CP diets. On the high-CP diets, milk urea nitrogen concentration was greater in cows fed the high-RDP diet compared with those fed the low-RDP diet, but it was unaffected by RDP concentration on the low-CP diets. Ruminal NH3-N concentration tended to be greater in cows fed the high-CP diet compared with those fed the low-CP diet, and it was greater in cows fed the high-RDP diet as compared with those fed the low-RDP diet. Nitrogen intake and both total N and urea-N excretion in urine were greater for cows fed the high-CP diet compared with those fed the low-CP diet. However, N balance and urinary excretion of purine derivatives were unaffected by dietary treatment. Urea-N entry rate (UER) was greater in cows fed the high-CP diet compared with those fed the low-CP diet; however, UER was unaffected by dietary RDP concentration. The proportion of urea-N recycled to the gastrointestinal tract (as a percentage of UER) was greater in cows fed the low-CP diet compared with those fed the high-CP diet. In summary, reducing dietary CP concentration decreased urinary N excretion but had no effect on milk yield, thus resulting in an overall improvement in milk N efficiency.
authors.affiliation: Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada. Electronic address: tim.mutsvan@usask.ca.
authors.forename: T
authors.initials: T
authors.lastname: Mutsvangwa
authors.affiliation: Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada.
authors.forename: K L
authors.initials: KL
authors.lastname: Davies
authors.affiliation: Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada.
authors.forename: J J
authors.initials: JJ
authors.lastname: McKinnon
authors.affiliation: Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada.
authors.forename: D A
authors.initials: DA
authors.lastname: Christensen
mesh_terms: D000818:Animals; D002417:Cattle; D004032:Diet / Q000662:veterinary; D004044:Dietary Proteins / Q000008:administration & dosage*; D004063:Digestion; D005260:Female; D007774:Lactation; D008892:Milk / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism; D014508:Urea / Q000378:metabolism
chemical_list: D004044:Dietary Proteins; D014508:Urea; D009584:Nitrogen
keywords: crude protein; dairy cow; microbial protein; rumen-degradable protein; urea-nitrogen recycling