title: Effects of varying ruminally undegradable protein supplementation on forage digestion, nitrogen metabolism, and urea kinetics in Nellore cattle fed low-quality tropical forage.
abstract: Effects of supplemental RDP and RUP on nutrient digestion, N metabolism, urea kinetics, and muscle protein degradation were evaluated in Nellore heifers () consuming low-quality signal grass hay (5% CP and 80% NDF, DM basis). Five ruminally and abomasally cannulated Nellore heifers (248 ± 9 kg) were used in a 5 × 5 Latin square. Treatments were the control (no supplement) and RDP supplementation to meet 100% of the RDP requirement plus RUP provision to supply 0, 50, 100, or 150% of the RUP requirement. Supplemental RDP (casein plus NPN) was ruminally dosed twice daily, and RUP supply (casein) was continuously infused abomasally. Jugular infusion of [NN]-urea with measurement of enrichment in urine was used to evaluate urea kinetics. The ratio of urinary 3-methylhistidine to creatinine was used to estimate skeletal muscle protein degradation. Forage NDF intake (2.48 kg/d) was not affected ( ≥ 0.37) by supplementation, but supplementation did increase ruminal NDF digestion ( < 0.01). Total N intake (by design) and N retention increased ( < 0.001) with supplementation and also linearly increased with RUP provision. Urea entry rate and gastrointestinal entry rate of urea were increased by supplementation ( < 0.001). Supplementation with RUP linearly increased ( = 0.02) urea entry rate and tended ( = 0.07) to linearly increase gastrointestinal entry rate of urea. Urea use for anabolic purposes tended ( = 0.07) to be increased by supplementation, and RUP provision also tended ( = 0.08) to linearly increase the amount of urea used for anabolism. The fraction of recycled urea N incorporated into microbial N was greater ( < 0.001) for control (22%) than for supplemented (9%) heifers. Urinary 3-methylhistidine:creatinine of control heifers was more than double that of supplemented heifers ( < 0.001). Control heifers reabsorbed a greater ( < 0.001) fraction of urea from the renal tubule than did supplemented heifers. Overall, unsupplemented heifers had greater mobilization of AA from myofibrillar protein, which provided N for urea synthesis and subsequent recycling. Supplemental RUP, when RDP was supplied, not only increased N retention but also supported increased urea N recycling and increased ruminal microbial protein synthesis.
authors.forename: E D
authors.initials: ED
authors.lastname: Batista
authors.forename: E
authors.initials: E
authors.lastname: Detmann
authors.forename: E C
authors.initials: EC
authors.lastname: Titgemeyer
authors.forename: S C
authors.initials: SC
authors.lastname: Valadares Filho
authors.forename: R F D
authors.initials: RF
authors.lastname: Valadares
authors.forename: L L
authors.initials: LL
authors.lastname: Prates
authors.forename: L N
authors.initials: LN
authors.lastname: Rennó
authors.forename: M F
authors.initials: MF
authors.lastname: Paulino
mesh_terms: D000641:Ammonia / Q000378:metabolism; D000821:Animal Feed / Q000032:analysis*; D000818:Animals; D002364:Caseins / Q000378:metabolism; D002417:Cattle; D004044:Dietary Proteins / Q000008:administration & dosage / Q000494:pharmacology*; D019587:Dietary Supplements; D004063:Digestion / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D005260:Female; D007700:Kinetics; D008762:Methylhistidines; D009584:Nitrogen / Q000378:metabolism*; D006109:Poaceae / Q000378:metabolism; D012417:Rumen / Q000502:physiology*; D014508:Urea / Q000378:metabolism*
chemical_list: D002364:Caseins; D004044:Dietary Proteins; D008762:Methylhistidines; D000641:Ammonia; D014508:Urea; C028118:3-methylhistidine; D009584:Nitrogen