title: Effects of dietary crude protein and supplemental urea levels on nitrogen and phosphorus utilization by feedlot cattle.
abstract: Three dietary CP concentrations (11.5, 13.0, and 14.5% of DM) and 3 supplemental urea levels (100, 50, and 0% of supplemental N) were used in a completely randomized block design experiment conducted at 2 locations to determine N and P balance and serum urea N (SUN) concentrations of feedlot cattle. Crossbred steers [British and British x Continental; initial BW = 315.0 +/- 3.2 kg at location 1 (n = 27) and initial BW = 353.2 +/- 8.4 kg at location 2 (n = 27)] were used in 3 nutrient balance sampling periods (SP) at the beginning, middle, and end of the feeding period (154 d in location 1 and 159 d in location 2). Fecal N (g/d; P = 0.03), urinary N (g/d; P < 0.01), urinary urea N (UUN; g/d; P < 0.01), apparent N absorption (g/d; P < 0.01), and SUN concentration (mg/dL; P < 0.01) increased linearly as dietary CP concentration increased. Nitrogen retention (g/d) was not affected (P = 0.61) by dietary CP concentration. Phosphorus intake (g/d; P = 0.02), fecal P (g/d; P = 0.04), and urinary P (g/d; P = 0.01) increased linearly as dietary CP increased, reflecting changes in diet composition with increasing CP concentrations. As dietary urea levels increased, urinary N (g/d; P = 0.04), UUN (g/d; P = 0.01), and apparent N absorption (g/d; P = 0.04) increased linearly, but P intake (g/d; P = 0.10) and urinary P (g/d; P = 0.02) decreased linearly. No interactions were observed between SP and dietary treatments for most variables. Evaluation of SP means, however, showed that as days on feed increased, fecal N (g/d; P = 0.01), urinary N (g/d; P < 0.01), UUN (g/d; P < 0.01), apparent absorption of N (g/d; P < 0.01), SUN (mg/dL; P < 0.01), and urinary P (g/d; P < 0.01) increased linearly, whereas retained N (g/d) decreased linearly (P < 0.01) with increasing days on feed. These data suggest that changes in dietary CP and urea levels, as well as stage of the feeding period, markedly alter N and P utilization by feedlot cattle.
authors.affiliation: Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409, USA. jvasconcelos2@unl.edu
authors.forename: J T
authors.initials: JT
authors.lastname: Vasconcelos
authors.forename: N A
authors.initials: NA
authors.lastname: Cole
authors.forename: K W
authors.initials: KW
authors.lastname: McBride
authors.forename: A
authors.initials: A
authors.lastname: Gueye
authors.forename: M L
authors.initials: ML
authors.lastname: Galyean
authors.forename: C R
authors.initials: CR
authors.lastname: Richardson
authors.forename: L W
authors.initials: LW
authors.lastname: Greene
mesh_terms: D000818:Animals; D001806:Blood Urea Nitrogen; D002417:Cattle / Q000378:metabolism / Q000502:physiology*; D004032:Diet / Q000662:veterinary*; D004044:Dietary Proteins / Q000008:administration & dosage*; D019587:Dietary Supplements; D008297:Male; D009584:Nitrogen / Q000378:metabolism*; D010758:Phosphorus / Q000378:metabolism*; D011897:Random Allocation; D013997:Time Factors; D014508:Urea / Q000008:administration & dosage*
chemical_list: D004044:Dietary Proteins; D010758:Phosphorus; D014508:Urea; D009584:Nitrogen