title: Effects of slow-release urea on ruminal digesta characteristics and growth performance in beef steers.
abstract: Two experiments were conducted to evaluate the effects of slow-release urea (SRU) versus feed-grade urea on ruminal metabolite characteristics in steers and DMI, gain, and G:F in growing beef steers. Experiment 1 used 12 ruminally cannulated steers (529 +/- 16 kg of BW) to monitor the behavior of SRU in the ruminal environment. Compared with feed-grade urea, SRU decreased ruminal ammonia concentration (P = 0.02) and tended to increase ruminal urease activity (P = 0.06) without affecting ruminal VFA molar proportions or total concentrations (P > 0.20). After 35 d of feeding, the in situ degradation rate of SRU was not different between animals fed urea or SRU (P = 0.48). Experiment 2 used 180 Angus-cross steers (330 +/- 2.3 kg) fed corn silage-based diets supplemented with urea or SRU for 56 d to evaluate the effects on feed intake, gain, and G:F. The design was a randomized complete block with a 2 x 4 + 1 factorial arrangement of treatments. Treatments included no supplemental urea (control) or urea or SRU at 0.4, 0.8, 1.2, or 1.6% of diet DM. Over the entire 56 d experiment, there were interactions of urea source x concentration for gain (P = 0.04) and G:F (P = 0.01) because SRU reduced ADG and G:F at the 0.4 and 1.6% supplementation concentrations but was equivalent to urea at the 0.8 and 1.2% supplementation concentrations; these effects were due to urea source x concentration interactions for gain (P = 0.06) and G:F (P = 0.05) during d 29 to 56 of the experiment. The SRU reduced DMI during d 29 to 56 (P = 0.01) but not during d 0 to 28, so that over the entire experiment there was no difference in DMI for urea source (P = 0.19). These collective results demonstrate that SRU releases N slowly in the rumen with no apparent adaptation within 35 d. Supplementation of SRU may limit N availability at low (0.4%) concentrations but is equivalent to urea at 0.8 and 1.2% concentrations.
authors.affiliation: Department of Animal and Food Sciences, University of Kentucky, Lexington 40546-0215, USA.
authors.forename: C C
authors.initials: CC
authors.lastname: Taylor-Edwards
authors.forename: G
authors.initials: G
authors.lastname: Hibbard
authors.forename: S E
authors.initials: SE
authors.lastname: Kitts
authors.forename: K R
authors.initials: KR
authors.lastname: McLeod
authors.forename: D E
authors.initials: DE
authors.lastname: Axe
authors.forename: E S
authors.initials: ES
authors.lastname: Vanzant
authors.forename: N B
authors.initials: NB
authors.lastname: Kristensen
authors.forename: D L
authors.initials: DL
authors.lastname: Harmon
mesh_terms: D000818:Animals; D001835:Body Weight / Q000502:physiology; D002417:Cattle / Q000254:growth & development* / Q000378:metabolism*; D004032:Diet / Q000662:veterinary; D019587:Dietary Supplements; D005766:Gastrointestinal Contents / Q000737:chemistry*; D008297:Male; D011897:Random Allocation; D012417:Rumen / Q000378:metabolism*; D014508:Urea / Q000008:administration & dosage*
chemical_list: D014508:Urea