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 BW) to monitor the behavior of SRU in the ruminal environment. Compared to 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 [REMOVED SEQ FIELD]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 level 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 levels but was equivalent to
urea at the 0.8 and 1.2% supplementation levels; these effects were due to
urea source x level 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%) levels but is equivalent to
urea at 0.8 and 1.2% levels.
