Abstract #864

# 864
The effect of zilpaterol hydrochloride supplementation on apparent nutrient digestibility and carbon-nitrogen retention of steers fed at maintenance intake.
Lee-Anne J. Walter*1, N. Andy Cole2, Jenny S. Jennings3, John P. Hutcheson4, Beverly E. Meyer2, Angela N. Schmitz1, DeMetris D. Reed1, Ty E. Lawrence1, 1West Texas A&M University, Canyon, TX, 2USDA ARS, Bushland, TX, 3Texas A&M AgriLife Research and Extension Center, Amarillo, TX, 4Merck Animal Health, Summit, NJ.

A trial was conducted to examine apparent nutrient digestibility and carbon (C) -nitrogen (N) retention of cattle supplemented zilpaterol hydrochloride (ZH). Beef steers (n = 20; 463 ± 14 kg) blocked (n = 5) by weight and source were individually fed and adapted to maintenance intake (MI; {[(BW×0.891)0.75 × 0.077]/diet NEm, BW d −1 and 1}) for 21 d before ZH (90 mg/hd/d) or non-ZH treatment for 20 d (455 ± 14 kg, SOT). During the 21d maintenance period, MI was adjusted based on BW (d −1, 1 vs. 4, 5, 9, 10, 14, 15, 19 and 20) after 10h water and feed withdrawal (no feed remained for any animals at start of withdrawal periods). Intakes were not adjusted during 20 d treatment period. Feces and urine was collected at MI (d12–16 of ZH period) with daily feed and fecal samples analyzed for proximate analysis, ADF, NDF, starch and C; urine was analyzed for C and N. Data were analyzed as a mixed model with fixed effect of ZH and random effects of block and chamber. Dry matter (DM) intake, DM digestibility, fecal or urine output did not differ (P > 0.10) between treatments. Intake, fecal excretion and apparent digestibility of NDF, starch or ether extract did not differ (P > 0.10) between treatments. Control cattle excreted more (P < 0.05) N in urine (39.8 vs. 32.4 g/d, respectively) and tended (P < 0.07) to excrete more N daily than ZH treated cattle. Cattle fed ZH tended to have increased absolute N retention (P = 0.07; 22.14 vs.14.12 g/d) and increased N retention as a percentage of digested N (P = 0.06; 39.73 vs. 25.49%) while apparent N digested did not differ (P > 0.10) between treatments. Total C loss via, urine, fecal, or CH4 did not differ (P > 0.10) between treatments but ZH treated cattle lost more C via CO2 than control cattle (P = 0.04; 1036.9 vs. 974.3 g, respectively). Total C digested and retained did not differ between treatments (P > 0.10). Results from this trial indicate that ZH treatment does not alter apparent digestibility of nutrients or C retention but does tend to increase N retention (P = 0.07) while increasing C loss through CO2 production (P = 0.04)

Key Words: beef, carbon nitrogen retention, zilpaterol