Abstract #865

# 865
The effect of zilpaterol hydrochloride supplementation on energy metabolism of steers at maintenance and fasting intake levels.
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.

An indirect calorimetry trial examined the energetic efficiency 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. d 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. Respiration chambers were used to quantify energetics for maintenance (d12–16 of ZH period) and fasting heat production (FHP; d19–20 of ZH period, total 4d fast). Daily feed and fecal samples were analyzed for gross energy (GE); urine was analyzed for nitrogen and GE was calculated from the energetic value of urea. Data were analyzed as a mixed model with fixed effect of ZH treatment and random effects of block and chamber. No difference in DMI, O2 consumption or CH4 production (P > 0.10) was detected between treatments but ZH cattle had higher CO2 production (P = 0.04; 2325 vs. 2185 L/steer; 23.6 vs. 22.4 L/kg BW0.75). During FHP, O2 consumption was not affected (P > 0.10) by treatment whereas CO2 production (L/steer) increased with ZH treatment (P = 0.04; 1423 vs. 1338 L/steer). Gross energy, fecal energy, digestible energy and methane energy did not differ between treatments (P > 0.10), however urinary energy was higher (P = 0.05; 0.091 vs. 0.074 Mcal) in control cattle. There was no difference in metabolizable energy between treatments (P > 0.10). Heat production tended (P = 0.09) to increase for ZH treated cattle (12.44 vs. 11.69 Mcal, respectively) on an absolute basis, but not on a BW0.75 basis (P = 0.12; 0.126 vs. 0.120, respectively). No difference in FHP was detected between treatments (P = 0.32). Results from this trial indicate that ZH treatment tends (P = 0.09) to modify heat production during maintenance by increasing CO2 production (P = 0.04).

Key Words: beef, energy metabolism, zilpaterol