Abstract #863

# 863
The effect of zilpaterol hydrochloride supplementation on gain efficiency, harvest yields and carcass grading of steers fed a 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 gain efficiency, harvest yields and grading characteristics of steers fed only to maintenance intake (MI) and supplemented with zilpaterol hydrochloride (ZH). Beef steers (n = 20; 463 ± 14 kg) blocked (n = 5) by weight and source were individually fed and adapted to 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 but steers were weighed on d1, 2, 11, 12, 17, and 18 of ZH treatment and on d 1, 2, 5, and 6 of withdrawal. Cattle were harvested after a 6 d withdrawal and carcasses graded 24 h post-harvest. Data were analyzed as a mixed model with fixed effect of ZH and random effect of block. Gain efficiency, ADG and end of ZH BW was not different between treatments (P > 0.10). Control cattle lost more BW (P < 0.01; 9 vs. 2 kg, respectively) than ZH fed cattle during the maintenance collection period (d12–16 of ZH) while BW loss of ZH fed cattle was greater (P < 0.01; 9 vs. 4 kg, respectively) during fasting heat production period (FHP; d18–19 of ZH, 4-d total fast). Harvest BW, exsanguinated BW, empty body weight (EBW) and harvest yields (g/kg EBW) were not different between treatments (P > 0.10). Supplementation of ZH did not improve HCW (P = 0.12) but did increase dressed carcass yield (P = 0.02; 62.12 vs. 60.65%, respectively). Feeding ZH improved LM area (P = 0.02; 77.81 vs. 70.90 cm2) and tended to lower USDA calculated yield grade (P = 0.06; 1.8 vs. 2.2). Marbling, 12th rib fat depth and calculated empty body fat did not differ (P > 0.10) between treatments. Results from this trial indicate that ZH supplementation alters carcass traits in steers fed reduced energy intakes by improving dressed carcass yield and LM area (P = 0.02) and exhibiting a tendency to improve yield grade (P = 0.06).

Key Words: feed efficiency, carcass grading, zilpaterol