The effects of technology use in feedlot production systems on the heat stress and blood metabolites of finishing steers

Abstract Text: Crossbred, black-hided steers (n=336; initial BW=379 ± 8 kg) were utilized in a RCBD (8 pens/treatment; 14 steers/pen) to determine the effects of technology use in feedlot production systems during the summer on heat stress.  Treatments consisted of an all-natural treatment (receiving no growth promoting technologies; NAT), a conventional treatment (implanted with 40 mg of estradiol and 200 mg of trenbolone acetate on d 0, and fed 33 and 9 mg/kg of monensin and tylosin daily, respectively; CONV), and a CONV treatment plus the addition of a beta-adrenergic agonist (zilpaterol hydrochloride at 6.76 g/ton for the last 20 DOF with a 3-4 d withdrawal; CONV-Z). Blood was collected every 10 d during the beta-agonist period (d 112 to 132) on 7 steers/pen to determine blood gases, pH, and metabolites. The same subset of steers was evaluated for respiration rates and panting scores during the final 23 DOF, and rumen temperatures were continuously measured. All blood metabolites measured were within clinically normal concentrations throughout the experiment. Blood pH was greater for the CONV-Z cattle compared to the NAT cattle (7.41 vs. 7.37; P<0.01) and CONV cattle intermediary (7.39) at d 122.  The CONV-Z cattle had reduced lactate concentrations compared to NAT and CONV cattle at d 122 (13.5 vs. 28.9 and 27.3 mg/dL, respectively) and 132 (12.5 vs. 25.1 and 27.2 mg/dL, respectively; P<0.01).  Conventional cattle exhibited greater glucose concentrations than NAT and CONV-Z (88.9 vs. 83.9 and 82.5 mg/dL, respectively; P<0.01).  The CONV-Z cattle exhibited greater potassium concentrations than NAT and CONV cattle at d 122 (4.40 vs. 4.16 and 4.23 mmol/L, respectively; P<0.01).  The CONV-Z cattle showed increased severity in the morning panting score compared to CONV and NAT cattle (1.23 vs. 1.00 and 1.08, respectively; P<0.03), and the afternoon panting score compared to CONV cattle (1.84 vs. 1.67; P<0.01), with NAT cattle intermediary (1.76).  Respiration rates were lowest for CONV cattle, intermediate for NAT cattle, and highest for CONV-Z cattle in the morning (99.5 vs. 105.0 vs. 112.8 breaths/min, respectively) and afternoon (120.1 vs. 125.8 vs. 133.8 breaths/min, respectively; P<0.01).  The NAT cattle had lower mean rumen temperatures compared to CONV cattle in the morning (39.56 vs. 39.71 °C; P<0.01) and afternoon (40.70 vs. 40.95 °C; P<0.01), with CONV-Z cattle intermediary (39.62 and 40.88 °C).  Based on these results, zilpaterol increased respiration score and rate, as noted on the product label.

Keywords: beta-adrenergic agonist, heat stress, respiration rate