Effects of zilpaterol hydrochloride on internal body temperature and respiration rate of black-hided feedlot steers and heifers during moderate heat stress

Abstract Text: The objective of this study was to examine the effects of zilpaterol hydrochloride (ZH) on the internal body temperature and respiration rate of feedlot cattle during moderate heat stress.  Black-hided steers and heifers (n=96) were sourced from a commercial feedlot and transported to the Texas Tech University Beef Center in New Deal, TX.  Cattle were weighed and scanned using real-time ultrasound.  Resulting data were used to predict empty body fat percentage (pEBF %).  Steers (n=48; BW = 520 ± 30.4 kg; pEBF % = 26.2 ± 1.9) and heifers (n=48; BW = 466 ± 29.5 kg; pEBF % = 26.7 ± 1.7) were blocked within gender by pEBF % in a completely randomized block design and randomly assigned to pen (2 pens/block; 4 hd/pen) and treatment (6 pens/treatment): 1) control heifers (HC), 2) ZH heifers (HZ), 3) control steers (SC), and 4) ZH steers (SZ).  During the ZH treatment period the climatic conditions were: mean maximum temperature, 29.67 °C; mean minimum temperature, 15.18 °C; mean relative humidity, 60.69%; mean wind velocity, 8.37 km/h.  Immediately preceding the ZH treatment period (d -1), cattle were fitted with an indwelling rectal temperature probe.  Rectal temperatures (RT) were recorded at 5-min intervals throughout the treatment period.  Panting scores (PS) were assigned to cattle every other day from 1500 to 1700  h during the ZH treatment period based on a scale ranging from 1 to 4 (1 = normal respiration, 4 = extremely elevated respiration).  Repeated measures analysis of RT revealed a significant treatment x gender interaction (P < 0.0001).  The HC group recorded higher RT relative to the HZ group (P ≤ 0.0009; 38.83 °C versus 38.76 °C, respectively).  Alternatively, the SC group recorded lower RT relative to the SZ group (P ≤ 0.0009; 38.96 °C versus 38.98 °C, respectively).  Although highly significant differences were detected for RT, the marginal effect size may be insufficient to deduce biologically significant implications.  Fisher’s exact test for count data was used to analyze the frequency distribution of PS.  No differences were detected in PS between treatments at any single point of data collection or for the entire treatment period (P ≥ 0.32).  Collectively, the variables measured in the present study do not provide compelling evidence to suggest that ZH treated black-hided cattle of either sex have more difficulty coping with moderate heat stress relative to their control counterparts.

Keywords: cattle, zilpaterol hydrochloride, heat stress