PSXIV-1 Effect of Ambient Temperature on Respiration Rate and Ruminal Temperature for Growing-Finishing Calf-fed Holstein Steers in the Feedlot During Summer in Southern California

Abstract

Abstract Objectives were to evaluate the relationships between ambient temperature (AT) and respiration rate (RR) and ruminal temperature (RT) of calf-fed Holstein steers during the extreme heat characteristic of the months of August and September in the Southern California desert region. Holstein steers (n = 21; one steer per pen; initial BW = 422 ±15 kg) were utilized in 28-d experiment (August 17 – September 14, 2021). Steers were blocked by initial shrunk weight (220 d before the initiation of the experiment) and randomly assigned to 21 pens (5 steers per pen). Pens were 62 m2 with 25 m2 overhead shade, automatic waterers and 2.4 m fence-line feed bunks. All steers were fed a steam-flaked corn-based growing-finishing diet. A bolus measuring temperature activity (smaXtec Basic, Smaxtec Animal Care GmbH, Austria) was placed in the rumen of each steer 55 d before the initiation of the experiment (June 22, 2021). During the experimental period, the respiration rate of one calf per pen was collected weekly. Respiration rate was obtained at 1100 h by counting the number of flank movements per minute. Ambient temperature at 1100 h averaged 37.6 ± 2.3° C for the experimental period. Temperature-humidity-index (THI) at 1100 h averaged 83.5 ± 1.7 (steers experienced “danger” or “emergency” THI categories through the entire 28 d trial). As AT increased from the minimum of 22.9° C to a maximum of 39.8° C, RR increased from 58.6 ± 12.9 breaths/min to 103.9 ± 16.9 breaths/min (RR = -54.9 + 4.13 AT, P < 0.01, r2 = 0.65). Likewise, RR increased with increasing RT (RR = -1259 + 33.7 RT, P < 0.01, r2 = 0.92) as RT increased. As ambient temperature increased, ruminal temperature increased (RT = 35.9 + 0.117 AT, P< 0.01, r2 = 0.65), though never exceeding 40.6° C.