PSVIII-18 - Effect of rumen temperature on the average daily gain of growing-finishing calf-fed Holstein steers in the feedlot

Objectives were to evaluate the relationship between ruminal temperature and average daily gain of calf-fed Holstein steers during the extreme summer months characteristic of the Southern California desert region. Thirty-eight calf-fed Holstein steers (395 ±25 kg) were utilized in the 84-d experiment (June 30 – September 20, 2020). Steers were blocked by initial shrunk weight (168 d before the initiation of the experiment) and randomly assigned to 38 pens with 4 other steers (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 and activity at ten-minute intervals (smaXtec Basic, SmaXtec Animal Care GmbH, Austria) was placed in the rumen of each steer 112 d before the initiation of the experiment (March 10, 2020). Air temperature for the trial averaged 34.90 ± 3.03°C (maximum = 43.8 ± 2.97° C; minimum 24.72 ± 4.00°C). Average temperature-humidity-index (THI) was 80.1 ±5.4. Maximum THI during the 84-d period averaged 89.29 ± 7.85 (steers experience “emergency” THI category for all 84 days of the trial). Maximum (RTmax) and mean (RTm) daily ruminal temperature averaged 40.93 ±0.24 and 40.21 ±0.23°C, respectively. Average daily gain (mean = 1.18 kg/d; maximum = 1.54 kg/d; minimum = 0.88 kg/d) increased (P < 0.01) as RTm decreased (ADG, kg = 19.13 – 0.45 RTm; r² = 0.28). There was a close association between RTmax and RTm (RTm = 4.57 + 0.87 RTmax; r² = 0.84). While there are numerous factors that influence ADG of feedlot cattle, differences in ruminal temperature during periods of elevated ambient conditions (ie. temperature, humidity, ration) accounts for an important portion of that variation.