Digestibility and Volatile Fatty Acids in Steers and Wethers at 21 and 32 C Ambient Temperature

Abstract

Four steers and four closely shorn wethers, each with ruminal fistula, were used in a reversal experiment to compare the direct effect of 21 and 32 C ambient temperatures (70% relative humidity) on dry matter digestibility and ruminal volatile fatty acid concentrations. Pelleted alfalfa intake was held constant at 2.3% of body weight. Orts were placed into the rumen via fistula. Dry matter digestibility was determined by total collection during the last 7 days of each trial. On one of the last 2 days, ruminal fluid was sampled 11 times within 13 h. At 32 C, the steers suffered considerable heat stress as evidenced by a mean 1.6 C increase in body temperature, reduced heart rate, and greatly increased respiration rate. The wethers, however, exhibited only increased respiration rate at 32 C. Extent of digestion increased five percentage units for the steers but did not change for the wethers at the high temperature. Temperature caused only slight shifts in ruminal volatile fatty acid concentrations. Volatile fatty acid concentrations were much higher in steers than in wethers. The frequently observed effects of heat stress on intake and digestibility are mediated by changes in rate of passage as a result of altered thyroid secretion rates. © 1975, American Dairy Science Association. All rights reserved.