Influence of Heat Stress on Rumen Acid Levels and Some Blood Constituents in Cattle

Abstract

As part of a series of investigations to determine some of the internal body adjustments made by cattle in response to heat stress, studies were conducted in a psychrometric chamber to examine changes in rumen acids, blood glucose, ketones and hematocrit. Holstein cows and yearling Hereford heifers exposed to 90° F. temperature were compared with pair-mates kept under prevailing ambient conditions (36–78° F.). There was also a group of Angus heifers exposed to 100° F. for 7 weeks. Total VFA concentrations were lower at 90° F., with lower acetic acid concentrations being largely responsible. Changes in total VFA concentrations were consistently correlated to a significant degree with rectal temperature (−.49 to −.78**), and rate of respiratory evaporative loss (−.64 to −.71**), but the relationships of VFA levels to dry matter and water intake, blood glucose and blood ketones were quite variable. In the low temperature groups, rectal temperature was positively correlated with hematocrit, but in the chamber groups there was a negative relationship, dependent on the level of ambient temperature (−.22 to −.70**). Hematocrit was also negatively associated with respiration rate (−.59), respiratory vaporization (−.65**), surface evaporation (−.43) and dry matter intake (−.26) in the Holstein chamber group. Hematocrit had a positive but quite variable relation to rumen volatile fatty acid levels. Indications are that the direct effect of the heat stress will bring about some changes in rumen acid concentrations. The extent can not be discerned because of the indirect effects—viz., decreased feed intake and increased water intake taking place concurrently. Results from these studies indicate that changes in rumen acid levels and blood constituents (viz., hematocrit) may be involved in the adjustment to heat stress.