Nycterohemeral patterns of acid-base status, mineral concentrations and digestive function of lactating cows in natural or chamber heat stress environments.

Effects of heat stress on multiparous lactating Holstein cows were evaluated in a continuous design in natural environments (a shade management system vs no shade, n = 6/treatment, early- to mid-lactation cows) and in a single-reversal design in climatic chambers (thermoneutral vs thermal stress, n = 4/treatment, mid- to late-lactation cows). Objectives were to compare effects of nycterohemeral heat stress vs nonheat stress environments on acid-base status, mineral concentrations and rates of ruminal liquid dilution and solid phase digesta turnover. In the shade vs no shade experiment, the complete mixed diet consisted of 38% corn silage and 62% corn-based concentrate mix. In the chamber experiment, the diet consisted of 40% alfalfa haylage and 60% corn-based concentrate mix. In both experiments, respiration rates, rectal temperatures, blood and urine were sampled hourly for 26 h. Measurements of rectal temperature, respiration rate and blood gas composition indicated that chamber heat stress simulated natural heat stress. In both experiments, cows exhibited signs of respiratory alkalosis only during hours of heat stress. Ruminal turnover rates of liquid were measured with chromium-ethylenediaminetetraacetate and solid turnover rates with ytterbium-marked fecal fiber, both dosed through the ruminal cannula. Turnover rates of liquid and solid digesta and total volatile fatty acids were lower in the heat stress vs the thermoneutral environment. The results illustrate the necessity of frequent sampling to characterize nycterohemeral patterns of physiological measurements during heat stress.