Influence of dietary calcium chloride on adaptive changes in acid-base status and mineral metabolism in lactating dairy cows fed a diet high in sodium bicarbonate

Eight mature, lactating Holstein cows were assigned to two treatments in a simple changeover design to examine physiological adaptations to dietary NaHCO3 over a 2-wk period. Treatments were diets containing, on a total diet DM basis, 1) 1.4% NaHCO3 and 2) 1.4% NaHCO3 with 1.3% CaCl2 substituted for limestone. Blood samples were collected hourly, and urine samples bihourly, for 8 h postfeeding on the 1st d of the experimental period and daily at 4 h post-feeding thereafter. Hourly analysis revealed that for most variables measured, the greatest response to dietary treatments occurred at or after 4 h post-feeding. Blood pH and blood HCO3- tended to be lower, and urinary excretion of chloride higher, on 1.4% NaHCO3 plus 1.3% CaCl2 throughout the trial. Blood HCO3- and blood partial pressure of CO2 fluctuated in response to the dietary treatments but functioned to maintain blood pH fairly constant. Plasma K concentration tended to increase during the experimental period on both treatments, but overall, plasma mineral concentration was relatively unresponsive to diet, and urinary mineral excretion was an excellent indicator of dietary mineral concentration. Inherent homeostatic mechanisms appear adequate to maintain a constant internal milieu when a high NaHCO3 diet is fed for 2 wk.