Lactating dairy cow responses to dietary sodium, chloride, and bicarbonate during hot weather.

Twelve Holstein and eight Jersey cows in the first half of lactation were assigned to one of four concentrate formulations to provide supplemental sodium salts in the total diet dry matter of 1) .5% sodium chloride, 2) .5% sodium chloride plus .72% sodium bicarbonate, 3) .5% sodium chloride plus .72% sodium bicarbonate, and 4) 1.44% sodium bicarbonate to measure effects on body temperature and respiration rate, milk yield and composition, and blood components that reflect acid-base balance. After data were adjusted for body weight, age, and a covariate based on differences within individuals in the standardization period, they were analyzed with a model that included breed, treatment, week, temperature-humidity index, and interactions. Cow fed sodium bicarbonate and no supplemental sodium chloride had lower body temperatures than the other groups. High sodium and chloride in basal diet and drinking water prevented the large change in dietary amount and ratio of these two electrolytes that treatments were designed to impose. More stringent control of dietary amounts and greater heat stress will be necessary to show effects of these elements on acid-base balance.