Dietary sodium bicarbonate and magnesium oxide for early postpartum lactating dairy cows: effects on production, acid-base metabolism, and digestion.

In two experiments with cows in early lactation fed 40% corn silage and 60% concentrate diets (dry basis), we examined effects of 1.0% dietary sodium bicarbonate and .8% magnesium oxide in a 2 x 2 factorial arrangement on performance, acid-base metabolism, and digestion. In Experiment 1 with 20 Holstein and 4 Jersey cows, intake and milk production were not affected by treatment through 12 wk postpartum. In digestion trials at 3, 6, 9, and 12 wk postpartum, addition of magnesium oxide increased digestibility of dry matter 1.8% units by sodium bicarbonate whereas acid detergent fiber digestion increased 9 to 12% units with addition of either buffer. In Experiment 2, with four fistulated Holstein cows in a Latin square design, intake per unit body weight was increased .18% units by dietary sodium bicarbonate while milk production was unaffected. Dry matter and acid detergent fiber digestion were increased slightly by sodium bicarbonate plus magnesium oxide addition whereas nitrogen balance increased 23 g/day in cows fed magnesium oxide alone. Because of the low fiber content of the ration, milk fat percent increased .5 to .9% by addition of either buffer in both experiments and resulted in corresponding increased in fat-corrected milk. Sodium bicarbonate and magnesium oxide prevented rapid declines in rumen pH associated with feeding in Experiment 2, which related to changes in acid detergent fiber digestion. Blood acid-base metabolites were relatively unchanged by treatment in either experiment. Urinary bicarbonate and net acid excretion rates were changed on the average by +41 and +43 meq/h by sodium bicarbonate or magnesium oxide but could not be related to changes in intake and production.