Dietary cation-anion balance and cation source effects on production and acid-base status of heat-stressed cows.

Two 4 x 4 replicated Latin square studies, each containing 8 lactating Holstein cows, were conducted simultaneously in south Georgia during the summer. The effects of dietary cation source (Na or K) and increasing dietary cation-anion balance (milliequivalents of Na + K - Cl per kilogram of feed DM) within cation source (control = 120.4 meq/kg of feed DM; Na source = 219.7, 347.8, 464.1 meq/kg of feed DM; K source = 231.2, 352.6, 456.0 meq/kg of feed DM) were determined on performance and acid-base chemistry during hot, humid weather. Cow body temperatures were elevated by environmental conditions but were not affected by dietary cation-anion balance. Differences in body temperature from dietary cation source probably were related to differences in cow BW. Intake of DM increased linearly, but yields of milk and FCM did not change with increasing dietary cation-anion balance, and cation source had no effect. Milk fat and protein percentages were not altered by dietary cation-anion balance, and greater milk fat and protein percentages from cows offered the Na versus K source diets probably were due to differences between cows in the two Latin squares. Alterations in blood acid-base chemistry with increasing dietary cation-anion balance were as expected. Greater blood buffering capacity, indicated by blood base excess and bicarbonate content, may be responsible for the improved feed intake.