DIETARY CATION-ANION DIFFERENCE, ACID-BASS STATUS, MINERAL METABOLISM, RENAL-FUNCTION, AND MILK-PRODUCTION OF LACTATING COWS

Three switchback experiments were conducted with 12 cows in early lact ation, 12 cows in midlactation, and 12 cows in late lactation. Each ex periment compared two dietary cation-anion differences. Increasing die tary cation-anion difference increased DMI and milk production in earl y and midlactation. These effects were not observed in late lactation. Production of milk protein and lactose and concentration of lactose w ere increased by the higher dietary cation-anion difference in early l actation. Higher dietary cation-anion difference reduced milk fat conc entration in midlactation because of the higher milk production. In la te lactation, none of the milk components were affected by dietary cat ion-anion difference. In early and midlactation, apparent absorption o f water and urine volume were increased by a higher dietary cation-ani on difference; these effects were not caused by higher intake of Na or by higher glomerular filtration rate. Intake, balance, and concentrat ion of S in plasma were increased by the lower cation-anion difference at all stages of lactation. Excretion of HCO3- in urine was reduced b y a lower dietary cation-anion difference at all stages of lactation, but secretion of protons in milk was reduced in late lactation only. I ncreasing dietary cation-anion difference does affect acid-base parame ters in urine at all stages of lactation, but DMI and milk production of cows were affected in early and midlactation only.