EFFECT OF LEVEL OF DIETARY POTASSIUM ON THE ABSORPTION AND EXCRETION OF CALCIUM AND MAGNESIUM BY LACTATING COWS

This study was undertaken with the objective of providing more basic i nformation on the absorption and excretion of calcium (Ca) and magnesi um (Mg) by lactating cows when high levels of potassium (K) are fed. E mploying potassium carbonate as the source of supplemental K, total mi xed rations were formulated to contain 1.6 (low), 3.1 (medium) and 4.6 % (high) K. The three rations were fed to 15 Holstein cows in mid-lact ation in a 3 x 3 Latin square experiment replicated five times. The ex perimental periods were 21 d in length. Blood, rumen and milk samples were obtained during the 2nd and 3rd wk of each experimental period an d two cows from each treatment group were subjected to total collectio n procedures for feces and urine during the last 4 d of each experimen tal period. Dry matter intake was 23.0, 23.8 and 22.3 kg d-1 for the c ows fed the low, medium and high K diets, respectively. Milk yield was lower (P < 0.05) for cows fed the high compared to the low and medium K diets. Level of K in diet did not influence the fat and protein con tent of the milk but Ca content was lower (P < 0.05) for cows fed the medium and high K diets compared with those fed the low K diet. Plasma K levels were 15.0, 15.7 and 16.2 mg dL-1 while plasma Mg levels were 2.35a, 2.25ab and 2.17b mg dL-1 for cows on the low, medium and high K diets, respectively. The acetate to propionate ratio in the rumen fl uid was wider (P < 0.05) for the cows fed the medium and high K diets compared with those fed the low K diet. The apparent absorption of Mg but not Ca was reduced with increasing levels of K in the diet. Both w ater consumption and urine output were greater (P < 0.05) for the cows fed the medium and high K diet compared with those fed the low K diet . The concentration of Ca and Mg in the urine dropped markedly and tot al excretion of Ca and Mg was lower (P < 0.05) for cows fed the medium and high K diets compared with those fed the low K diet. It was concl uded from the results of this study that high K levels in the diet int erfere with absorption of Mg but not Ca. However, K did appear to inte rfere with the utilization of Ca as indicated by the lower amount of C a in the milk and urine. Increasing K levels in the diet increased wat er consumption and urine output which has major implications for waste management.