Effect of level of dietary potassium on the absorption and excretion of calcium and magnesium by lactating cows

Abstract

This study was undertaken with the objective of providing more basic information on the absorption and excretion of calcium (Ca) and magnesium (Mg) by lactating cows when high levels of potassium (K) are fed. Employing potassium carbonate as the source of supplemental K, total mixed 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-lactation in a 3 × 3 Latin square experiment replicated five times. The experimental periods were 21 d in length. Blood, rumen and milk samples were obtained during the 2nd and 3rd wk of each experimental period and two cows from each treatment group were subjected to total collection procedures for feces and urine during the last 4 d of each experimental period. Dry matter intake was 23.0, 23.8 and 22.3 kg d−1 for the cows 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 the diet did not influence the fat and protein content 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 fluid 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 water 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 total 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 concluded from the results of this study that high K levels in the diet interfere with absorption of Mg but not Ca. However, K did appear to interfere with the utilization of Ca as indicated by the lower amount of Ca in the milk and urine. Increasing K levels in the diet increased water consumption and urine output which has major implications for waste management. Key words: Lactating cows, excess potassium, calcium, magnesium