Magnesium absorption by wethers fed potassium bicarbonate in combination with different dietary magnesium concentrations

We hypothesized that the decrease in the absolute amount of Mg absorbed in the total digestive tract, as induced by K, would remain constant if Mg intake by ruminants was increased. This hypothesis was based on earlier studies that used temporarily isolated rumens of sheep and the fact that the rumen is the major site of Mg absorption in ruminants. To test the hypothesis, six rumen-fistulated wethers were fed diets at two concentrations of K and three concentrations of Mg in a 6 x 6 Latin square design. Diets contained either 10 or 36 g of K/kg of dry matter and 1.3, 2.5, or 3.7 g of Mg/kg of dry matter. Extra K was added in the form of KHCO3, and Mg was added in the form of MgO. For wethers fed the low K diets, absolute Mg absorption rose by 0.32 g/d for each 1 g/d of Mg intake that was in excess of requirements. The high K diets reduced absolute Mg absorption by a mean of 0.36 g/d; this reduction was independent of Mg intake. Magnesium intake and Mg concentrations in rumen liquid were positively related. Extra KHCO3 in the diet increased K concentrations in rumen liquid, but the concentrations of Mg remained unchanged. Rumen pH was elevated by a mean of 0.45 units when the high K diets were fed. This study indicated that, in practical ruminant feeding, the supplementation of Mg to either low or high K diets increased absolute Mg absorption to the same extent.