Abstract
High dietary concentrations of K are known to inhibit Mg absorption in ruminants, but the influence of variation in concentrations higher than 30 g/kg dry matter was unknown, whereas such high concentrations are encountered in grass silages from various countries. Thus, the question addressed was whether K intrinsically prsent in grass silages, within the range of 30–45 g/kg dry matter, would be inversely related with apparent Mg absorption (percentage of intake) in dry cows. In a Latin-Square design, six non-pregnant and non-lactating cows were fed on six different grass silages that were supplemented with a low K, high Mg concentrate so that the amount of K in the whole ration ranged between 29 and 44 and that of Mg between 5.7 and 7.5 g/kg dry matter. Mg absorption was found not to be related with the level of K in the ration. Multiple regression analysis showed that only the dietary level of Mg contributed significantly to the explained variance in Mg absorption. For the observed lack of effect of dietary K on the low Mg absorption (approximately 12%), three explanations can be offered. First, the high levels of Mg in the whole ration (four times requirement) counteracted the inhibitory action of K on Mg absorption. Secondly, the high range of K levels in the rations may have caused maximum inhibition of Mg absorption so that no differences between rations emerged. Thirdly, the typical anion moiety of K salts occurring in grass silages might not allow for the inhibitory effect of K on Mg absorption to become expressed. In any event, this study indicates that high Mg dry cow ratios with K contents higher than 30 g/kg dry matter do not need to be further proportionally supplemented with extra Mg so as to prevent hypomagnesemia.