Magnesium metabolism in the rumens of lactating dairy cows fed on spring grass

Six rumen cannulated lactating Jersey cows were used to study the effects of an abrupt change from a dry diet to young spring grass on mineral metabolism in the rumen, on plasma Mg2+ levels and on water intake. There was an increase in the intake of dietary Mg2+ and concentration of ultrafilterable Mg2+ in rumen fluid when the grass was fed associated with a fall in plasma Mg2+ concentration and percentage urinary Mg2+ excretion. The pH of rumen fluid fell when the cows were fed grass and there was a close inverse relationship with the concentration of ultrafilterable Mg2+ and Ca2+. Changes occurred in the mineral composition of rumen fluid that interact to affect the absorption of Mg2+. The concentrations of NH3, ultrafilterable K+ and ultrafilterable Ca2+ increased and the concentration of ultrafilterable Na+ decreased. These changes were associated with an increase in the potential difference between rumen fluid and blood (positive). Water consumption fell abruptly when the grass was fed and some cows drank no water on several days. However, total water intake was almost doubled resulting in an increased rumen volume.