MAGNESIUM METABOLISM IN THE RUMENS OF LACTATING DAIRY COWS FED ON SPRING GRASS

SUMMARY Sixrumencannulated lactating Jersey cowswereusedtostudy theeffects ofanabrupt change fromadrydiet toyoungspring grass onmineral metabolism intherumen, onplasma Mg2" levels andonwaterintake. Therewasanincrease intheintake ofdietary Mg2+and concentration ofultrafilterable Mg2+inrumenfluid whenthegrass wasfedassociated witha fall inplasma Mg2+concentration andpercentage urinary Mg2+excretion. ThepHofrumen fluid fell whenthecowswerefedgrass andthere wasaclose inverse relationship withthe concentration ofultrafilterable Mg2+andCa2+. Changes occurred inthemineral composition ofrumenfluid thatinteract toaffect theabsorption ofMg2+. Theconcentrations ofNH3, ultrafilterable K+andultrafilterable Caa2+ increased andtheconcentration ofultrafilterable Na+decreased. Thesechanges wereassociated withanincrease inthepotential difference between rumenfluid andblood (positive). Waterconsumption fell abruptly whenthegrass was fedandsomecowsdrank nowater onseveral days. However, total waterintake wasalmost doubled resulting inanincreased rumenvolume.