Nutritional Problems Affecting Calcium and Magnesium Metabolism in Grazing Ruminants

Complex disorders arise when the calcium and magnesium requirements of ruminants are not met during pregnancy and lactation. Hypocalcaemia and hpagnesemia occur when the losses of Ca and Mg in milk cannot be supplied from the diet or mobilized from body reserves. Older cows and ewes, which canrvot mobilize enough minerals from bone or body fluids, are particularly susceptible when there is reduced absorption of Ca and Mg from the gut. This may arise simply from inadequate intakes, or as a result of complex interactions that decrease Ca and Mg absorption from the rumen. Increased concentrations of ammonium ions and, particularly, potassium in the rumen act independently to reduce net Mg absorption. Intrarumen K concentrations may be increased either through dietary intake, or from reductions in salivary sodium secretion. Factors affecting salivary secretion, which is the main source of Na and P for the rumen, may indirectly affect the absorption of Mg and also of Ca. The absorption of Ca from the rumen is unaffected by intrarumen K concentration, but increases as intrarumen P concentrations increase. Ewes! which normally mobilize skeletal Ca reserves to meet the fetal demands during late pregnancy, are particularly susceptible to hypocalcaemia when intake or absorption of Ca decreases. The young lamb will maintain its plasma Ca concentration at the expense of bone structure when dietary Ca intake is inadequate during growth. Young lambs are unable to absorb sufficient Ca from pasture. Milk intakes that result in growth rates greater than 150 g/day during the first six weeks are necessary to prevent osteoporosis by the time lambs are weaned at 12 weeks. Dietary management offers the most practical way of preventing most nutritional disorders affecting Ca and Mg metabolism in ruminants.