Phosphorus Nutrition in Ruminants Grazing Tropical Rangelands ^†

Abstract

Nutritional deficiency of phosphorus (P) is a major constraint to productivity of cattle grazing many tropical rangelands with low P soils, particularly in northern Australia, South America and Africa. Cattle growth and reproductive rates may be severely reduced. Such P deficiency is usually addressed by providing supplements containing calcium phosphates. In the seasonally dry tropics such supplements are most effective when fed during the summer rainy season when the pasture quality as energy and protein are highest. Young cattle often continue to grow slowly when P deficient, but with reduced bone mineralization. Cows with normally high bone mineral reserves (from previously P-adequate diets) can mobilize bone P during late pregnancy and lactation when diet P is insufficient. This mobilization may contribute up to ca. one-third of the P requirements and allow P-deficient cows to maintain milk production and calf growth, but is associated with reduced pasture intake and severe loss of cow liveweight. Cows can replenish bone minerals when P intake exceeds immediate requirements for growth and milk. Since on large commercial farms in rangelands it is often difficult to effectively implement P supplementation of cattle during the rainy season the mobilization and replenishment of body mineral reserves are important for managing P nutrition through the annual cycle. Biochemical markers in blood are valuable for diagnosis of P deficiency in grazing cattle. In conclusion, understanding of the nutritional physiology of cattle provides opportunities to improve management of P nutrition of cattle grazing P deficient rangelands and alleviating production losses.