Phosphorus requirements of tropical grazing systems: the northern Australian experience

Abstract

Background and aims: Extensive grazing is common on low phosphorus (P) soils in tropical areas. In this study we aimed to use experience and research results from northern Australia to investigate methods to manage low P status to efficiently raise animals without exploiting soil P resources. Scope: Simple "farm-gate" P balances were calculated for four cattle breeding and growing operations. Estimated P balances were slightly negative (outputs>inputs) for extensive breeding operations (0.02 to 0.04 kg/ha/year depending on P supplementation), slightly positive for a system growing young animals with small annual P fertilizer applications, and negative for a mixed grazing-cropping enterprise. In northern tablelands pastoral environments, responses to P application have remained unchanged over the last 50 years, with >80% of paddocks remaining P limited. Liveweight gain responses to P fertilizer in most experiments have been <4 kg LWG/kg P applied as fertilizer. Conclusions: Little or no P fertilizer is used in most grazing systems. Supplying P directly to animals as a feed supplement can economically improve animal performance and contribute to an improvement in the P balances of grazing systems. Guidelines for supplement use have been devised. Further research is required to determine the best application strategies to overcome P deficits, and the best farming systems to minimise P requirements.