Phosphorus diagnostic tool - validation and demonstration of a diagnostic tool for phosphorus status of beef cattle

Summary

Much of northern Australia is based on soils that are deficient in phosphorus (P), an essential nutrient for growing and breeder cattle. A robust test is required by industry to determine the P content of diets grazed by cattle.

This project aimed to validate the use of the ratio of the concentration of P in the faeces (FecP) to the metabolisable energy (ME) content (FecP:ME; mg P/MJ ME) of the diet as a diagnostic test for the adequacy of dietary P content in relation to energy intake and to determine the likely response of cattle to P supplementation. Feed intake, liveweight gain, the concentration of P in plasma (PiP), FecP and FecP:ME increased with increasing diet P content and P intake. Plasma inorganic P was the best indicator of P intake of steers consuming a high N, high ME diet and under grazing conditions.

The responses of FecP:ME and FecP:dietary N content to increasing diet P were similar when dietary ME and N content were constant. In a field experiment auto-drafters were used to draft cattle to allocated P supplement treatments in a single paddock. Under these extensive grazing conditions the FecP:ME indicated that indicator steers and breeders were P deficient but no response to P supplementation was evident and serum inorganic P concentration suggested that the indicator steers were not P deficient.
The FecP:ME may require further validation across a range of land types/pasture bases before it can be recommended for wide use by industry.