Phosphorus management for breeding cattle in northern Australia

Summary

Most of the cattle in Australia north of about 21oS graze phosphorus-deficient native pastures growing on phosphorus-deficient soils. Cattle grazing these pastures often suffer sub-clinical phosphorus deficiency, leading to slow growth and poor fertility. Across northern Australia, the losses might total $80 million annually. Where no clinical symptoms of phosphorus deficiency are apparent, producers are often reluctant, for reasons of cost, to feed supplementary phosphorus.

As a result of this project, the following conclusions and recommendations can be made: - Concentrating phosphorus supplementation on young females would, on properties with adequate segregation, greatly reduce costs and effort compared to supplementing a larger fraction of the herd. - Growing heifers and first-calf cows should be targeted for aggressive and comprehensive phosphorus management. - Blood phosphorus is a useful diagnostic indicator of phosphorus deficiency when used with care. Indicators of bone turnover are potentially very sensitive and reliable indicators of phosphorus status. Tail-bone mineral density is an unreliable assay of phosphorus status. - Blood phosphorus and bone turnover indicators for different cow classes should be tested and validated. Sources of variation in bone turnover indicators need to be located and the assays simplified. - Phosphorus requirements of grazing, breeding cows are quite modest by published standards.

The indicative requirements published by Ternouth and Coates in 1997 are valid for use in northern Australia in the pasture growing season. There are still no good estimates of dry season/pregnancy/out-of-season lactation requirements for cows. - There needs to be published an addendum to the McCosker and Winks manual detailing new wet season phosphorus requirements and revised blood diagnostic levels. The requirements should estimate supplementation requirements for different pasture zones, taking account of phosphorus supplied from forage. - Cow phosphorus requirements for dry season growth, pregnancy, and out-of-season lactation need to be determined.