Variation in bone composition of grazing sheep in south-western Queensland, related to lactation and type of country

Abstract

Samples of rib bone were obtained by biopsy from Merino ewes on four properties in the Charleville district of south-western Queensland, before joining and after lambing in 1970. Comparisons were made within and between properties of bone composition and blood inorganic phosphate (Pi) concentrations in ewes which were classified according to whether they had reared a lamb just before the sampling or not. The study confirmed earlier work showing that the expression of bone composition per unit of volume is a more sensitive criterion than that per unit of dry fat-free weight. Within properties, the level of skeletal mineral was significantly lower after lactation, but the concentration of blood Pi was not affected. Significant differences were found in bone composition between areas studied. The ranking order of the properties in terms of bone composition was comparable with that on the basis of recorded lamb-marking percentages. Blood Pi was of no value as an index of skeletal mineralization in these sheep. The fresh bone of ewes that were grazing the most favourable area in this study, and had recently lactated, had a specific gravity in excess of 1.60, and a phosphorus content between 130 and 140 mg cc-1. It is tentatively suggested that these figures be regarded as evidence of satisfactory mineralization. The methods of bone biopsy and analysis employed in this work appear to have potential application in the assessment of the phosphorus status of grazing animals.