Forelimb bone growth and mineral maturation as potential indices of skeletal maturity in sheep

Abstract

The aim of this study was to characterise the allometric growth and bone mineral maturation of forelimb bones in sheep throughout the growth phase. Forelimb bones (scapula to proximal phalanx) were measured in 84 Merino sheep from similar genetic stock of approximately 12, 32, 64, 84, 116, and 168 weeks of age, with approximately equal numbers of wethers and ewes in each age cohort (n = 14). Sheep were selected for divergence of size, body weight, and condition, in order that the effects of age and body size could be assessed independently. Bone magnesium was measured in the metacarpal and humerus. Results demonstrate the highly coordinated, allometric nature of linear bone growth within the ovine forelimb, though allometric growth patterns differed from those previously published for bone weights. We propose that estimates of maturity proportion (M) based on relative limb bone length or limb proportions may present significant advantages over weight- or composition-based maturity indices, or qualitative variables such as dental eruption or USDA-type maturity scores. Sex differences in growth gradients were minimal, although the higher variability and greater gender divergence of metacarpal bone length casts doubt on the use of its growth plate (breakjoint) to indicate maturity. Bone magnesium content was found to decrease rapidly during the growth period and may represent a useful independent estimate of physiological maturity.