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.

Genotype and gender effects on sheep limb bone growth and maturation: selection for loin depth causes bone hypotrophy

“…Allometric analysis of length v. total limb bone length (Table 2) confirmed that, as shown previously (Cake et al 2006b), most limb bones obey a strict allometric relationship with the greater limb length, though this relationship is less consistent (i.e. higher residual error and lower r 2 values) for the fore and hind cannon bones.…”

“…This is only partly consistent with tooth eruption data from the same study (Hopkins et al 2007), showing that BL × M and PD g × BLM, but not PD m × M, lambs showed earlier eruption of permanent dentition. In agreement with this discrepancy, bone magnesium content of PD m × M lambs at earlier timepoints was also consistent with the mineral profile of physiologically less mature animals, despite their more relatively advanced skeletal maturity (Ravaglioli et al 1996;Cake et al 2006b). Similarly, although ewes are known to mature earlier in terms of metacarpal growth plate closure (Ho et al 1989;Jeremiah et al 1997) and were clearly demonstrated in this study to be earlier maturing skeletally, the absence of a gender difference in bone magnesium content and the later eruption of permanent dentition in ewes shown by Hopkins et al (2007) suggests this does not necessarily reflect earlier physiological maturation.…”

“…We have previously advocated the use of maturity proportions based on limb bone lengths indexed to a known or estimated mature endpoint (Cake et al 2006b). This index demonstrates that ewes mature skeletally earlier than wethers, and that PD m × M and BL × M wethers mature earlier compared with wethers of other genotypes.…”

“…Bone ash calcium, phosphorus and magnesium content were determined to assess the 'mineral maturity' of bone (Grynpas 1993;Ravaglioli et al 1996;Cake et al 2006b). …”

“…However, it is not clear to what extent alternative, qualitative maturity indices such as physeal closure or dental eruption accurately reflect physiological or even skeletal maturation. An alternative index of maturity may be bone mineral profile, in particular magnesium content which steadily declines in a manner which appears to reflect physiological rather than chronological age (Ho et al 1989;Ravaglioli et al 1996;Cake et al 2006b). …”