Broken shoulders in dairy heifers in New Zealand : investigating the relationship between live weight and bone morphology in the bovine forelimb : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand

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Spontaneous humeral fractures in first lactation dairy heifers are a significant economic and welfare issue in the New Zealand dairy industry. With approximately 4% of dairy farms being affected and, within farm, up to 25% of replacement heifers lost, the cost to the dairy industry is estimated to be over $10 million. Preliminary observations from Dittmer et al. (2016), showed the presence of growth arrest lines in bones from affected heifers, indicating a period of malnutrition in the heifer’s life. However, with little known about how heifer’s bones grow within the New Zealand pastoral system, it is difficult to identify developmental time periods that have the greatest impact on long bone growth and, therefore, greatest potential to affect the risk of fracture. Therefore, the aim of this thesis was to investigate the relationship between live weight and bone morphology in the bovine forelimb to identify potential risk periods for spontaneous humeral fractures in first-lactation dairy heifers. In this thesis, the metacarpus and humerus were opportunistically collected from cattle from a variety of experiments carried out at Massey University and from samples submitted from farms throughout New Zealand. Measures of bone morphology in the metacarpus were highly correlated with the same measure in the humerus (R²=0.89-0.98). Measures of the humerus and metacarpus were also highly correlated with live weight at all ages. However, the relationship between bone and live weight changed over time demonstrating differences in bone maturity. Distal regions of the forelimb such as the metacarpus matured by one year of age as indicated by a different relationship between live weight and pQCT measures of the humerus at 24 months compared with 8-12 months of age. However, proximal regions of the forelimb such as the humerus continued to grow after a year of age. Differences in proximal and distal bone growth were reflected in the pattern of stature growth. A cohort of dairy heifers (Holstein Friesian, Jersey, and Holstein-Friesian cross Jersey) were measured for live weight, height, wither-rump length, girth, and distal and proximal forelimb length (stature) from birth to 15 months of age. As live weight increased, the contribution of the distal limb to increases in height decreased, which is consistent with knowledge that physeal closure in cattle occurs earlier in distal limb bones, such as the metacarpus, than in proximal limb bones, such as the humerus. Finally, bones collected from heifers affected and unaffected by spontaneous humeral fractures were compared and the effect of copper status on bone measures was examined. The mid diaphysis of the humerus of the affected group had reduced cortical bone mineral density (p<0.05) and a trend for reduced cortical content and total bone content which contributed to a reduced stress-strain index. A trend for reduced bone length in affected humeri provides additional support for the hypothesis of inhibited humeral growth. Heifers with low liver copper concentration had reduced humeral length and reduced cortical bone mineral density (p<0.05). Growth and fracture data presented in this thesis support the hypothesis that the most likely developmental period in which inadequate bone growth occurs in heifers subsequently affected by humeral fracture is in the months leading up to the event, which coincides with the animal’s second winter.
Figure 1.2 is re-used with the publisher's permission. Parts of Chapter 2 & Chapter 5 are republished under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC-BY-NC-ND).
Dairy cattle, Heifers, Weight, Bones, Growth, Forelimb, Wounds and injuries, New Zealand