Insights into the pathogenesis of catastrophic spontaneous humeral fractures in first-lactation dairy cows

Findings in bones that indicate issues with bone formation in affected cows include:

These findings suggest that affected cows may have failed to reach peak bone mass. Growth arrest lines were present in 21% of cows with humeral fractures. These can form when bone growth temporarily stops due to factors such as systemic illness, major stress, trauma or nutritional deprivation, leaving a transverse bone trabecula. Nutritional deprivation due to protein or energy restriction (undernutrition or inadequate feed quality) is the most likely contributor to the formation of growth arrest lines, considering the histological and histomorphometry findings reported in affected cows (Wehrle-Martinez et al. 2023a).

Subsequent bone resorption and remodelling, both observed in many affected humeri, can lead to the disappearance of these lines, likely obscuring their true prevalence (Wehrle-Martinez et al. 2023a). While the presence of growth arrest lines in 21% of affected cows provides convincing evidence of interrupted bone growth, their absence in other affected cows does not rule out similar periods of growth disruption, as remodelling may have removed these lines.

The presence of growth arrest lines in the examined sections also suggests that the changes leading to inadequate bone formation occurred recently. In New Zealand, the second winter of a heifer’s life, leading up to calving at 2 years of age in the early spring, coincides with a period of lower pasture quality and quantity and the potential for reduced growth rates if additional supplementary feed is not provided (Handcock et al. 2019). Therefore, it is plausible that this seasonal challenge contributes to skeletal growth deficits in cows that graze on pasture (Gibson et al. 2024). However, this hypothesis has not been prospectively studied and requires further investigation.

It is important to acknowledge that not all rising 2-year-old dairy cows experience poor pasture conditions, as many are fed on crop-based diets during their second winter. A recent study by Wehrle-Martinez et al. (2023a) found that osteoporosis and bone changes indicative of decreased and irregular bone formation and increased bone resorption were present in rising 2-year-old dairy cows regardless of winter-feeding strategy, including those grazing on fodder beet and those on other diets (swedes, maize, silage oats, and/or kale). However, the impact of specific crop-based diets on bone quality and fracture risk remains unclear and requires further investigation.

Moreover, studies investigating the relationship between bovine live weight, stature, and bone morphology have provided further evidence on why the humerus might be at risk of fractures, particularly following recent events such as the second winter nutrition. Notably, the humerus continues its longitudinal and appositional growth even after the metacarpus has ceased growing at 1 year of age, making it more susceptible to growth issues in the second winter (Gibson et al. 2019; Gibson et al. 2022; Gibson et al. 2025). These findings agree with measures of stature, whereby 90% of height is achieved in the first year but girth continues to grow after 2 years of age (Gibson et al. 2022). A case-control study using peripheral computed tomography to compare bone morphology between affected and control cows found that affected cows had a significant reduction in cortical bone mineral density. Additionally, there was a trend towards reduced bone length, cortical content, total bone content, and stress-strain index, suggesting these results reflect a recent period of compromised humeral bone growth, such as can occur during the second winter for New Zealand dairy cows (Gibson et al. 2021).

The second process observed in the bones from affected cows was increased bone resorption (high bone turnover). Findings associated with this process in affected cows included:

Bone resorption is a normal physiological process that occurs in cows during gestation and lactation, mainly due to increased demands for calcium (Rowland et al. 1972; Horst et al. 2005). This process is a significant risk factor for spontaneous humeral fractures, considering the reported three fold increase in bone resorption during lactation (Rowland et al. 1972; Horst et al. 2005), and the greater bone resorption observed in first-lactation cows compared to multiparous cows (Liesegang et al. 1998; Kim et al. 2010). In cows with spontaneous humeral fractures, the amount of bone resorption was found to be excessive and likely occurred for a longer time than in control cows (Wehrle-Martinez et al. 2023a). This is supported by the presence of abnormal cortical resorption, fewer, thinner, and less connected bone trabeculae, as well as reduced cortical thickness and the presence of large resorption canals in the cortex, including the distal humerus in affected cows (Wehrle-Martinez et al. 2023a). This finding raises questions about potential metabolic or nutritional imbalances exacerbating bone loss.

Further evidence of compromised bone strength was observed in bones from affected cows using Raman and Fourier transform infrared spectroscopy (FTIR) (Wehrle-Martinez et al. 2023c). These techniques are useful for the evaluation of bone chemical composition and hence can provide information about bone quality (Compston 2006; Paschalis et al. 2017). Raman and FTIR band ratios measured and studied for the first time in pooled bones from affected and control cows showed that the proximal humerus from affected cows, on average, is immature and more recently formed compared to the same bone from control cows (Wehrle-Martinez et al. 2023c). The formation of this new bone could also be a compensatory response for poor bone strength due to osteoporosis (Wehrle-Martinez et al. 2023c).

Copper deficiency has been proposed as a contributing factor to spontaneous humeral fractures, but the evidence is complex. Low liver copper concentrations are commonly measured in affected cows and consequently, copper deficiency is frequently suggested as a cause of humeral fractures in dairy cows (Suttle and Angus 1978; Grace and Knowles 2010; Thompson et al. 2012). This led to a second research question: what is the role of copper in humeral fractures in 2-year-old first-lactation dairy cows?

A significant number of affected cows have low liver copper concentrations (up to 80% of affected cows) and/or low serum copper concentrations (up to 21% of affected cows) at the time of fracture (Thompson et al. 2012; Dittmer et al. 2016; Wehrle-Martinez et al. 2023b). These findings suggest that copper deficiency may be a contributing factor to the occurrence of fractures (Thompson et al. 2012; Dittmer et al. 2016). During bone formation, copper plays a central role by facilitating collagen synthesis and crosslinking, both of which contribute to the mechanical properties of bone (Viguet-Carrin et al. 2006; Saito and Marumo 2010).

The role of copper deficiency in the pathogenesis of spontaneous humeral fractures in affected cows was investigated by assessing the total collagen and its crosslink content in the humerus of affected cows compared to control cows (Wehrle-Martinez et al. 2022). This study aimed to determine if there was a strong link between low liver copper concentrations and fracture risk, and to identify why affected cows have low liver copper concentrations. Twenty-six humeri from affected cows and 14 humeri from control cows were processed, and the total collagen and collagen crosslink content were quantified using mass spectrometry and the hydroxyproline assay, respectively (Wehrle-Martinez et al. 2022). This novel study showed that the low liver copper concentrations found in affected cows are likely the result of the mobilisation of stored copper (from the liver) to the bone. This was evidenced by the association of higher bone copper concentration (mean of 0.69 ± 0.27 mg/kg in affected cows compared to 0.41 ± 0.12 mg/kg in control cows; mean ± s.d.) and total collagen crosslink content (mean of 3979.07 ± 797.72 mg/mol in affected cows compared to 3058.71 ± 548.70 mg/mol in control cows; mean ± s.d.) observed in affected cows. More importantly, the lower total collagen content in humeri from affected cows (mean of 1.71 ± 0.43 mg in affected cows compared to 2.18 ± 0.46 mg in control cows; mean ± s.d.) could also evidence that periods of protein and/or energy deficiency during important growth periods are a significant contributory factor in the pathogenesis of spontaneous humeral fractures (Wehrle-Martinez et al. 2022). Protein provides the necessary amino acids for collagen synthesis, a key component of the bone matrix (Babraj et al. 2005), while sufficient caloric intake ensures the energy required for bone modelling and remodelling (Karsenty and Oury 2010). Although it is expected that bones with osteoporosis will have overall lower collagen content, these findings underscore the need for further research into the interplay between nutrition, copper metabolism, and bone integrity.

Liver samples from the same 26 affected and 14 control cows used for collagen and collagen crosslink quantification were analysed to determine the concentration of cadmium, iron, molybdenum and zinc, which are known to be antagonists of copper absorption (Grace and Knowles 2010). No significant accumulation of these were found in the liver of affected cows, suggesting a minimal antagonist impact to the liver copper concentrations (Wehrle-Martinez et al. 2022).

In recent years, a significant increase in fluoride concentrations in New Zealand topsoils due to the use of phosphate fertilisers has possibly resulted in an increase in ingestion of fluoride by grazing animals (Healy 1968; Hedley et al. 2007; Loganathan et al. 2008; Kim et al. 2016). We hypothesised that the use of phosphate fertilisers could have led to the accumulation of toxic amounts of fluoride in the bones of affected cows, significantly contributing to the decline in bone quality and strength (Ranjan and Ranjan 2015). This led to a third research question: Is fluoride toxicity a contributing factor to humeral fractures?

Bone fluoride concentration was measured in 35 affected cows and 21 control bones. The study concluded that although affected cows had significantly higher bone fluoride concentrations compared with control cows, the concentration remained well below the thresholds that cause significant adverse effects on bone (Grace 2010), making its role in fracture risk uncertain (Wehrle-Martinez et al. 2025). However, fluoride is reported to reduce collagen synthesis and collagen crosslink formation (Susheela and Mukerjee 1981; Ranjan and Ranjan 2015). While low total bone collagen is described in affected cows, there were no low collagen crosslinks, making bone fluoride concentration a less likely contributing risk factor in the pathogenesis of spontaneous humeral fractures.

The final question investigated whether there were any farm management and husbandry practices that were associated with humeral fractures. To do this, a case-control study approach was used to compare farm management and husbandry practices between farms across New Zealand with and without humeral fractures, using a farm-based questionnaire. To maximise the response rate, the authors employed diverse methods for questionnaire distribution (Wehrle-Martinez et al. 2024). Despite these efforts, responses were received from only 0.6% of dairy farms in New Zealand (68/11,179 farms for the 2019/2020 season), based on data from LIC, DairyNZ (2021). This was a major limitation of the study, as the low response rate may have introduced bias. The survey results suggested that Holstein-Friesian Jersey crossbred cows (KiwiCross) may be at higher risk of spontaneous humeral fractures. While fractures have also been reported in Holstein-Friesian, Jersey, and Ayrshire breeds, KiwiCross cows are the predominant breed in the New Zealand dairy herd, making up a significant proportion of the national herd (Wehrle-Martinez et al. 2024a). It is unclear whether this increased risk is due to breed susceptibility or simply reflects the higher proportion of crossbred cows in the national herd.

The survey also indicated that delaying calves’ access to pasture for a week after weaning appeared to reduce fracture risk (Wehrle-Martinez et al. 2024a). However, the specific reasons why some farms allowed or restricted calves’ access to pasture were not surveyed but are likely influenced by factors such as nutritional management and environmental conditions. Notably, case farms in the study were generally located further north, where calves may be placed on pasture earlier due to better weather. This regional variation could contribute to differences in the duration of milk feeding and may have implications for growth and health outcomes.

Is it important to note that while these findings offer preliminary insights, the study was underpowered to identify definitive risk factors and was limited by potential response bias. Furthermore, the proportion of fracture cases explained by the identified risk factors was not assessed, leaving significant gaps in our understanding of this condition. As highlighted by Hunnam et al. (2024), further research is urgently needed to clarify the aetiology and contributing factors of humeral fractures in dairy heifers.