Large animal - June 2023

Good calf health is important for production efficiency of a dairy farm. Research from UCD School of Veterinary Medicine has shown that calves in poor health grow at a slower rate (Rhodes et al., 2021) and are therefore expected to have negative fertility (Hayes et al., 2019) and production impacts as adults (Hayes et al., 2021). In addition, if replacement calves die, more calves need to be born to maintain a given replacement rate, exacerbating the environmental impacts of dairy production. The welfare of dairy calves is under increasing scrutiny from both industry and consumers. Calf mortality metrics are recognised as crude indicators of calf welfare: whilst low calf mortality alone should not be considered evidence of good welfare, high calf mortality is undoubtedly an indication of poor welfare.

Quantifying calf mortality

To accurately measure calf mortality and to facilitate between-herd comparisons, it is important that it is measured using appropriate metrics. In epidemiology, the terms ‘risk’ and ‘rate’ have specific definitions but are often used incorrectly. Risk is the number of cases occurring over a specified time period, divided by the number at risk and therefore ranges from 0 to 1 (or 0% to 100%). On the other hand, rate is the number of cases occurring per unit time at risk (Dettori et al., 2021). As an example, 100-day calf mortality risk might be 5% (or 0.05), whereas calf mortality rate might be 0.06 deaths per 100 calf days at risk.

In Irish dairy systems, quantifying calf mortality using risk is problematic since a significant proportion of calves are sold from some farms. As a consequence, using the number of calves born on the farm as a denominator will invariably underestimate mortality on the farm. This is demonstrated in the schematic in Figure 1 which shows how mortality risk may be the same for four calves in each of two example farms, yet true mortality rate varies significantly. In the Netherlands, recent work comparing a range of calf mortality metrics has shown that results can change dramatically with slight differences in definitions, e.g., changing farm ranking in between-herd comparisons (Santman-Berends et al., 2019). Therefore, some herds will exceed a particular ‘acceptable’ threshold for calf mortality on one metric yet be below an ‘acceptable’ threshold for calf mortality when the definition is changed slightly.

Johne’s control and calf health

The Irish Johne’s control programme (IJCP) led by Animal Health Ireland was introduced as a pilot programme in 2013, progressing to a voluntary national programme subsequently (Gavey et al., 2021). One of the four programme objectives of the IJCP is to improve calf health. This is possible since many of the interventions to reduce within-herd transmission of paratuberculosis are considered ‘calf protective’ measures that can benefit calf health generally (Field et al., 2022; McAloon et al., 2016). However, to date there has been relatively little peer-reviewed evidence to support and quantify this positive impact on calf health.

Recent Irish research on calf mortality

In 2017, the Department of Agriculture Food and the Marine funded the Surveillance, Welfare and Biosecurity (SWAB) research project led by UCD School of Veterinary Medicine and Teagasc. Recent research from this study (McAloon et al., 2023) has quantified calf mortality (from 1-100 days old) in Irish dairy herds using multiple metrics. In addition, this work has examined the relationship between paratuberculosis control measures on farm and calf mortality.

Overall, this study found a calf mortality rate of 0.041 deaths per 100 calf days at risk between 2016 and 2020. As expected, 100-day calf mortality risk (0.032) significantly underestimated mortality. The top 5% of herds achieved a mortality rate of 0 deaths per 100 calf days at risk, whereas in the worst 5% of herds the rate was 0.106 deaths per 100 calf days at risk. Mortality was associated with several animal and herd-level factors including calf sex and breed, and herd size and type. 

Comparing calf mortality between countries is notoriously difficult owing to differences in methodology and definitions. Nevertheless, this Irish value (0.041 deaths per 100 calf days at risk) is among the lowest reported recently in major dairy producing countries. The reasons for this difference are unclear: it could simply reflect differences in methodology and/or data capture but may also reflect a lower disease burden in Irish dairy calves. Recent work from our group has shown that the prevalence of respiratory disease in pre-weaned Irish dairy calves is approximately 4% (95% Bayesian probability intervals 0-8%) which also appears to be lower than dairy systems in other countries (Donlon et al., 2023). However, while these findings are positive, the large disparity between male and female mortality (male calves were 1.45-times as likely to die after correcting for all other factors), in the absence of reasonable biological rationale for this difference, indicates that there is substantial room for improvement in the management and health of our male dairy calves.

Between 2016 and 2020 there was a decline in calf mortality on Irish dairy farms nationally: calves born in 2020 were 0.83-times as likely to die compared with those born in 2016, after correcting for other variables. For IJCP herds, initial calf mortality was higher than for non-IJCP herds. However, the decline in calf mortality observed nationally occurred to a greater extent in IJCP herds. In addition, for those farms conducting Johne’s disease Veterinary Risk Assessment and Management Plans (VRAMP), higher risk assessment scores (poorer biosecurity) were associated with increased mortality. This suggests that implementation of recommended biocontainment practices to control paratuberculosis in IJCP herds was associated with a reduction in calf mortality. For the first time, this work has provided robust evidence that efforts to reduce the transmission of paratuberculosis, through veterinary-led risk assessments leading to agreed management plans, result in reductions in calf mortality beyond what is generally achieved in non-IJCP herds.

Acknowledgements

The study by McAloon et al. (2023) was funded by the Irish Department of Agriculture Food and the Marine as part of the Surveillance, Welfare and Biosecurity (SWAB) project (RSF 17/2/230).

The studies by Rhodes et al. (2021) and Donlon et al. (2023) were funded by Dairy Research Ireland.

References

Available on request