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  • Writer's picturePaul Dillon

Low calf mortality rates, a meaningless figure?

It is human nature to compare ourselves to others. To determine the relative success of our business, we often look for industry specific benchmarks to gauge how our animals are performing compared to other farms. Mortality rate is a commonly used figure when comparing the overall quality of animal husbandry, or when researchers are discussing the efficacy of a treatment in their study, but how useful is it really? While there is no argument that a high mortality rate in a calf rearing program is seriously concerning. If 10% of your calves are dying, you definitely have a problem, or probably multiple problems, that need to be addressed. But, does a low mortality rate (<5%) really mean your calves are doing well? Research published by Teagasc in Ireland adds weight to the argument that low mortality figures are meaningless.

Researchers ¹ at Teagasc Moorepark ran a trial to establish the effect of feeding colostrum that was stored in different conditions prior to feeding to dairy heifer calves.

The colostrum was subject to 5 different treatments and each treatment was fed to 15 dairy heifer calves. 75 calves in total participated in the trial.

(1) fresh pasteurized colostrum (pasteurized and fed to calves immediately after collection).

(2) fresh colostrum (not pasteurized but fed immediately after collection).

(3) colostrum stored at 4°C in a temperature-controlled unit for 2 days before being fed to calves.

(4) colostrum stored at 13°C in a temperature-controlled unit for 2 days before being fed to calves.

(5) colostrum stored at 22°C in a temperature-controlled unit for 2 days before being fed to calves.

The colostrum fed to the calves was tested and only the highest quality (IgG>50 g/L) used, each calf received 8.5% of body weight for first feed and was subsequently given 4 feedings of unpasteurized transition milk.

At 3 days, each calf was transferred to a group pen (5m × 7m) of 15 calves; calves from different treatments were housed together. The maximum age difference between calves in each group was 2 weeks.

All calves were inspected twice daily for general demeanour and signs of clinical illness (e.g., diarrhoea, pneumonia, or navel ill). Clinical illness was determined as any case in which a calf received treatment for disease.

A total of 60 disease episodes (i.e., treatments for clinical illness) were recorded during the experimental period, which involved a total of 40 calves experiencing some form of illness.

No calf died during the trial; Mortality rate was 0% which in isolation looks good but disease incidence was 80%, which is clearly not so good.

The overall finding in relation to colostrum storage (“Colostral treatment did not affect the likelihood of a calf receiving treatment for a disease”) of the trial have been questioned by some experts because of the high incidence of disease and subsequent veterinary treatments of all 5 groups of calves.

However, this trial should not be dismissed in its entirety as it gives very interesting details regarding incidence of disease in group reared calves. The scale and housing of the calves would have a lot of similarities to a typical Irish farm.

Despite feeding adequate quantities of high-quality colostrum and transition milk to the first 2 groups, these 30 calves still had multiple disease incidents. What may have caused such high levels of disease?

1) The calves were housed in groups of 15 which is more than Teagasc own recommendations of maximum group size. According to The Animal Health Ireland Expert Group “A review of group housing of dairy calves with different feeding systems concluded that group housing increases the risk of infection, especially in larger groups and thus requires more skills and poses more challenges to management.”

2) There is not much detail on how the calves were fed except that it was twice daily for the first 4 weeks and once daily until they were weaned. This rules out the possibility that they were fed with an automatic feeder so it is most likely that they were fed with a communal teat bar. These communal teat systems will inevitably allow some calves to over feed and others will be underfed. Overfed calves run the risk of diarrhoea while undernourished calves will become susceptible to pneumonia.

3) 12 calves were treated for navel problems. It can be difficult to prevent navel sucking in groups and this can often lead to navel infections.

4) Even calves with the high level of acquired passive immunity delivered from quality colostrum will eventually succumb to disease when the challenge is large and persistent.

Some people will still argue that in spite of the 80% disease incidents that no calf died and that the time saved in labour requirements with grouping of calves compensates for the costs of increased treatments.

Calves in individual pens can and do also get disease, albeit at a much-reduced rate compared to group housing. What are the real costs of shifting calves from individual housing to group housing?

How much does respiratory disease effect milk production?

215 female calves from 3 dairy herds in southwestern Ontario were enrolled in a trial to assess if lung damage in young dairy calves influenced first-lactation milk production. Calves were clinically assessed for elevated temperature, nasal or ocular discharge, cough and ear position. The researchers also used thoracic ultrasonography, more commonly known as lung scanning, to create a more objective diagnosis of lung damage.

123 (57%) calves were diagnosed with lung damage by thoracic ultrasound within 56 days of birth. Only 92 (43%) of the calves had displayed clinical signs of Bovine Respiratory Disease during this period. 31 calves out of the 215 in the trial had suffered lung damage visible to ultrasound without displaying any clinical signs.

Roll on to 3 years later to the end of first lactation.

5% of healthy calves died before the end of first lactation, whereas 9% of lung damaged calves died before the end of first lactation. Completed lactation data was collected for 140 animals, average milk production was 8,626kg. The heifers who had been diagnosed with lung damage as calves averaged 525kg less milk in the first lactation than the calves that did not have lung damage. This trial only quantified the effect over the first lactation but it would be reasonable to assume that this lung damage could affect performance in subsequent lactations also.

123 calves were diagnosed with lung damage by thoracic ultrasound. Week 2-3 were the peak periods for diagnosis.

Lung consolidation is the term used to describe damage to the lung tissue, and once the lung has been damaged, consolidation cannot typically be reversed. Consolidation can be thought of as scar tissue within the lung, reducing both lung capacity and the ability to properly oxygenate blood

Preweaned calves are especially vulnerable as their passive immunity is declining and their endogenous humoral immunity has not yet compensated for this decline

The peak frequency of diagnosis of lung damage in the Ontario trial (2-3 weeks old) coincides with decline of passive immunity.

The lesson from this trial is that while calves can survive lung disease, their lifetime profitability has been seriously compromised.

Dr. Theresa Ollivett, a veterinarian at the University of Wisconsin-Madison has been working with ultrasound to scan calves to determine the health of the lungs. Dr Ollivett's experience is consistent with the findings of the Ontario trial that many cases of lung damage do not display symptoms, "pneumonia is an iceberg-effect, and that the clinical symptoms producers see are just the tip of the iceberg, producers are likely under-estimating how many calves actually have disease"


Dr Ollivett said about one-third of new lung disease cases are subclinical, and that the severity of the lung disease is not well-correlated with the severity of clinical signs that might be presented.

Organic dairies are more sensitive to management strategies to minimize the risk of infectious disease because they minimize the use of antibiotics. Thus, it’s not surprising that Zwald et al. (2004) reported that 63% of organic dairies in Wisconsin (20 of 32 herds) housed preweaned calves so that there was no contact with other calves. This compares to only 13% of conventional dairies.

Human costs of disease in calves.

There is growing concern in public health services with increasing levels of AMR (antimicrobial resistance). The WHO have declared AMR to be “a catastrophe that must be managed with the utmost urgency”. Following the experience of Covid-19 we can now expect that declarations of this nature from The WHO will be taken very seriously in the future. We can expect closer monitoring and tighter restrictions to be applied to the use of antibiotics for both human and veterinary medicine. Disease prevention is about to become even more important.

The Animal Health Ireland Expert Group have strongly endorsed individual pens as the healthiest way to rear pre-weaned calves "Individual housing of dairy calves, either indoors or outside, is generally linked with improved calf health. There is long-term recognition of the benefit to dairy calf health of outdoor housing in hutches, especially for the prevention of diarrhoea and respiratory disease". However, individual pens can never replace good animal husbandry. The basic principles of quality colostrum early, warm bedding, consistent feeding etc must still be applied.

We are all well aware of how infections spread. Give calves a bit more space during their high risk pre-weaning period.

¹Cummins, C., D. P. Berry, J. P. Murphy, I. Lorenz, and E. Kennedy. 2017. The effect of colostrum storage conditions on dairy heifer calf serum immunoglobulin G concentration and preweaning health and growth rate. J. Dairy Sci. 100:525–535.

² TR. Dunn,TL. Ollivett,DL. Renaud,KE. Leslie,SJ LeBlanc,TF. Duffield,DF. Kelton The effect of lung consolidation, as determined by ultrasonography,on first-lactation milk production in Holstein dairy calves J. Dairy Sci. 101:5404–5410

3 Zwald, A. G., P. L. Ruegg1, J. B. Kaneene, L. D. Warnick, S. J. Wells, C. Fossler and L. W. Halbert. 2004. Management practices and reported antimicrobial usage on conventional and organic dairy farms. J. Dairy Sci. 87:191-201.


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