Dairy calf health – review of main concerns and how to counter them

The health and performance of calves – specifically those prior to and around weaning – can have a significant impact on overall performance on dairy farms. This time period, with rapidly growing animals and developing immunity, can present a particular challenge to performance, but also an opportunity to take advantage of a high-potential feed conversion and epigenetic programming for future performance.

As such, calves from birth to four months can be seen as either a challenge or an opportunity to vets and farmers alike.

The University of Nottingham’s herd health toolkit provides industry-level benchmarking for calf mortality, with a national average mortality rate of 4% to two months and 6% to five months.

This is despite an industry-recognised target of 3%. This correlates well with British Cattle Movement Service data showing 6% calf mortality rate, and 15% of dairy bred heifers failing to reach first lactation¹, showing that significant losses continue to occur in calf rearing.

The main disease syndromes responsible for disease, mortality and losses continue to be diarrhoea and respiratory disease, each being recorded in nearly 50% of calves in a recent large cohort study². The losses often most associated with these conditions are the deaths and severe clinical cases. However, the largest financial element is from lost performance and production from the more prevalent milder or subclinical cases.

A recently published study from the RVC³ followed 476 calves on 16 farms from birth until eight weeks of age. During this time, calves were assessed weekly using a range of systems for respiratory or intestinal health issues.

During this period, 55 calves (12%) received any antimicrobial or anti-inflammatory treatment, with 10 requiring at least one further treatment. These are the calves that often indicate veterinary intervention or hit the farmer’s attention. However, in the past week of assessment, nearly 30% of calves had thoracic consolidation of more than 1cm, as identified by ultrasound scanning, and more than 18% of all scans performed showed lesions at this level or worse.

Similarly, during the second week of life, between 37% and 44% of calves dependent upon scoring system showed evidence of scour.

The high prevalence of disease in this in-depth study of a small number of herds is not atypical, and is in line with other studies showing both respiratory disease and diarrhoea being highly prevalent within artificially reared calves. This demonstrates the ongoing challenge of improving calf health, but also the opportunity to intervene and facilitate improved performance.

Both of these disease syndromes follow a similar control system, whereby disease occurs when pathogen load exceeds immunity, and the severity of disease is likely correlated to the magnitude of overload of immunity by the challenge.

As such, the focus should be two-fold: improving immunity and reducing challenge. These are not mutually exclusive – often, measures to reduce the challenge lead to improved environmental conditions and improved immunity.

Immune response

Innate immunity

The immune response to challenges can be split into innate or adaptive, with adaptive immunity further split into active or passive. The innate immune system is the immediate, non-specific response or barrier to threats. With regards to scour, this relates primarily to the integrity of the gastrointestinal tract as a barrier to pathogens, as well as the ability of the intestinal contents to maintain a stable microbiota, preventing overgrowth.

Essential oil extracts from specific plants have shown promising potential to improve the quality of the intestinal barrier, while also preventing bacterial overgrowth. While some papers correlate this with improved growth, feed conversion efficiency and reduced diarrhoea incidence⁴, others are less clear cut. Essential oil inclusion into starter mixes and/or commercial milk replacer is an area which warrants further consideration.

For both scour and respiratory disease, the innate immune system is optimised through optimised nutrition. When considering an outbreak of disease in calves, it is important to look at dietary sufficiency.

This not only correlates to macronutrients such as energy and protein supply, but also to trace element supply. Vitamin E and selenium act as potent anti-oxidants, minimising the damage during the immune response.

The respiratory innate immune system involves many complex adaptations, such as the cough reflex, cilia and turbinate structures, to reduce the opportunity for pathogens to reach the lower respiratory tract. One of the biggest challenges to this is from ammonia, which builds up as a reaction by-product from urea. Ammonia reduces mucociliary clearance while also increasing oxidative stress, challenging the innate immune system’s ability to defend against pathogens.

Reducing the levels of ammonia in the environment through good drainage, regular cleaning out and good ventilation will help to maintain a strong innate immunity.

Adaptive immunity

Adaptive immunity is the specific response to individual threats, leading to longer-term “memory”, allowing more rapid responses to repeated exposure.

Active adaptive immunity is a factor of overall health and performance, similar to the innate system. The active immune response can be boosted through vaccinal priming. It is important to remember that vaccines should not be expected to provide complete protection – by stimulating the immune system with controlled doses (sometimes killed or inactivated) – of known threats; when re-exposed, the immune system will respond in a rapid, targeted fashion. This will lead to a reduced duration and severity of infection.

While this may lead to fewer clinical cases, less shedding and less severe disease, use of vaccines alone will not lead to complete disease control.

With regards to calf vaccination, two main approaches are considered: vaccination of calves themselves, and vaccination of dams. Vaccinating calves will lead to development of a primed immune system in a number of weeks. This immunity will often be long lasting; however, due to the delay in developing the immunity, in some cases it is not feasible to achieve adequate protection before disease.

This is particularly true in the case of neonatal scours. This necessitates the vaccination of dams to utilise passive immunity.

Passive immunity is the transfer of antibodies, usually from colostrum to calves. Calves are born immune-naive, with no specific antibodies against threats, and so rely on antibodies in colostrum, transferred into the calves’ blood, for their adaptive immune priming until their own systems develop.

Maximising passive immunity is key for the control of calf disease – particularly scour, navel ill and joint ill. Maximising passive immunity through colostrum has been summarised as the “5Qs”.

Quickly: the permeability of the intestine to antibodies is a time-related parameter; the older the calf, the less permeable the barrier, with complete closure by 24 hours. The aim is to ensure colostrum intakes ideally in the first one to two hours, but by six hours maximum. Colostrum quality also reduces significantly in the cow with time calved, so rapid harvesting is also important. Feeding colostrum, even if not from the first milking, to calves for an extended period has advantages in local immunity: the antibodies present will bind to pathogens, ensuring they are either excreted or broken down in the gut, reducing disease.

Quality: colostrum quality is essential to ensure good passive transfer of immunity. Various target levels of colostrum quality have been reported, with a minimum of 50g/L of immunoglobulin G (IgG) being an absolute minimum – this correlates to a Brix reading of 22%. Given the variability in absorption, time delays after calving and other measures that will reduce absorption, a minimum threshold of 25% Brix (75g/L) is often more reliable. Where natural colostrum does not meet this level, it can be supplemented with freeze-dried, powdered colostrum that is commercially available. A balance exists between colostrum that is specific to the pathogens on farm from calving cows and ensuring suitable general protection from commercial colostrum. Colostrum quality is influenced by speed of collection, prepartum nutrition – in particular, supply of metabolisable protein – calcium metabolism, heat stress and other management factors. Prepartum vaccination, in particular for neonatal scour, aims to boost specific antibody levels within the colostrum. Doing this while failing to produce or deliver quality colostrum is a waste of money and effort.

Quantity: quantity and quality interact to give a total yield of antibodies. The target should be 150g to 200g IgG total in the first feed to allow for variability in absorption. For 22% Brix colostrum, this equates to four litres. For better-quality colostrum, three litres is likely to suffice. Current recommendations are for colostrum volumes to be at least 10% bodyweight in the first six hours. Colostrum quantity is influenced by similar factors as colostrum quality.

Quietly: stress and inconsiderate handling reduces the absorption of antibodies from colostrum. Teat drinking will lead to better absorption, but getting calves to drink suitable quantities through a teat is time consuming and often impractical in large herds. Whatever method for feeding is used, the calves should be handled quietly and in a considerate manner.

sQueaky clean: contamination of colostrum will cause antibodies to be bound to material in the colostrum prior to feeding. This reduces the effective quality of the colostrum. The pathogen load also leads to premature closing of the intestinal barrier, localised inflammation and potentially disease. Colostrum should be harvested, stored and fed in a hygienic manner. Pasteurisation is an option to ensure clean colostrum, but care must be taken to pasteurise at a lower temperature for longer to avoid protein denaturation. Colostrum can be sampled and tested for pathogen load with a target total viable count of less than 100,000cfu/ml, and a target coliform count of less than 10,000cfu/ml.

Reducing the challenge to calves can lead to significant improvements in disease incidence. Many measures can be undertaken to ensure calves are exposed to lower pathogen loads, and interventions should be specific to individual farm scenarios.

Each farm is unique in its layout and management and, as such, disease control is unique.

However, a few common threads can easily be overlooked with regards to pathogen loads, and which are not always considered when things look “fairly clean”. These are worth having in your mind for potential areas for improvement.

Cleansing and disinfection. All-in, all-out units lend themselves to appropriate between-batch cleansing and disinfection. This is harder to achieve in continuous throughput settings. When looking at adequacy of post-disinfection hygiene, whether considering feed/milk buckets, housing, pens and so forth, it is important to consider the following points:

Does cleaning to place before disinfection? Without first physically removing organic debris, disinfectants will not be effective.

Is the disinfectant used suitable for the pathogens it is attempting to kill, and at the right concentration? Defra maintains a list of disinfectants certified for common pathogens that is worth referencing.

Do you have enough time for drying? For most disinfectants, dehydration is a part of their mode of action; for this to work, items must be allowed to dry.

What role may water be playing? Given most farms are on private water supplies, extracting water from the land around which slurry is spread, it is not uncommon to see high pathogen loads in the water. Together with biofilm build up within pipes, and often no or inadequate disinfection, high pathogen loads can be found in water. The author has tested multiple farm water supplies this year, and coliform counts in excess of 100,000cfu/mL are not unusual. Water is also considered a major source of cryptosporidiosis infection in people.

How often are fans cleaned? Remember GCSE physics: any build-up of dust or physical debris, either on fans or ventilation tubes, will significantly affect both direction and quantity of air flow. Farmers have invested thousands of pounds on mechanical ventilation that is not performing to the same level, for want of a clean. Every fan or tube should be cleaned at least every six months.

Water is bad. Increased humidity in a shed leads to longer pathogen survival outside the host. As such, any measure that reduces humidity is likely to be beneficial. This may be those fans and tubes, or may be as simple as moving water troughs to the front of sheds, and fixing gutters.

Conclusion

Respiratory disease and diarrhoea continue to be the two main concerns for calves, responsible for widespread disease and above-target mortality. Controlling these two syndromes will allow vets and farmers to start looking at youngstock rearing as an opportunity for optimisation rather than a challenge.

Narratives can then progress to increasing growth rates when feed-conversion efficiency is at its best, allowing for cheaper targeted growth later, and switching in gene expression for future performance in the milking herd. Controlling these diseases is a factor of optimising immunity – innate and adaptive, active and passive – as well as limiting the pathogen challenge.

• This article appeared in Vet Times (2025), Volume 55, Issue 44, Pages 6-12