By: Jason Lombard, DVM, MS, Natalie Urie, MPH, DVM, Chelsey Shivley, DVM, PhD, DACAW, USDA:APHIS:VS:Center for Epidemiology and Animal Health and Chloe Stenkamp-Strahm, PhD.
Contributors to these reports also include: C. A. Kopral, A. E. Adams, T. J. Earleywine, J. D. Olson, F. B. Garry, D. M. Haines, R. Sargent, M. Santin, R. J. Magnuson, L. M. Linke, S. Magzamen, and C. S. McConnel.
On any dairy, a young heifer calf typically carries some of the herd’s best genetics and represents future potential for performance and efficiency. She also presents challenges, with disease or management shortfalls affecting her future milk production, milk quality and food safety, reproduction and longevity. Understanding the management factors associated with calf morbidity and food-safety problems could help veterinarians and mangers refine practices for life-long productivity.
In January 2014, the USDA’s National Animal Health Monitoring System (NAHMS) launched its sixth national dairy study. As with all NAHMS national studies, objectives for the Dairy 2014 study were identified through focus groups and a needs assessment survey completed by various stakeholders in the U.S. dairy industry.
The study encompassed a broad range of dairy practices for managing cows, heifers and heifer calves, and generated a series of summary reports, available online at the NAHMS website.
In addition, researchers from USDA and Colorado State University drew upon data from the Dairy 2014 study to generate six research reports specifically on pre-weaned heifer management in the United States. The six reports were published in the Journal of Dairy Science in 2018. This article summarizes key points from the final three reports.
These summaries report on the following management issues affecting pre-weaned dairy heifers:
- Factors associated with the presence of Escherichia coli O157 in pre-weaned dairy heifers.
- Factors associated with morbidity and mortality in pre-weaned dairy heifer calves.
- Factors associated with average daily gain in pre-weaned dairy heifer calves.
Factors associated with the presence of Escherichia coli O157 in pre-weaned dairy heifers
As part of the National Animal Health Monitoring System’s (NAHMS) Dairy 2014 study, researchers evaluated farm, animal, and environmental factors associated with O157 presence in dairy heifer calves.
Dairy calves shed pathogenic Escherichia coli O157 (O157) in feces and are a potential route of exposure for human infections. E. coli O157:H7 is a gram-negative, pathogenic bacterium capable of causing bloody diarrhea, abdominal cramps, and hemolytic uremic syndrome in humans. The most recent data available estimate that up to 149,000 domestically acquired human O157 infections occur annually in the United States and are associated with ingestion of contaminated bovine products and other food sources.
Previous studies have revealed that dairy cow shedding patterns are intermittent and complex, but can be influenced by environmental, farm management, host health, and lactation factors. Although risk factors in lactating cows have been measured, less is known about O157 shedding in pre-weaned calves. Calves may play a pivotal role in both maintaining O157 within the farm environment and directly spreading the pathogen to humans. Calves may shed greater numbers of O157 for prolonged periods, and both on- and off-farm human contact tend to be greater with calves than with adult cows.
For this O157 study, calves were enrolled from 100 dairy operations in 13 states. Each operation collected data from calves from birth to weaning over an 18-month period. A single fecal sample was collected from 487 calves in western states and from 871 calves in eastern states for a total of 1,358 total calves, and O157 was detected in 2.5% (n = 34) of fecal samples.
The final O157 main effects model included variables for source of colostrum, temperature-humidity index (THI), and serum IgG concentration.
Findings from this study include:
- Feeding colostrum from the calf’s dam, compared with pooled sources, increased the likelihood of the presence of O157 in the calf. This may be related to time in the calving area and exposure to their dam and the calving environment.
- Calves that drank colostrum from their own first-lactation dam during heat stress THI had an even greater risk of shedding.
- Lower serum IgG concentrations increased the likelihood of O157 presence at different THI levels. Specifically, calves with poor or moderate serum IgG had increased odds of shedding during thermoneutral or heat stress THI compared with calves with excellent serum IgG.
On-farm management strategies that mitigate shedding in calves may only modestly reduce environmental contamination with the pathogen because the prevalence of pre-weaned calves shedding O157 at any given time is low. Regardless, the potential to limit the number of human infections due to direct animal and manure contact makes pursuing these strategies valuable.
Factors associated with morbidity and mortality in pre-weaned dairy heifer calves
The objective of this study was to evaluate morbidity and mortality in pre-weaned dairy heifer calves based on different health, feeding, and management practices, as well as environmental factors.
The pre-weaning phase of dairy heifer calves represents a critical period in the life of replacement females, as they are very susceptible to disease and death. Calves are at the greatest risk of dying during the first 21 days of life. Overall, the health status of a pre-weaned dairy heifer can greatly affect lifelong production, including growth, reproductive efficiency and milk production.
Few recently published studies have examined risk factors for morbidity and mortality in pre-weaned dairy heifer calves. The most recent estimates of national morbidity and mortality in pre-weaned dairy heifers were reported in 1992 and 2007. The percentage morbidity in the 1992 National Dairy Heifer Evaluation project was 36.1%, and the percentage mortality was 8.4%. These numbers stayed relatively consistent with the USDA NAHMS Dairy 2007 study, which reported a morbidity of 38.5% and a mortality of 7.8%.
Both studies had morbidity and mortality above the Dairy Calf and Heifer Association’s 2010 target rates for pre-weaned dairy heifers, which are 25% and 5% for morbidity and mortality, respectively.
In this component of the 2014 NAHMS study, researchers collected data on 2,545 calves. The percentage morbidity for all calves enrolled in the study was 33.9%, and the percentage mortality was 5.0%.
The final morbidity model included birth weight, serum IgG concentration, ventilation type, and average temperature-humidity index (THI) during the pre-weaning period. The final mortality model included birth weight, serum IgG concentration, amount of fat/day in the liquid diet, and morbidity.
Results and conclusions from this portion of the study include:
- Calves born at a higher birth weight had a lower predicted risk of morbidity than calves with a lower birth weight.
- An increase in serum IgG concentration was associated with decreased morbidity.
- Calves housed in positive- or cross-ventilated systems had a 2.2 times higher odds of developing disease compared with calves housed in natural ventilation systems.
- Average THI during the pre-weaning period was inversely correlated with morbidity; as THI increased, the predicted morbidity risk decreased.
- Calves born at a higher birth weight had a lower predicted risk of mortality.
- An increase in serum IgG concentration decreased the predicted risk of mortality.
- The odds of mortality were 3.1 times higher in calves fed ≤0.15 kg of fat/d in the liquid diet compared with calves fed ≥0.22 kg of fat/d.
- The odds of mortality were 4.7 times higher in calves that experienced any disease throughout the pre-weaning period than in calves with no disease.
The morbidity rate of 33.9% and mortality rate of 5.0% in this study suggest that US dairy operations have improved in terms of overall calf care. However, areas for improvement remain, including properly treating calves with antibiotics, feeding calves through scours events, and classifying causes of death. To continue reducing morbidity and mortality, the minimum serum IgG concentration target should be increased compared with the current industry standard of 10 g/L.
In summary, morbidity and mortality were both associated with birth weight and serum IgG concentration. Additionally, morbidity was associated with ventilation type and average monthly THI, and mortality was associated with amount of fat per day in the liquid diet and morbidity.
Factors associated with average daily gain in pre-weaned dairy heifer calves
The objective of this portion of the study was to evaluate average daily gain (ADG) in dairy heifer calves based on health, feeding, management practices, and environmental factors.
The pre-weaning phase is a critical period in the life of a dairy heifer, and optimal conditions increase the likelihood of success as a lactating cow. Growth determines the age at first breeding and age and weight at first calving, and is associated with lifetime productivity.
Growth during the pre-weaning period, and specifically ADG, is affected by many different factors, including passive transfer of immunity, disease, nutrition, management practices, and environmental factors. Therefore, ADG is an appropriate metric to evaluate growth and health during the pre-weaning period.
Proper calf nutrition, defined by the energy and protein available in the diet, is essential for growth and rumen development. At birth, calves are functional monogastrics, and they rely exclusively on fluid milk diets for nutrients.
During the first two weeks of life, calves typically just nibble calf starter; by about three weeks of age, calves should increase their intake of calf starter to approximately 1.4 kg/d to stimulate rumen development. Industry recommendations state that calf starter should be offered by four days of age. Calves must have access to fresh, clean water in addition to the milk diet to stimulate starter feed intake and rumen development.
Protein is considered the rate-limiting nutrient for calf growth. Feeding 4 L of a 20% protein, 20% fat milk replacer (MR) provides enough protein and energy for a 50-kg calf under thermoneutral conditions to gain 0.3 kg/d; feeding the same calf fed 4 L of a 25% protein, 20% fat MR would result in gain of 0.8 kg/d.
The data in this study are based on monitoring 1,410 Holstein heifer calves from birth to weaning. The final ADG model included dam lactation number, singleton versus twin birth, bedding type, Giardia and Cryptosporidium fecal shedding, disease events, a categorized average temperature humidity index for the pre-weaning period (pTHI), amount of protein in the liquid diet (kg/d), milk pasteurization, direct-fed microbials, and the interaction between milk pasteurization and direct-fed microbials.
Results and conclusions include:
- The mean ADG from birth to final weight was 0.74 kg/d. Calves were fed liquid diets an average of 64.7 days. Average birth weight was 43.2 kg, average final weight was 90.9 kg, and average pre-weaning weight gain was 48.4 kg. Overall, 33.3% of calves had a mean ADG of greater than 0.82 kg/d.
- Calves born to first-lactation dams gained less (0.60 kg/d) than calves from second- (0.65 kg/d) or third- or greater-lactation (0.64 kg/d) dams.
- Singleton calves gained 0.07 kg/d more than twins.
- Calves bedded with sand or no bedding gained less (0.49 kg/d) than calves on all other bedding types.
- Calves negative for Cryptosporidium or Giardia at the time of sampling gained 0.03 or 0.02 kg/d more, respectively, than calves that were positive for Cryptosporidium or Giardia.
- Calves with no disease events gained 0.07 kg/d more than calves with one or more disease events.
- Calves experiencing an average pTHI <50 gained more (0.67 kg/d) than calves experiencing an average pTHI from 50 to 69 (0.62 kg/d), or ≥70 (0.59 kg/d).
- Within the range of observed kilograms of protein fed per day in the liquid diet, every additional 0.1 kg of protein fed per day equated to 0.02 kg/d of gain.
- Calves fed milk replacer with a direct-fed microbial gained less (0.44 kg/d) than calves fed milk replacer without a direct-fed microbial (0.60 kg/d) and calves fed pasteurized or unpasteurized milk regardless of direct-fed microbial use.
The two most commonly reported clinical signs of disease in this study were digestive and respiratory signs. Disease during the pre-weaning period can affect ADG by nutrient loss from the diet via diarrhea, diversion of energy to the immune system and away from growth, dehydration, and suppressed appetite and feed intake.
Disease events were self-reported by producers for this study, and it is possible that some operations were more vigilant about detecting and reporting disease than others. Therefore, the real impact of disease on ADG was probably greater than the results of this study indicate. Ensuring calves are fed the recommended amount of high-quality colostrum immediately after birth for adequate passive transfer of immunity, housing calves in a clean environment, and providing proper nutrition can all help reduce disease incidence.
The results of this study highlight the importance of feeding an appropriate quantity and quality of a liquid diet, keeping calves free from disease and comfortable, and mitigating the effects of temperature and humidity during the preweaning period, on ADG.
Part 1 of this series, titled “Identify Opportunities for Better Heifer Health,” is available on BovineVetOnline.