Ionophores, such as monensin, are used in dairy calf rearing systems to control coccidiosis and aid in growth and health of the calf. However, the need for alternative growth-promoting and disease preventing feed additives to ionophores in calf diets has become a great area of interest. Many European Union (EU) countries have banned the subtherapeutic use of antimicrobial drugs in livestock and increased legislation on therapeutic antibiotic use. The relevant issue of the misuse of antibiotics that may lead to increased antimicrobial resistant pathogens present on farm level, has gained public concern. Since some pathogenic bacteria found in food products stem from the farm, the livestock sector is often viewed as a reservoir for antibiotic resistant pathogens that can easily be transferred to humans through contamination. Essential oil compounds carvacrol, capsaicin, and cinnamaldehyde have shown promising antimicrobial properties and overall health benefits to poultry, pigs, cattle, and pre-weaned calves.

    The present study investigated the benefit of adding an essential oil blend or monensin to the diet of pre-weaned calves on subsequent growth, health, and rumen development. Furthermore, the abundance of Escherichia coli and Salmonella spp. in the faeces of the calf was investigated to determine if the blend of essential oil compounds or monensin may increase the prevalence of antibiotic resistant bacteria. Two animal trials were conducted in two separate locations. In both trials, the calves were randomly allocated to one of three treatment groups: control (CON), a garlic extract in the liquid diet (0.6 g/calf/day) and a blend of essential oil compounds (150 mg/kg DM) in the starter feed (EOC), and monensin (30 mg/kg DM) in the starter feed (MON).

    Calves were weaned at 60 days of age. In Trial 1, Jersey bull calves (N = 24; 22.105 ± 1.383 kg birth weight) were raised in an outdoor, sheltered unit to investigate the effect of the treatment groups on rumen development, calf growth and performance, and the prevalence of antibiotic resistant pathogens. In Trial 2, Ayrshire heifer calves (N = 39; 35.5 ± 5.2 kg birth weight) were raised in an intensive indoor rearing unit to assess the effect of the treatment groups on calf health, growth, and performance, as well as the abundance of antibiotic resistant pathogens.

    No difference in overall growth, health and rumen development was observed between treatment groups, indicating that essential oil compounds can successfully replace monensin in calf diets. However, during Trial 2 an increase in antibiotic administration for treatment of disease was associated with the MON treatment. Furthermore, an increase in multidrug resistant E. coli (71%) was associated with the MON treatment, confirming that monensin contributes to the risk associated with subtherapeutic antimicrobial use in dairy calves. Resistance profiles also differed between farms, with the intensive indoor calf rearing facility having the highest prevalence of multidrug resistance.

    Similar average daily gain and faecal consistency indexes showed by the CON treatment group, together with an increased abundance of multidrug resistant E. coli associated with the MON treatment, raises the question of the necessity of the supplementation of either monensin or an essential oil compound blend in calf diets. As this study further confirms the association between subtherapeutic antibiotic use and increased multidrug resistant bacteria, the implementation of surveillance programmes for antibiotic use in large, commercial dairy operations is needed. Further research into suitable replacements to subtherapeutic antimicrobial drugs in animal feed are also needed, such as the use of essential oil compounds, and should be prioritized in future endeavors.

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The full thesis can be viewed here.