Plant-derived essential oils as natural additives for pre-weaned calves: impact on immune system function, health, performance and ruminal parameters

In recent years, interest in natural products, particularly in calf rearing, has grown due to the search for alternatives to antibiotics. In this context, essential oils (EO) attract considerable attention for their antimicrobial, anti-inflammatory, antioxidant and immune-supporting properties. The majority of the studies conducted on pre-weaned calves have shown that EOs used in appropriate doses in the milk, milk replacer (MR) or starter are effective in increasing the feed intake, average daily gain (ADG) and feed efficiency (FE) of calves. Despite their antimicrobial properties and reported improvements in fecal scores, evidence supporting the effectiveness of EOs in reducing calf diarrhea incidence remains limited. In majority of the studies EOs have also been reported to show immunomodulatory effect in calves in the pre-weaning period especially by increasing the serum immunoglobulin levels. Also, studies evaluating the effects of EOs on ruminal parameters have revealed that some EO combinations may increase the volatile fatty acids (VFA) and decrease the NH3-N concentration in rumen. However, there are also conflicting results that require further investigation. Therefore, the aim of this review was to compile and critically evaluate the available literature regarding the use of EOs in calf rearing in the pre-weaning period, to discuss their effects on growth performance, immune system function, diarrhea and ruminal parameters.