Effect of Essential Oil Supplementation on Growth and Health in Pre-weaning Dairy Calves: A Meta-analysis

Essential oils ( EOs ) are widely used as phytogenic feed additives in livestock systems; however, evidence for their benefits in pre-weaning dairy calves remains inconsistent. We conducted a systematic review and meta-analysis to quantify the effects of EO supplementation on growth performance, feed utilization, rumen fermentation, and blood immune and biochemical indices in pre-weaning dairy calves, and to explore potential sources of heterogeneity. Web of Science, ScienceDirect, PubMed, Scopus, China National Knowledge Infrastructure ( CNKI ), VIP Chinese Journal Database ( CQVIP ), and WanFang Data were searched from inception to 8 October 2024. Twenty-nine studies (48 treatment arms; 1 048 calves) met the inclusion criteria. Risk of bias was assessed using the Systematic Review Centre for Laboratory Animal Experimentation tool. Random-effects meta-analyses were performed, with prespecified subgroup analyses by supplementation route (liquid feed versus starter feed), dominant bioactive group (phenolic, cineole/terpene, aldehyde-containing, and mixed/other), supplementation duration, and estimated harmonized dose category. Leave-one-out sensitivity analyses were used to assess robustness; publication bias was evaluated using Egger’s and Begg’s tests when ≥10 effect sizes were available. Overall, EO supplementation was associated with higher average daily gain and several body measurements, higher dry matter intake, lower feed conversion ratio, improved apparent nutrient digestibility, and lower fecal score. In rumen fermentation, acetate decreased and butyrate increased, while other rumen parameters showed no consistent changes. In blood indices, EO supplementation increased immunoglobulin G and immunoglobulin A as well as total protein and triglycerides, and decreased albumin, total cholesterol, and blood urea nitrogen, whereas glucose and β-hydroxybutyrate were not significantly affected. Exploratory subgroup analyses suggested that the magnitude and direction of responses may vary with supplementation route, dominant bioactive group, duration, and estimated harmonized dose category; however, several strata were supported by limited numbers of studies and should be interpreted cautiously. In conclusion, EO supplementation is associated with improved growth and feed utilization in pre-weaning dairy calves, with response patterns that may vary according to EO composition and feeding strategy; further well-designed trials are needed to refine practical recommendations.