This study evaluated the effects of thymol and cinnamaldehyde (T+C) supplementation on growth performance, selected blood metabolites, and diarrhoea occurrence in pre-weaned Holstein calves. Forty Holstein calves (20 females and 20 males) were initially enrolled, with 10 calves assigned to each treatment × sex combination. Three calves died during the experimental period, leaving 37 calves for the final growth-performance analyses (control: 9 females and 9 males; T+C: 9 females and 10 males). Growth performance (average daily gain, solid-feed dry matter intake, solid-feed conversion ratio, and weaning body weight) was not affected by treatment, sex, or their interaction. Serum phosphorus at birth and blood urea nitrogen at weaning showed nominal sex-associated differences, but neither remained significant after Benjamini–Hochberg false discovery rate correction. The proportion of calves with diarrhoea was numerically lower in the T+C group than in the control group (8/19 versus 11/18), but the difference was not statistically significant. Thus, under the conditions of this study, T+C supplementation did not significantly affect growth performance, measured biochemical variables, or diarrhoea occurrence.

(Submitted 04 March 2026; Accepted 13 October 2026; Published 10 November 2026)