Abstract
<title>Abstract</title> <p> Poor immune system and the incidence of gastrointestinal diseases are major challenges in the rearing of pre-weaning calves. Dietary supplementation with probiotics and biochar as feed additives could offer a promising strategy to mitigate these adverse conditions by potentially enhancing gut health and modulating the immune response. This study evaluated the effects of a combined dietary supplement of a multi-species probiotic and biochar on growth performance and health status in Holstein suckling calves over a 67-day period. Twenty-eight newborn female calves (initial body weight: 41.2 ± 3.8 kg) were randomly allocated to four experimental groups in a 2×2 factorial arrangement under a completely randomized design. The dietary treatments were as follows: 1) control (no additive), 2) probiotic (2 g/day per calf), 3) biochar (1% of dietary DM), and 4) probiotic + biochar (at the same levels as in treatments 2 and 3). The results demonstrated that weaning weight was significantly higher (P < 0.05) in calves receiving biochar compared to the control and probiotic-only groups. Dietary biochar supplementation also increased (P = 0.05) urinary allantoin excretion and microbial nitrogen synthesis. Serum albumin concentration was elevated (P < 0.05) in response to both probiotic and biochar supplementation. Furthermore, the inclusion of these additives significantly reduced (P < 0.05) rectal <italic>E</italic> . coli counts and improved fecal consistency scores. In conclusion, the simultaneous dietary inclusion of probiotic (2 g/calf/day) and biochar (1% of dietary DM) enhanced overall growth performance, microbial nitrogen synthesis, and key health indicators in Holstein suckling calves. </p>