Role of Calcium Propionate and Monensin on Performance, Rumen Fermentation Patterns, and Ruminal Bacterial Populations in Growing Lambs

The objective of this study was to determine the effects of dietary calcium propionate (CaPr) and monensin (MON) on lamb growth performance, ruminal volatile fatty acids (VFAs), methane (CH₄) production, and ruminal bacteria and archaea populations. Forty crossbred ram lambs (initial weight 23.4 ± 2.8 kg) were randomly assigned to one of four diets (n = 10 per treatment): (a) control diet (CONT); (b) MON diet, which contained 30 mg/kg dry matter (DM) of monensin + CONT; (c) CaPr diet, which contained 10 g/kg DM of CaPr + CONT; and (d) MCA diet, which contained both additives (30 mg/kg DM of monensin plus 10 g/kg DM of CaPr) + CONT. Dietary MON or CaPr in lamb diets did not affect (<i>p</i> ≥ 0.05) dry matter intake (DMI), average daily gain (ADG), or DMI/ADG. The main effect (<i>p</i> ≥ 0.05) of CaPr supplementation was observed in acetate concentration and CH₄ production. The addition of calcium propionate increased (<i>p</i> < 0.05) <i>Fibrobacter succinogenes</i>, <i>Ruminococcus albus</i>, and <i>Selenomonas ruminantium</i>, whereas CaPr reduced (<i>p</i> < 0.05) methanogenic archaea. The addition of MON and CaPr did not affect lamb performance; however, CaPr improved fermentation kinetics towards more efficient routes, reflected by a reduction in the proportion of methane and methanogenic archaea.