Effects of In Ovo Injection of Mannan-Oligosaccharides on Early Growth Performance, Intestinal Morphology, and Cecal Microbiota in Broilers

Mannan oligosaccharides (MOS) are functional prebiotics that selectively promote the proliferation of beneficial bacteria in the chicken intestinal tract. This study evaluated the effects of in ovo MOS injection on early growth, intestinal morphology, and cecal microbiota of broilers at 7, 14, and 21 days post-hatch (dph). In total, 700 Cobb 500 fertilized eggs received either no injection or 200 μL of PBS diluent, low (0.5 mg/egg), medium (5 mg/egg), or high (10 mg/egg) doses of MOS in the amnion. At hatch, approximately 280 male hatchlings from injection groups were distributed into 28 cages and raised until 21 dph. Our results indicated that MOS injection linearly increased feed intake and body weight during 0–7 dph and improved duodenal villus height and width, with no persistent improvements observed at older age. Moreover, microbiota shifts triggered by MOS were only observed at 7 dph. The MOS injection at 0.5 mg/egg reduced genus-level Sobs, and PCoA coupled with ADONIS analysis confirmed microbial separation across treatments (p = 0.017). MOS selectively enriched obligate anaerobic taxa, including Lachnospiraceae, Pseudoflavonifractor, and Collidextribactor (0.5 mg/egg), and Ruminococcus gauvreauii (5 mg/egg). Conversely, control-enriched genera (Enterococcus and Litchfieldia) were negatively correlated with body weight gain. In conclusion, in ovo administration of MOS at 0.5 mg/chick transiently improved the abundance of beneficial anaerobes, and injecting 10 mg MOS could improve villus morphology and growth performance in chicks in the first week, suggesting a potential role for MOS in shaping early microbial establishment in broilers.