Abstract
The optimal timing for initiating starter feeding in lambs remains controversial, warranting a systematic evaluation of its effects across multiple indicators. This study investigated the effects of initiating starter feeding at 7, 14, or 21 days of age on growth performance, rumen fermentation, serum immunity, gastrointestinal development, and microbiota in Hu lambs. Forty-five newborn lambs were randomly assigned to three groups (n = 15 per group) and fed starter until slaughter at weaning (60 days). The 14-day group exhibited significantly higher body weight (BW) at 49 and 60 days compared with the 7-day group (p < 0.05), and greater average daily gain (ADG) during 28-35 days than the 21-day group. Rumen ammonia nitrogen (NH(3)-N), acetate, propionate, valerate, and total volatile fatty acids (VFAs) were higher in the 7-day and 14-day groups than in the 21-day group (p < 0.05). The 14-day group showed lower pro-inflammatory cytokines (IL-1β, TNF-α) and higher immunoglobulins (IgA, IgG, IgM) and anti-inflammatory cytokines (IL-2, IL-4) (p < 0.05). This group also displayed improved rumen papilla width, jejunal villus dimensions, and reduced crypt depth. Beneficial microbes such as Christensenellaceae_R-7_group and Butyrivibrio were enriched in the 14-day and 21-day groups. In conclusion, initiating starter feeding at 14 days of age optimizes growth, rumen function, immune response, and colonization of beneficial microbiota in Hu lambs.