Spirulina (Limnospira platensis) has gained attention as a functional feed additive in animal nutrition. However, its effects on the cecal microbiota and systemic metabolic responses in lambs remain unclear. Here, twelve 5-month-old male Hu lambs were randomly assigned to a control group (Control) or a Spirulina-supplemented group (SPI; 1.5% of dietary dry matter) and fed for 60 days (n = 6 per group). We measured serum biochemical indices, energy-metabolism variables, and immune and antioxidant parameters and characterized the cecal microbiota using 16S rRNA gene sequencing. Spirulina supplementation increased serum triglycerides (TG), glucose (GLU), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) (p < 0.05). In the cecum, α-diversity indices were lower in SPI than in Control (p < 0.01), and Principal Coordinates Analysis (PCoA) indicated distinct community separation between groups (p < 0.01). Spearman correlation analyses further linked dominant genera to host metabolic and antioxidant traits. Collectively, Spirulina altered lamb metabolism and antioxidant status and reshaped the cecal microbial community, with microbial shifts associated with changes in serum indices.

Atrazine is a triazine herbicide widely used in maize production, and residues in corn stover intended for feed may compromise forage safety. However, the