Abstract

Dairy calves rely on a well-developed immune system and healthy gut to grow strong and stay productive later in life. We investigated whether supplementing galacto-oligosaccharides in the early diet of calves influences immunoglobulin levels, gut microbiota, and metabolism after supplementation stops. Calves that received this ingredient showed higher levels of immunoglobulin in their blood, and these benefits continued even after the supplement was no longer given. Although overall microbial diversity was not greatly altered, galacto-oligosaccharides promoted the enrichment of beneficial bacterial taxa and metabolic pathways linked to amino acid synthesis, lipid metabolism, and cofactor production. By day 70, calves supplemented with galacto-oligosaccharides early in life displayed distinct fecal and serum metabolite profiles, particularly in pathways related to vitamin B6, folate and cobalamin metabolism, branched-chain amino acid biosynthesis, and purine and arginine metabolism. These findings demonstrate that early galacto-oligosaccharides supplementation can influence host immunity, gut microbial functions, and metabolic profiles. Such early nutritional strategies may help improve calf growth performance, reduce disease susceptibility, and support more sustainable dairy production. Early-life development of immune functions is crucial for calf health, growth, and future productivity. Galacto-oligosaccharides (GOSs) have been reported to facilitate ruminal microbial establishment and improve growth in Holstein dairy calves, but their prolonged influence on immunoglobulin levels, hindgut microbiota, and metabolic regulation remains insufficiently understood. This study evaluated the effects of early-life GOS supplementation on immune-related indicators, intestinal microbial ecology, and metabolic profiles in Holstein calves. Twenty-four newborn Holstein female dair