IMPACT OF LONG-TERM ANTIBIOTIC USE ON GUT MICROBIOTA AND GROWTH PERFORMANCE IN COMMERCIAL CALVES

Abstract

The research compared the implications of prolonged antibiotic use on the composition of gut microbiota, metabolic activity, growth performance, and development of antimicrobial resistance in commercial calves under field conditions.  Over a year, 200 Holstein-Friesian calves were observed and divided into two categories, including one group given antibiotics and the other one none.  Performance data on growth revealed minor improvements in the average daily growth (0.93 kg/day vs. 0.88 kg/day) and feed conversion ratio (1.95 vs. 2.10) of antibiotic-treated calves.  But gut microbiota 16S rRNA gene sequencing revealed that microbial diversity had decreased significantly, with large losses of beneficial genera such as Lactobacillus and Bifidobacterium and increased opportunistic pathogens Escherichia/Shigella and Clostridium sensu stricto.  The profiling of short-chain fatty acids revealed that the concentrations of butyrate and acetate were significantly reduced in antibiotic-exposed calves (p < 0.001), which suggested impaired microbial fermentation.  The positive relation between butyrate level and growth performance was evident.  The quantitative PCR analysis also demonstrated that the abundance of antimicrobial resistance genes (tetA, tetM, aac(6)-Ib and blaTEM) was significantly greater in calves administered with antibiotics (p < 0.001).  The findings indicate that chronic antibiotic use can result in modest increases in growth, which are not enough to offset the significant microbial dysbiosis, reduced metabolic capability, and a concerning expansion of antimicrobial resistance reservoirs.  These results require an urgent reconsideration of routine antibiotic use in calf rearing and highlight the urgency of microbiome-sparing management approaches to maintain animal productivity and reduce the risks to public health.