The objective of this study was to track the dynamic variations in fecal bacterial composition and fermentation profile of finishing steers in response to three stepwise diets varied in energy and protein density. A total of 18 Holstein steers were divided into three groups in such a way that each group contained six animals and received one of three stepwise dietary treatments. Dietary treatments were C = standard energy and protein diet, H = high energy and protein diet, and L = low energy and protein diet. Animals were fattened for 11 months with a three-phase fattening strategy. Fecal samples were collected to evaluate the dynamics of fecal fermentation and bacterial composition in response to dietary treatments and fattening phases using 16S rRNA gene sequencing. Fecal acetate, propionate, and butyrate increased with increasing density of diet and as the fattening phase continued. The relative abundances of Firmicutes and Bacteroidetes dominated and showed 56.19% and 33.58%, respectively. Higher dietary density decreased the fecal bacterial diversity, Firmicutes to Bacteroidetes ratio, and the relative abundances of Ruminococcaceae_UCG-005, Rikenellaceae_RC9_gut_group, and Bacteroides, whereas higher dietary density increased the abundance of Prevotella_9. Our results indicated that both fecal fermentation profile and bacterial composition share a time-dependent variation in response to different dietary densities. This knowledge highlights that both diet and fattening phase impact fecal fermentation profile and bacterial composition, and may provide insight into strategies to reduce fecal contamination from the origin by optimizing diet and fattening time.
Keywords: dietary density; dynamic variation; fecal bacteria; fermentation profile.