In the past, biological feed was explored and broadly used to improve the animal gut health, feed efficiency and production. This special issue is to exchange the research discovery, progress, and ideas in the field of biological feed and animal gut health, such as probiotics and prebiotics for improving the animal production. The papers focused on the mechanism of Lactobacillus and other direct-fed microbial supplementation, and resistant starch to modulate the gut immunity, barrier function and health and growth in livestock. Topics ranged from basic discovery research on how bio-feed modulate intestinal function to applied principles and practical feeding of bio-feed in livestock. Journal of Animal Science and Biotechnology is a leading open access journal publishing innovative research across agriculture, dairy and animal science.