This document discusses rumen development in young ruminants. It notes that a newborn ruminant's rumen is initially sterile but quickly colonizes with microbes through exposure. Various microbe types appear at different stages in the first few weeks. Proper rumen development relies on establishment of diverse microbial populations and stimulation of rumen wall growth. Papillae on the rumen wall aid nutrient absorption; their development is encouraged by volatile fatty acids from grain feeding. Transitioning calves requires a developed, functioning rumen capable of digesting solid feed.