The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. Ruminant stomachs have four compartments: the rumen, the reticulum, the omasum, and the abomasum. The rumen is composed of several muscular sacs, the cranial sac, ventral sac, ventral blind sac, and reticulum. The rumens microbial favoring environment allows it to serve as the primary site for microbial fermentation of ingested feed. Rumen microbes ferment feed and produce volatile fatty acids, which is the cow’s main energy source. Rumen microbes also produce B vitamins, vitamin K, and amino acids. The feed contained within the reticulorumen, known as "paunch waste," has been studied as a fertilizer for use in sustainable agriculture. Developing the rumen after birth is a critical aspect of the overall development of ruminants. Generally, the most receptive time for rumen development is between the postnatal and weaning periods. Over this period, rumen organ and epithelium growth, along with the establishment of rumen microbiota, will prove to be essential to rumen development.