I have been writing blog posts with lots of scientific information so I wanted to back it up and give a basic overview of ruminant physiology before getting into depth on the metabolism of nutrients. Ruminants are very complex animals when it comes to the anatomy of the digestive tract as well as the physiology of nutrient digestion.
Cattle are classified as ruminants due to their four-chambered stomach. Each chamber has its own name and function.
The reticulum is the first chamber and has a honeycomb-like structure and represents 5% of the total mass of the stomach. This is often a storage pouch for heavy feeds as well as any metals or other heavy objects consumed by the cow. Some microbial fermentation occurs in this compartment promoting absorption of volatile fatty acids. In calves, there is a closed reticular groove that allows milk and water to bypass the other compartments and go directly to the abomasum. The groove will change with development in response to change in diet and microbial growth in the reticulum and rumen.
The next compartment is the rumen and is the largest compartment (80% of the total capacity). The microbial fermentation function of the rumen is directly related to the reticulum. The rumen is filled will many different types of microbes that degrade different nutrients depending on their structure. These microbes create volatile fatty acids which are the main energy source for ruminants. This microbial fermentation also produces vitamins and amino acids for protein synthesis. The rumen can transfer feed products to the reticulum or straight to the omasum if microbial fermentation has already occurred.
The omasum is a circular type structure with "leaves" that form a structure similar to pages of a book. These leaves function to squeeze fluid from the feed bolus which facilitates movement, titrates feed, and promotes the absorption of nutrients. When feed particles are moved from the omasum to the abomasum, the lack of water allows gastric secretions to penetrate all feed particles to promote digestion.
The abomasum is the "true stomach" of a ruminant animal. There are glands in the lining of the abomasum that release acid and digestive enzymes. The function of this compartment is mechanical digestion and storage of ingested feed. The abomasum functions similar to the human stomach and mixes food with gastric secretions to promote digestion.
I like to take a few lines to describe how horses can digest fibrous feeds because they are monogastric animals (same as humans). This means they only have one stomach that uses mechanical digestion and gastric secretions to digest feed products. However, unlike humans, horses have a large cecum that has microbes (much like rumen microbes) that allow fiber digestion. The cecum is located between the small intestine and the large intestine. Humans have a cecum but the size is very small relative to horses decreasing our ability to digest plant structures such as cellulose.