What are the four compartments of a ruminant stomach and what is their function?

The rumen, reticulum, omasum and abomasum.

Rumen and reticulum are responsible for microbial fermentation of carbohydrates, degradation of protein and non-protein nitrogen, and partial hydrogenation of unsaturated lipids from feedstuffs eaten by the ruminant. Fermentation in the rumen and reticulum enable break-down if cellulose, fibre, and lignin to digest VFAs and amino acids. Microbes comprising of protozoa and bacteria live in these chambers which help with the fermentation process.

In the Omasum, the main function is the absorption of water and some nutrients from the digesta of the rumen.

In the Abomasum, digesta is redigested, where amino acids and starches undergo further digestion.