The abomasum, also known as the maw,^[1] and the rennet-bag,^[1] and the read,^[1] is the fourth and final stomach compartment in ruminants. It secretes rennet, an ingredient used in cheese creation.

The word abomasum is from New Latin and it was first used in English in 1706. It comes from Latin ab- + omasum "intestine of an ox," and it is possibly from the Gaulish language.

The abomasum's normal anatomical location is along ventral midline. It is a secretory stomach similar in anatomy and function as the monogastric stomach. It serves primarily in the acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine.

Dairy cattle on high production diets are susceptible to a number of pathologies, most commonly after calving. A gas filled abomasum can move into an abnormal location resulting in left displaced abomasum (LDA) or right displaced abomasum (RDA). If the abomasum displaces to the right, it is at risk of torsion and becoming a right torsioned abomasum (RTA). A displaced abomasum will cause cows to present all or some of the following signs: loss of appetite, decrease rumen contractions, decrease cud chewing, and drop in milk production. While an LDA and RDA are not immediately life threatening, veterinary care is required for surgical correction. Abomasitis is a relatively rare, but serious, disease of the abomsum whose causes are currently unknown.

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