MSD Manual

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Professional Version

The Ruminant Digestive System


Derek Foster

, DVM, PhD, DACVIM, College of Veterinary Medicine, NC State University

Reviewed/Revised May 2023 | Modified Jun 2023

Ruminants differ notably from other mammals in that much of their feed undergoes microbial fermentation in the forestomachs (rumen, reticulum, omasum), chiefly in the rumen. There is also postgastric fermentation in the cecum and colon; however, postgastric fermentation is much less important than in other herbivores (eg, horses).

Maintaining normal fermentation and motility of the forestomachs is critical to the health and productivity of ruminants, prompting research to provide pharmacological enhancement or support of the fermentative and mixing actions of the rumen. Also important to consider are both the impact of this fermentative environment on orally administered drugs and the impact of all administered agents on this crucial microbial community.

Other than the forestomachs, the components of the ruminant GI tract are similar to those of monogastric mammals, and the use of pharmacological agents to treat diseases of the glandular stomach (abomasum) and intestine has historically been extrapolated from principles of treatment in monogastric species, with varying success.

Ruminoreticular motility and fermentation are decreased in many conditions, including improper feeding (overload or deficiency of specific nutrients), lack of water, infectious diseases, intoxications, lesions of any part of the upper GI tract, metabolic states (eg, hypocalcemia), and decreased flow of alkaline saliva that allows pH to fall and the microbial population to be altered to an extent that is harmful to the animal (Also see Diseases of the Ruminant Forestomach Diseases of the Ruminant Forestomach ). Because ruminoreticular activity is inhibited by sympathetic tone, most painful conditions will decrease motility and subsequently decrease fermentation (because normal rumen contraction patterns are crucial to support microbial activity).

The primary objectives of pharmacotherapy are to remove the cause of dysfunction and to promote the return of normal digestive actions by meeting or reestablishing the requirements for optimal ruminoreticular function as quickly as possible. Means of achieving these objectives may include any of the following:

  • Ensuring an appropriate substrate for microbial fermentation

  • Providing any cofactors (eg, phosphorus, sulfur) necessary for microbial fermentative processes

  • Removing any soluble end products, undigested solid residues, and gas

  • Maintaining continuous flow culture of ruminal microorganisms

  • Ensuring that the contents of the ruminoreticulum are fluid

  • Maintaining optimal intraruminal pH (generally between 6 and 7)

  • Promoting normal ruminoreticular motility patterns

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