Ruminants

Leslee Maasdorp wrote The Impala of Rifa. Within this hugely informative booklet is an article by Graham Smith entitled “Detailed Account of Ruminant Digestion and Absorption in the Impala” which is included below. This post provides an introduction to the class of animal called “ruminant” and to the characteristics of ruminant digestion.

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Ruminants are animals that have more than one stomach and that swallow food and then bring it back up again to continue chewing it.

Ruminants represent the most diverse group of living ungulates.  There are roughly 200 species of ruminants that include all domesticated and wild cattle (such as buffalo), goats and sheep as well as giraffes, deer, gazelles and antelopes.   

Also, some mammals are what is called “pseudoruminants” and have a three-compartment stomach instead of four like ruminants. Hippos and camels are examples. Pseudoruminants, like traditional ruminants, are foregut fermenters (see explanation below) and most ruminate or chew cud. However, their anatomy and method of digestion differs significantly from that of a four-chambered ruminant.

The opposite of a ruminant is a non-ruminant or monogastric (literally “one-stomach”) herbivore which has a simple single-chambered stomach. They are what is called “hindgut fermenters” (digestive fermentation occurs in the large intestine, the last stage of the digestive process) as opposed to “foregut fermenters” (digestive fermentation happens in the stomach, the first chamber of the digestive tract). They include rhinos, horses, guineapigs and rabbits.

Ruminants are always:

• Mammal – any member of the group of vertebrate animals in which the young are nourished with milk from special mammary glands of the mother
• Ungulate – hoofed herbivorous quadruped mammal (such as a pig, cow, deer, horse, elephant or rhinoceros)
• Artiodactyl – any ungulates (such as the camel or pig) with an even number of functional toes on each foot
• Herbiverous – plant-eating i.e. grazing or browsing. (Non-ruminants can be herbivorous, carnivorous or omnivorous.)

Ruminating animals have physiological features that enable them to adapt to their particular environment.

1. Continuously growing teeth. During grazing, the silica content in forage causes abrasion of the teeth. This is compensated for by continuous tooth growth throughout the ruminant’s life, as opposed to the case with humans or other nonruminants, whose teeth stop growing after a particular age.
2. Most ruminants do not have upper incisors. Instead, they have a thick dental pad against which they grind plant-based food.
3. Large ruminal (first stomach) storage capacity that gives them the ability to consume food rapidly and complete the chewing process later when they feel safe. Ruminants in the wild are mostly prey animals. They feed in a series of quick bites, giving the food no more than a cursory grind between the molars, mixing it with large quantities of saliva (several hundred liters per day in domestic cattle) and then swallowing it into the rumen. They are unable to digest grasses, foliage, and other types of plant material here but it is stored and sent back up for “rumination” when they are ready.
4. Animals that normally consume tannin-rich leaves, twigs and bark (e.g. giraffe) have developed defensive mechanisms against the negative effects of tannins due to the presence in their saliva of tannin-processing proteins.

To go further into the description of what is involved in ruminant digestion, ruminants acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The process, which takes place in the front part of the digestive system and therefore is called” foregut fermentation”, typically requires the lump of solid fermented food (known as “cud”) to be regurgitated (brought back up to the mouth) and re-chewed, re-supplied with saliva, and re-swallowed. (The word “ruminant” comes from the Latin ruminare, which means “to chew over again”.)

The process of rechewing the cud to further break down plant matter and stimulate digestion is called rumination[1]. 

Simple summary of ruminant stomachs and digestion process

(more detail below in the article on impala digestion)

1. Rumen – primary site of microbial fermentation
2. Reticulum (together with the rumen – the “reticulo-rumen”) – makes up the “fermentation vat” breaking down complex carbohydrates such as cellulose. Food mixed with saliva (the cud) is regurgitated from here and re-chewed to break down particle size further.
3. Omasum – receives chewed cud and absorbs volatile fatty acids
4. Abomasum – true stomach (the direct equivalent of the monogastric stomach) where digestion happens as in all mammals with acids and enzymes that further digest the material passing through.)

The food then passes from the abomasum into the small intestine which is the main site of nutrient absorption and then to the large intestine where fermentation with microbes further breaks down fibre. The caecum is the bulge from the side of the lower part of the animal’s digestive tract. Any material which has not been broken down by the digestive process – especially tough plant material – has one further opportunity for fermentation and absorption here. Any material not digested by this stage in the process will be excreted from the body.  

Detailed account of ruminant digestion and absorption in the Impala

Graham Smith (Bsc Agriculture. BVSc.)

Introduction

The impala, a medium sized antelope, may be characterised by having a ruminant digestive system. What makes ruminant animals unique is their four stomach compartments: the reticulum, the rumen, the omasum and the abomasum. The reticulum and rumen are often discussed together as the rumino-reticulum because they are adjoining compartments and share some functions. A second characteristic which makes ruminants unique is their ability to ingest large amounts of coarse foodstuffs without having to chew these foods completely and then regurgitate these coarse particles and remasticate (rechew) these particles, the “cud”, at a more suitable time which could be when the environment is perceived to be safer. This process is known as rumination or “chewing the cud”.  What follows here will emphasise the role played by the ruminant stomachs in digestion and nutrition, with particular reference to the impala.

Reticulum

• Structure: The reticulum, often called the “blind pouch”, is the first stomach compartment. Ln the impala it has a capacity of approximately two litres. Looking at the wall of the reticulum it is immediately apparent that a regular “honeycomb” appearance of depressions characterises this organ. Looking closely at these depressions one sees small conical papillae within the depressions and on the walls making up these depressions. The surface of these structures in lined by a fine layer of keratin which makes these surfaces very hard wearing.
• Functions: The reticulum traps heavy objects, such as half-chewed seeds or pods, and the tough “honeycomb” walls have a grinding action on these objects. Thus the reticulum helps digestion by grinding and breaking down large particles. The reticulum also carries out a storage function which it shares with the larger rumen.

Rumen

• Structure: The rumen is a large hollow muscular organ comprising a number of sacs formed by the ruminal pillars. In the impala the rumen has a capacity of approximately 20 litres. The walls of the rumen are covered by papillae of varying lengths, being longer where absorption is greatest and shorter and few in number where absorption is minimal.
• Functions: The rumen’s primary function is that of microbial fermentation of ingested carbohydrates. Bacteria, protozoa and fungi occur in very high concentrations (approximately  10¹⁰/gram) in the ruminal ingesta. The main products of fermentation are the volatile fatty acids (VFA), namely acetate, butyrate and propionate. These volatile fatty acids are absorbed by the large surface area constituted by the ruminal papillae, and are then converted to glucose in the liver providing up to 80% of the energy needs of the animal. As well as storing ingesta and a large quantity of fluid, the rumen also carries out regular mixing of these contents. Muscular contractions occur at a rate of 2 – 3 contractions every five minutes and these ensure that the microbes are able to access all components of the feed in the rumen. The rumino-reticulum is responsible for forming the bolus of coarse food, from the floating fiber mat, and propelling it into the oesophagus during regurgitation (rumination). It is the coarse heavy material from the reticulum and the light long fibers of the fiber mat in the rumen which then undergo re-mastication, re-ensalivation and are swallowed when completely chewed.

During fermentation large quantities of carbon dioxide and methane are formed as by-products. These gases are relieved into the atmosphere via a reflex which allows these gases to pass through the oesophagus and mouth, a process known as eructation. This prevents a fatal condition known as bloat occurring where the gases accumulate in the rumen causing it to expand excessively and press on the lungs, thus suffocating the animal. The rumen pH is determined by the animal diet and is ideally between pH 5,8 – 6,4. Different micro-organisms in the rumen function at different pH’s; with the coarse fiber digestec preferring pH 6,2 – 6,8 while the fine starch digestec prefer pH 5,2 – 6,0. Therefore, by measuring the rumen pH it is possible to determine the predominant type of microbial digesters and from that speculate on the components of the diet.

Reticular Groove

The reticular groove (formerly oesophageal groove) is the opening from the rumino-reticulum into the omasum.

• Structure: The reticular groove is a twisted spiral of fleshy muscular lips which form a gutter from the cardia (opening of oesophagus into the rumino-reticulum) to the omasum.
• Function: In the unweaned animal the reticular groove is converted into a closed tube, forming a channel that conveys milk directly from the oesophagus into the omasal canal whence it drops into the abomasum. The muscular contractions that draw the lips together are reflexively stimulated by the young suckling. As the animal gets older and its diet includes more solid material, there is less use of this by-pass route. However, recent research in the goat indicates that a portion of the nutrients released into the saliva during remastication may utilise the reticular groove in the adult animal thus by-passing the ruminoreticulum. This may also occur in the impala. Further research needs to be conducted in this field.

Omasum

• Structure: The omasum is a bean-shaped organ with a capacity of one liter. Its interior is composed of up to 100 crescentic laminae or parallel sheets.
• Function: The large surface area of the omasum, made up by the laminae, allows it to carry out its primary function, that of fluid absorption efficiency. This ensures that only the smallest volume of food material enters the abomasum.

Abomasum

• Structure: The abomasum of an impala has a capacity of approximately three litres. It is similar in structure and function to that of the simple stomach. The surface of the abomasal wall contains numerous glands. Those in the pale pinkish part secrete mucus while those in the darker part secrete enzymes which function in protein digestion.

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In conclusion, the ruminant digestive system is a remarkable example of evolutionary adaptation, allowing these animals to thrive on a diet of plant material that would be toxic to many other species.

Sources

Leslee Maasdorp’s references included the following three:

lshler, U. (1997). Nutrition of ruminants. Lecture notes. Univ. of Tennessee.

McDonald, P. and Edwards, R.A. (1987). Animal nutrition. Longman, London.

Surenson, M.J. (ed.) (1990). Duke’s physiology of domestic animals. Comell Univ. Press.

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“How do ruminants digest” Open University – https://www.open.edu/openlearn/science-maths-technology/biology/how-do-ruminants-digest

“Ruminant Wikipedia, https://en.wikipedia.org/wiki/Ruminant

“Ruminant” – https://www.britannica.com/animal/ruminant

“Ruminant” https://www.vedantu.com/animal/ruminant

“Ruminant Animals” https://www.animalspot.net/ruminant-animals

[1] The verb ‘to ruminate’ has been extended metaphorically to mean to ponder thoughtfully or to meditate on some topic. Similarly, ideas may be ‘chewed on’ or ‘digested’.