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Sheep and goats are widely known as small ruminants; they are part of a larger group of herbivores who all possess a rumen and ruminate (or chew the cud). And that means their digestive process is quite unfamiliar to many of us! We’ve probably vaguely heard of a rumen… but perhaps that’s about it. But if you have sheep or goats, and want to understand their dietary needs (or if you’re just curious!), read on, as our farm vet Cassandra explains the mysteries of ruminant digestion.
What is a rumen?
It’s a large forestomach that evolved alongside two more, called reticulum and omasum; they all sit along the digestive tract before the abomasum, or “true” stomach. From the oesophagus (the soft pipe that carries food from the mouth through the neck), feed enters the rumen, then reticulum, omasum and abomasum. Ruminants have evolved this complex system of forestomachs in order to digest fibrous plant material, which is indigestible for most animals that do not have the ability to host the necessary microorganisms.
The rumen is effectively a large fermentation vat, containing billions of bacteria, protozoa, yeasts and other species that can digest the fibre. They proliferate, releasing useful nutrients for the cow, sheep or goat, and when they die provide additional protein. Thanks to their digestive system, ruminants are excellent users of poor land which is not suitable to be cultivated intensively; many types of grass, rough grazing, moorland – the rumen can handle them all.
The microorganisms live in a symbiotic relationship within the rumen and they can dictate the health of the whole animal. If the rumen bugs are upset – incorrect fermentations, one population overwhelming the others, a general imbalance – the whole animal will become sick.
Parts of the digestive system
Each compartment of the gastrointestinal tract has a different appearance; the lining has evolved for their specific function. The rumen is a very large sac-like organ that can take up to 75% of the abdominal cavity. Its lining has papillae (short stubby “petal”-like structures) that absorb nutrients produced or digested by the rumen microbes. It is a truly amazing relationship between the ruminant host and the microbes within; the many microbial populations can digest cellulose from the plant cells, digesting simple and complex sugar like starch, producing microbial protein by using simple amino acids and Nitrogen, synthesise vitamins that will be used by the host. The ruminant provides them with a safe environment in which to proliferate and abundance of nutrients. If the rumen microbes are healthy, the host is also healthy.
Next to the rumen is the reticulum; also called honeycomb because it’s subdivided into sections by folds of soft tissue that make it look like the cells of honeycomb. The omasum is the next one; roughly ball-shaped, it has many layers of soft tissue (mucosa) layered up to form “leaves”, almost like a book; water is absorbed here. The abomasum is the true glandular stomach. The lining has the same acid-producing glands that we find in other animals.
From the stomach, digested feed particles pass into the intestinal tract. The small intestines are lined with villi (thin structures that make the inner surface resemble velvet). And the large intestines have lining that is very efficient at absorbing water.
Not to be forgotten are the accessory glands and organs; the salivary glands produce the saliva necessary for rumination, the pancreas produces enzymes needed for digestion and the liver produces bile in the gallbladder, plus processes many nutrients coming through from the bloodstream.
How does digestion happen in a ruminant?
It all starts from eating forage (grass, hay, silage). Ruminants have two eating phases. The first one is usually more intense; they are standing and moving about ingesting grass, taking large mouthfuls in a short period of time. They chew roughly, a few quick jaw movements, before it’s swallowed; then onto the next juicy patch. They spend a fair amount of time per day quickly filling the rumen this way, up to a third of the day. When they are full, it’s time to slow down and get onto the second phase.
This is the ruminating part, when they usually lie down and seem to meditate.
It’s a quieter time; the cud (the grass ingested earlier and now partially digested by rumen microbes) is regurgitated back up from the rumen and slowly chewed, getting soaked in saliva, very thoroughly. Each mouthful of cud is chewed up to 100 times before being swallowed again. This time it doesn’t go back into the rumen but is directed further into the next forestomach, the reticulum. Here the smaller particles are sieved and directed forward into the next compartment, the omasum, in a sort of sludge. The larger particles, still fairly solid, are taken back into the rumen. The reticulum also functions as a sort of “safety net”, for larger objects that may have been accidentally ingested and could cause damage if progressing further.
Rumen and reticulum are closely connected and work alongside each other in moving feed particles back and forth. The “sludge” of feed particles arrives in the omasum where between the many mucosal layers water is absorbed and a more solid paste, which then reaches the abomasum. Here in the glandular stomach acid is produced to complete digestion of protein and other nutrients, before it passes in the small intestine for absorption.
Ruminants have very, very long intestines
And they’re all cleverly arranged and packed tightly in the abdomen. Both small and large intestines have the main function of nutrient and water absorption, before the waste – dung – is passed. Sheep and goats have very efficient large intestines that can remove the majority of water from the dung; producing hard pellets.
Ruminants for the win?
Ruminants have evolved over millions of years to become the highly efficient users of rough forage that they are today. As farming techniques have changed and the animals’ diet changed alongside, from 100% all year round grazing to housing and concentrate feeds, the rumens have had to adapt.
Not all herbivores are ruminants; some like horses and donkeys are monogastric; that means they have a single stomach like humans and the essential fermentations necessary to digest forage take place in the hind gut. In the wild, there are many different shapes and sizes of ruminants, such as deer and elk, antelopes and buffaloes, to mountain goats, ibex and mouflons.
Young ruminant animals (lambs, kids and calves) up to 2-3 months old when they start eating solid feed are immature ruminants and functionally monogastrics. Their oesophagus is able to let milk bypass the rumen/reticulum, which at this age are not developed yet. And it will go straight through to omasum and abomasum for acid digestion. If a very young ruminant is unwell or stomach tubed incorrectly, may deteriorate because of milk ending in the rumen and causing inappropriate fermentations. They usually become bloated and go off their feed; speak to your vet for advice if you have unwell animals.
The microbial biome changes with the introduction of different feeds and each change needs to be gradually made in order to maintain a healthy balance of microorganisms. Highly specialised animals such as high yielding dairy animals (cattle, goats and sheep) need a higher plane of nutrition that is not possible with just grass or silage; as they rely on concentrate feeds alongside forage, the rumen must adapt and utilise the hard feed without sharp changes in pH (acidity). The pH is one of the parameters to keep in mind for a healthy rumen. A ruminant is designed to thrive on forage; a diet consisting only of concentrates and ready available starch will not suit their physiology. Forage must therefore always be provided.
There are conditions that can originate or worsen if the digestive system slows down or malfunctions; the digestive tract is such an important part of a ruminant that if we don’t manage to look after it as much as the whole host animal, we would be in for trouble. Listing all the many diseases that can cause digestive symptoms is beyond the remit of this article; however, if you have a ruminant that is off their feed and/or does not appear to be chewing the cud you should contact your farm vet for advice and examinations.