Arshine takes you through the digestive system of ruminants
Ruminantlivestock include cattle, sheep, and goats. Ruminants are hoofed mammals that have a unique digestive system that allows them to better use energy from fibrous plant material than other herbivores. Unlike monogastrics such as swine and poultry, ruminants have a digestive system designed to ferment feedstuffs and provide precursors for energy for the animal to use. By better understanding how the digestive system of the ruminant works, livestock producers can better understand how to care for and feed ruminant animals.
Ruminant FeedTypes
Based on the diets they prefer, ruminants can be classified into distinct feeding types: concentrate selectors, grass/roughage eaters, and intermediate types. The relative sizes of various digestive system organs differ by ruminant feeding type, creating differences in feeding adaptations. Knowledge of grazing preferences and adaptations amongst ruminant livestock species helps in planning grazing systems for each individual species and also for multiple species grazed together or on the same acreage.
Concentrate selectors have a small reticulorumen in relation to body size and selectively browse trees and shrubs. Deer and giraffes are examples of concentrate selectors. Animals in this group of ruminants select plants and plant parts high in easily digestible, nutrient dense substances such as plant starch, protein, and fat. For example, deer prefer legumes over grasses. Concentrate selectors are very limited in their ability to digest the fibers and cellulose in plant cell walls.
Grass/roughage eaters (bulk and roughage eaters) include cattle and sheep. These ruminants depend on diets of grasses and other fibrous plant material. They prefer diets of fresh grasses over legumes but can adequately manage rapidly fermenting feedstuffs. Grass/roughage eaters have much longer intestines relative to body length and a shorter proportion of large intestine to small intestine as compared with concentrate selectors.
Goats are classified as intermediate types and prefer forbs and browse such as woody, shrubby type plants. This group of ruminants has adaptations of both concentrate selectors and grass/roughage eaters. They have a fair though limited capacity to digest cellulose in plant cell walls.
Carbohydrate Digestion
Forages
On high-forage diets ruminants often ruminate or regurgitate ingested forage. This allows them to “chew their cud” to reduce particle size and improve digestibility. As ruminants are transitioned to higher concentrate (grain-based) diets, they ruminate less.
Once inside the reticulorumen, forage is exposed to a unique population of microbes that begin to ferment and digest the plant cell wall components and break these components down into carbohydrates and sugars. Rumen microbes use carbohydrates along with ammonia and amino acids to grow. The microbes ferment sugars to produce VFAs (acetate, propionate, butyrate), methane, hydrogen sulfide, and carbon dioxide. The VFAs are then absorbed across the rumen wall, where they go to the liver.
Once at the liver, the VFAs are converted to glucose via gluconeogenesis. Because plant cell walls are slow to digest, this acid production is very slow. Coupled with routine rumination (chewing and rechewing of the cud) that increases salivary flow, this makes for a rather stable pH environment (around 6).
High-Concentrate Feedstuffs (Grains)
When ruminants are fed high-grain or concentrate rations, the digestion process is similar to forage digestion, with a few exceptions. Typically, on a high-grain diet, there is less chewing and ruminating, which leads to less salivary production and buffering agents’ being produced. Additionally, most grains have a high concentration of readily digestible carbohydrates, unlike the more structural carbohydrates found in plant cell walls. This readily digestible carbohydrate is rapidly digested, resulting in an increase in VFA production.
The relative concentrations of the VFAs are also changed, with propionate being produced in the greatest quantity, followed by acetate and butyrate. Less methane and heat are produced as well. The increase in VFA production leads to a more acidic environment (pH 5.5). It also causes a shift in the microbial population by decreasing the forage using microbial population and potentially leading to a decrease in digestibility of forages.
Lactic acid, a strong acid, is a byproduct of starch fermentation. Lactic acid production, coupled with the increased VFA production, can overwhelm the ruminant’s ability to buffer and absorb these acids and lead to metabolic acidosis. The acidic environment leads to tissue damage within the rumen and can lead to ulcerations of the rumen wall. Take care to provide adequate forage and avoid situations that might lead to acidosis when feeding ruminants high-concentrate diets.
Two sources of protein are available for the ruminant to use: protein from feed and microbial protein from the microbes that inhabit its rumen. A ruminant is unique in that it has a symbiotic relationship with these microbes. Like other living creatures, these microbes have requirements for protein and energy to facilitate growth and reproduction. During digestive contractions, some of these microorganisms are “washed” out of the rumen into the abomasum where they are digested like other proteins, thereby creating a source of protein for the animal.
All crude protein (CP) the animal ingests is divided into two fractions, degradable intake protein (DIP) and undegradable intake protein (UIP, also called “rumen bypass protein”). Each feedstuff (such as cottonseed meal, soybean hulls, and annual ryegrass forage) has different proportions of each protein type. Rumen microbes break down the DIP into ammonia (NH3) amino acids, and peptides, which are used by the microbes along with energy from carbohydrate digestion for growth and reproduction.
Excess ammonia is absorbed via the rumen wall and converted into urea in the liver, where it returns in the blood to the saliva or is excreted by the body. Urea toxicity comes from overfeeding urea to ruminants. Ingested urea is immediately degraded to ammonia in the rumen.
When more ammonia than energy is available for building protein from the nitrogen supplied by urea, the excess ammonia is absorbed through the rumen wall. Toxicity occurs when the excess ammonia overwhelms the liver’s ability to detoxify it into urea. This can kill the animal. However, with sufficient energy, microbes use ammonia and amino acids to grow and reproduce.
The rumen does not degrade the UIP component of feedstuffs. The UIP “bypasses” the rumen and makes its way from the omasum to the abomasum. In the abomasum, the ruminant uses UIP along with microorganisms washed out of the rumen as a protein source.
Importance ofRuminant Livestock
The digestive system of ruminants optimizes use of rumen microbe fermentation products. This adaptation lets ruminants use resources (such as high-fiber forage) that cannot be used by or are not available to other animals. Ruminants are in a unique position of being able to use such resources that are not in demand by humans but in turn provide man with a vital food source. Ruminants are also useful in converting vast renewable resources from pasture into other products for human use such as hides, fertilizer, and other inedible products (such as horns and bone).
One of the best ways to improve agricultural sustainability is by developing and using effective ruminant livestock grazing systems. More than 60 percent of the land area in the world is too poor or erodible for cultivation but can become productive when used for ruminant grazing. Ruminant livestock can use land for grazing that would otherwise not be suitable for crop production. Ruminant livestock production also complements crop production, because ruminants can use the byproducts of these crop systems that are not in demand for human use or consumption. Developing a good understanding of ruminant digestive anatomy and function can help livestock producers better plan appropriate nutritional programs and properly manage ruminant animals in various production systems.
