Re: Forum: Avoiding Grains to minimal level to Ruminants
27/01/2014 | Ruminant Feeding Types
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
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.
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 6.0).
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
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. Acidosis is discussed in detail in Mississippi
State University Extension Service Publication 2519,
“Beef Cattle Nutritional Disorders.” In addition, energy
as a nutrient in ruminant diets is discussed in detail
in Mississippi State University Extension Service
Publication 2504, “Energy in Beef Cattle 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.
Protein digestion in the ruminant
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
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
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. Protein as a nutrient in ruminant diets is discussed
in detail in Mississippi State University
Extension Service Publication 2499, “Protein in Beef
Importance of Ruminant 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.
Church, D. C. ed. 1993. The Ruminant Animal Digestive Physiology and Nutrition. Waveland Press, Inc. Prospect Heights, IL.
Oltjen, J. W., and J. L. Beckett. 1996. Role of ruminant livestock in sustainable agricultural systems. J. Anim. Sci. 74:1406-1409.
Parish, J. A., M. A. McCann, R. H. Watson, N. N. Paiva, C. S. Hoveland, A. H. Parks, B. L. Upchurch, N. S. Hill, and J. H. Bouton. 2003. Use
of non-ergot alkaloid-producing endophytes for alleviating tall fescue toxicosis in stocker cattle. J. Anim. Sci. 81:2856-2868.
Van Soest, P. J. 1987. Nutritional Ecology of the Ruminant. Cornell University Press. Ithaca, NY.