• Proteins provide the amino acids for vital functions,
reproduction, growth and lactation.
• Non-ruminant animals need pre-formed amino acids in their
diets, but ruminants can utilize many other nitrogen sources
because of their rare ability to synthesize amino acids and
protein from non-protein nitrogen sources.
• In addition, ruminants possess a mechanism to spare
nitrogen. When a diet is low in nitrogen, large amounts of
urea (from urine) return in the rumen where it can be used by
• In non-ruminants, urea is always entirely lost in the urine. It is
possible to feed cows with diets containing non-protein
nitrogen as the only nitrogen source and still obtain a
IN THE RUMEN
• Feed proteins degraded by microorganisms in the rumen via amino acids
into ammonia and branched chain fatty acids .
• Non-protein nitrogen from the feed and the urea recycled into the rumen
through the saliva or the rumen wall contribute also to the pool of
ammonia in the rumen.
• Too much ammonia in the rumen leads to wastage, ammonia toxicity, and
in extreme cases, death of the animal. The bacterial population uses
ammonia in order to grow.
• Use of ammonia to synthesize microbial protein is dependent upon the
availability of energy generated by the fermentation of carbohydrates. On
the average, 20 grams of bacterial protein is synthesized per 100 grams of
organic matter fermented in the rumen.
• Bacterial protein synthesis may range from less than 400 g/day to about
1500 g/day depending primarily on the digestibility of the diet.
• The percentage of protein in bacteria varies from 38 to 55% .
• A portion of the dietary protein resists ruminal
degradation and passes undegraded to the small
intestine. Forage proteins degraded 60 to 80%
Concentrates or industrial by-products degraded
20 to 60%.
• Major part of the bacterial protein flows to the
abomasum attached to feed particles.
• The strong acids secreted by the abomasum stop
all microbial activity and the digestive enzymes
start breaking down the protein into amino acids.
Table 1: Composition and intestinal
nitrogen digestibility of ruminal microbes
• Amino acids absorbed Approximately 60% from bacterial
protein, 40% from undegraded dietary protein.
• The amino acid composition of bacterial protein is relatively
constant regardless of the composition of dietary protein.
• All amino acids, including the essential ones are present in
bacterial protein in proportion that is fairly close to the
proportion of amino acids required by the mammary gland
for milk synthesis.
• The conversion of dietary protein to bacterial protein is
usually a beneficial process. The exception occurs when
high quality protein is fed
PROTEIN IN FECES
1. Undigested protein About 80% of the protein
reaching the small intestine is digested, 20% goes into
2. Digestive enzymes secreted into the intestine also
major source of nitrogen in the feces
3. Fecal metabolic protein the rapid replacement of
intestinal cells (On the average, for every increment of
1 kg of dry matter ingested by the cow, there is an
increase of 33 g of body protein lost in the intestine
and excreted in the feces.
• Ruminant feces rich in nitrogen (2.2 to 2.6% N) as
compared to the feces of non-ruminant animals.
LIVER METABOLISM AND UREA
• Not all the ammonia produced in the rumen may be converted to
microbial protein. Excess ammonia cross the ruminal wall and is
transported to the liver.
• The liver converts the ammonia to urea which is released in the
blood. Urea in the blood can follow two routes:
1) Return to the rumen through the saliva or through the rumen
2) Excreted into the urine by the kidneys.
• When urea returns to the rumen, it is converted back to ammonia
and can serve as a nitrogen source for bacterial growth.
• Urea excreted in the urine is lost to the animal. With rations low in
crude protein, most of the urea is recycled and little is lost in the
• However, as crude protein increases in the ration, less urea is
recycled and more is excreted in the urine.
MILK PROTEIN SYNTHESIS
• The mammary gland needs large amounts of
amino acids to syntesize milk protein.
• The metabolism of amino acids in the mammary
gland is extremely complex.
• Amino acids may be converted into other amino
acids or oxidized to produce energy. Most of the
amino acids absorbed by the mammary gland are
used to synthesize milk proteins.
• Milk contains about 30 g of protein per kg, but
vary between cows /breed and among breeds.
Milk Protein Composition
• About 90% of the protein in milk is casein.
• There are many types of casein and they contribute to the high
nutritive value of many dairy products.
• Whey proteins are also synthesized from amino acids in the
• The enzyme á-Lactalbumin is essential for the synthesis of lactose
and â−lactoglobulin is important in curd formation during cheese
• Some proteins found in the milk (immunoglobulins) play a role in
immunity of newborn calf. The immunoglobulins absorbed directly
from the blood and not synthesized within the mammary gland, so
their concentration in the colostrum is high.
• Milk also contains non-protein nitrogen compounds in very small
amount (e.g., urea: 0.08 g/kg).