Babcock Institute for International
Dairy Research and Development
Dairy
Essentials
University of Wisconsin-Madison
4) LIPID METABOLISM IN DAIRY COWS
Michel A. Wattiaux
Babcock Institute
Ric R. Grummer
Department of Dairy Science
structure similar to the triglycerides except
that one of the three fatty acid has been
replaced by a sugar (usually galactose).
When one of the fatty acids is replaced by a
phosphate bound to another complex
structure the lipid is referred to as
phospholipid. Phospholipids are minor
components in feedstuffs, but they are
found in a high concentration in ruminal
bacteria.
Common fatty acids found in plant lipids
range from 14 to 18 carbons (Table 1). The
melting point determines whether a lipid is
in a liquid or a solid form at room
temperature. Melting point is influenced
primarily by the degree of saturation and to
a lesser extent by the length of the carbon
chain. Plant lipids typically contain 70 to
80% unsaturated fatty acids and they tend
to remain in the liquid state (oils). On the
other hand, animal fats contain 40 to 50%
saturated fatty acids and they tend to
remain in the solid state (fats). The degree
of unsaturation has a marked effect on how
well it is digested by an animal and, in the
case of ruminants, whether or not it
interferes with the fermentation of
Triglycerides
carbohydrates in the rumen.
TYPES OF LIPIDS
Usually, the diet eaten by cows contains
only 2 to 4% lipids. However, lipids are an
important part of the ration of dairy cows
because they contribute directly to about
50% of the fat in milk and they are the most
concentrated source of energy in feed.
Only small amounts of lipids are found in
forage and seed. However, some plants
(cotton, soybean) have seeds referred to as
oilseeds that contain more than 20% lipids.
Lipids are usually extracted from oilseeds
which may be used unextracted in diets of
cows.
Lipids are substances which are water
insoluble, but are soluble in organic
solvents (ether, chloroform, hexane, etc.).
Triglycerides are found primarily in cereal
grains, oilseeds and animal fats. The basic
structure of triglycerides consist of one unit
of glycerol (a 3 carbon sugar) and three
units of fatty acids (Figure 1).
Glycolipids form a second class of lipids
found primarily in forage (grasses and
legumes).
These compounds have a
Fatty Acids
Glycerol
O
H2
C
OH
H
C
OH
H2
C
OH
+
OH
C
OH
C
OH
C
O
R1
H2
C
O
C
R2
H
C
O
C
R3
H2
C
O
C
O
R1
O
O
R2
O
R3
Figure 1: Basic structure of triglycerides. The radicals
(R1, R2, and R3) are made of a carbon chain with variable
lengths and degrees of saturation.
HYDROLYSIS AND
SATURATION OF LIPIDS IN
THE RUMEN
In the rumen, the majority of the
lipids are hydrolyzed. The bonds
between the glycerol and the fatty
acids are broken down to give rise
to glycerol and three fatty acids.
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13
Dairy Essentials – Nutrition and Feeding
Table 1: Common fatty acids found in the diet of dairy cows
Common
Name
Structure
Melting
Abbreviation* point (°C)
................................................ Saturated acids .............................................................
Myristic
Palmitic
Stearic
CH3-(CH2 )12-COOH
CH3-(CH2 )14-COOH
CH3-(CH2 )16-COOH
(C14:0)
(C16:0)
(C18:0)
54
63
70
.............................................. Unsaturated acids ..........................................................
Palmitoleic
Oleic
Linoleic
Linolenic
CH3-(CH2 )5-CH=CH-(CH2)7-COOH
(C16:1)
61
CH3-(CH2 )7-CH=CH-(CH2)7-COOH
(C18:1)
13
CH3-(CH2 )4-CH=CH-CH2 -CH=CH-(CH2)7-COOH
(C18:2)
-5
CH3-CH2 -CH=CH-CH2 -CH=CH-CH2 -CH=CH-(CH2)7-COOH
(C18:3)
-11
* The first number denotes the total number of carbons and the second number denotes the number of double
bonds in the molecule.
Glycerol is fermented rapidly into volatile
fatty acids (see carbohydrate metabolism).
Some fatty acids are used by bacteria for the
synthesis of phospholipids that are needed
to build cell membranes.
Another important action of ruminal
microbes is to hydrogenate unsaturated
fatty acids. During hydrogenation, a fatty
acid becomes saturated because a double
bond is replaced by two hydrogen atoms.
For example, hydrogenation converts oleic
acid into stearic acid (Table 1).
Free fatty acids in the rumen tend to
attach to feed and microbial particles and
impede normal fermentation, especially of
fibrous carbohydrates. Excess lipids in the
diet (more that 8%) may have a negative
effect on milk production and fat
percentage in the milk. Unsaturated lipids
have a more negative effect than saturated
lipids. However, lipids may be "protected"
to slow down the rate of hydrolysis and
make them more "inert" in the rumen. The
seed coat tends to protect lipids within the
seeds and make them less rapidly accessible
for ruminal hydrolysis compared to free oil.
Also, industrial treatments that usually
involve the formation of soaps (calcium
salts of fatty acids) make the fatty acids
insoluble and thus inert in the rumen.
Microbial phospholipids make up 10 to
15% of the lipids leaving the rumen, the
remaining 85 to 90% are saturated free fatty
acids found primarily in the form of
14
palmitic and stearic acids bound to feed
and microbial particles.
INTESTINAL ABSORPTION OF LIPIDS
Microbial phospholipids are digested in
the small intestine and contribute to the
pool of fatty acids that are processed and
absorbed through the intestinal wall. The
bile secreted by the liver and the pancreatic
juice (rich in enzymes and bicarbonate) are
mixed with the contents of the small
intestine. These secretions are essential to
prepare the lipids for absorption by
forming water miscible particles called
micelles that can enter the intestinal cells.
In the intestinal cells, a major portion of
fatty acids are bound to glycerol (coming
from blood glucose) to form triglycerides.
Triglycerides, some free fatty acids,
cholesterol and other lipid-like substances
are coated with protein to form
triglyceride-rich lipoproteins (TG-rich LP)
also called chylomicrons or very low
density lipoproteins. The TG-rich LP enter
lymph vessels and flow to the thoracic duct
(the junction of the lymphatic system with
the blood system) where they enter the
blood system. In contrast to most nutrients
absorbed from the gastro intestinal tract,
the absorbed lipids enter the general
circulation directly and are used by all body
tissues without a preliminary processing by
the liver.
The Babcock Institute
4 - Lipid Metabolism in Dairy Cows
DIET
RUMEN
Triglycerides
Phospholipids
Glycolipids
Grains
INTESTINE
FECES
Undigested
bacterial
Lipids
Bacterial
Phospholipids
Bacterial
Phospholipids
Saturated
Free Fatty acids
Saturated
Free Fatty acids
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Forage
Sugar
RUMINAL
WALL
VFA
INTESTINAL
CELLS
Triglyceride rich
Lipoprotein
(TG-rich LP)
Free Fatty
acids
TG-r
ich L
LYMPH
P VESSEL
PORTAL
BLOOD
(To the liver)
carbohydrate
metabolism
Glucose
LIVER
Ketones
Glycerol
Fatty Acids
Energy
Triglyceride
storage
TG-rich LP
TG-rich LP
BLOOD
(General
circulation)
Gly
Tissue
Mobilisation
cer Fatty Acids
TG-rich LP Glycerol
ol
Glucose
Fatty Acids
Energy
Glycerol
Glycerol
Triglycerides
ADIPOSE TISSUES
Figure 2: Overview of lipid metabolism in dairy cows
University of Wisconsin-Madison
Glycerol TG-rich LP
Glucose
Fatty Acids
Energy
Glycerol
Milk Fat
(Long chains)
MAMMARY GLAND
15
Dairy Essentials – Nutrition and Feeding
UTILIZATION OF DIETARY LIPIDS
BY THE UDDER
About half the fat in the milk is derived
from the uptake of fatty acids by the
mammary gland. These fatty acids come
primarily from the triglyceride-rich
lipoproteins formed during the intestinal
absorption of lipids. An increase in long
chain fatty acids (i.e., acids made of more
than 16 carbons) in the diet increases their
secretion in milk, but it also inhibits the
synthesis of short- and medium-chain fatty
acids in the mammary tissue. Thus, the
marked depression in fat secretion when
cows are fed low fiber diets can be
compensated only partially by increasing
fat in the diet.
THE ROLE OF LIVER AND FAT
MOBILIZATION
During periods of under feeding or in
early lactation, cows meet their energy
demand by mobilizing fat from adipose
tissues to obtain energy in addition to that
provided by the diet. Fatty acids coming
from the triglycerides stored in the adipose
tissues (located primarily beneath the hide,
in the abdomen and over the kidneys) are
released into the blood. Mobilized fatty
acids are taken up by the liver where they
can be used as an energy source or be
converted to ketones that may be released
in the blood and used as an energy source
by many tissues. The liver does not have a
high capacity to form and to export TG-rich
LP and the excess mobilized fatty acids are
stored as triglycerides within the liver cells.
16
The fat deposited in the liver contributes to
development of metabolic disorders (e.g.,
ketosis and fatty liver) in early lactation.
ADDED LIPIDS IN DAIRY RATIONS
Lipids contain about 2.25 times more
energy than carbohydrates. Also, lipids are
sometimes referred to as a "cold" nutrients
because during digestion and utilization by
the body they produce less heat than
carbohydrates and proteins.
Thus,
increasing lipids in dairy cow rations may
have several potential benefits:
• Increase the caloric (energy) density of
the ration, especially when intake may
be limited as in high forage diet;
• Limit the need for carbohydrate-rich
concentrates which are usually
required in early lactation when a cow
is in negative energy balance;
• In hot weather, lipids may help to
reduce the heat stress of a lactating
cow.
Feed intake and milk production responses
vary greatly according to the type of lipids
added in a diet. Cows should not be fed
more than about 1.5 kg/day of lipids in
addition to the lipids present in feedstuffs.
This amount of lipids translates into a total
of about 6 to 8% lipids in the diet before
negative effects become evident. Milk
production is maximized when lipids
comprise 5% of the dietary dry matter.
Added dietary fat usually decreases milk
protein by about 0.1%. In addition, excess
lipids may depress feed intake, milk
production and milk fat composition.
The Babcock Institute
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