Chapter 4
Physiology of
Milk Synthesis
De novo synthesis
30
Preformed
W eig ht %
25
20
15
10
5
0
4:0
6:0 10:0 12:0 14:0 16:0
F a tty ac id
16:1 18:0 18:1 18:2
Milk Fat Synthesis
Circulation
Synthesis
Translocation
Secretion
UFA
TAG
LPL
FABP
NEFA
+
Glycerol
SFA (C16 - C18)
TAG
MFGM
synthesis
Glycerol
Glycerol-P
Glucose
Glucose
Acetate
ßHBA
Basal membrane
de novo FA synthesis (C4 - C16)
ACC
FAS
ER membrane
Luminal membrane
Epithelial Cell
Lumen of alveolus
Fat
Droplet
Fat Droplet
migrating
Golgi Body
Lysosome
Nucleus
Mitochondria
Mitochondria
E.R
.
Blood Vessel
Epithelial Cell
Lumen of alveolus
Fat
Droplet
Fat Droplet
migrating
Golgi Body
Lysosome
Nucleus
Mitochondria
Mitochondria
E.R
.
Blood Vessel
Epithelial Cell
Lumen of alveolus
Cell pinches off
Fat
Droplet
Fat Droplet
migrating
Golgi Body
Lysosome
Nucleus
Mitochondria
Mitochondria
E.R
.
Blood Vessel
Epithelial Cell
Lumen of alveolus
Cell pinches off
Fat
Droplet
Fat Droplet
migrating
Golgi Body
Lysosome
Nucleus
Mitochondria
Mitochondria
E.R
.
Blood Vessel
Major Fatty Acids (wt%) of Bovine Milk
Saturated Fatty Acids
4:0
6:0
8:0
10:0
12:0
14:0
16:0
18:0
Butyric acid
Hexanoic acid
Octanoic acid
Decanoic acid
Lauric acid
Myristic acid
Palmitic acid
Stearic acid
4.5
2.3
1.3
2.7
3.0
10.6
28.2
12.6
Major Fatty Acids (wt%) of Bovine Milk
Monounsaturated
Fatty Acids
16:1
18:1
Palmitoleic acid
Oleic acid
1.6
21.4
Major Fatty Acids (wt%) of Bovine Milk
Polyunsaturated Fatty
Acids
18:2
Linoleic acid
18:3
Linolenic acid
2.9
0.3
Effect of hydrogenation by rumen
microorganisms on dietary lipids
Wt% of Fatty Acids
Fatty acid
16:0
18:0
18:2
18:3
Diet
Abomasal digesta
26
3
17
31
29
45
4
6
Changes in rumen conditions
that lead to milk fat depression
linoleic acid
(cis-9, cis-12 C18:2 )
conjugated linoleic acid
(cis-9, transtrans-11 C18:2)
vaccenic acid
(trans-11 C18:1)
stearic acid (C18:0 )
conjugated linoleic acid
trans-10, ciscis-12 C18:2
trans-10 C18:1
MFD induced by diet
MFD induced by Ca-salts of CLA
5
5
2
y = 1.19x 2 - 3.37x + 4.47
y = 1.04x - 2.76x + 3.93
R2 = 0.63
3
3
2
2
1
1
0.0
0.5
1.0
1.5
trans -10, cis -12 CLA, mg/g milk fat
R2 = 0.66
4
Milk fat%
Milk fat, %
4
2.0
0.0
0.5
1.0
1.5
trans -10, cis -12 CLA, mg/g milk fat
2.0
Milk Fat Concentration

Most variable component of milk
 Increased during periods of energy deficit
 Decreased when fed diets containing
unsaturated fatty acids (most grains) and low
effective fiber (change rumen conditions)
– Milk fat depression (<2.8% fat)

Fatty acid changes also occur
Milk Proteins
Caseins
(80%)
beta-lactoglobulin
(10%)
Non-casein (Whey)
(20%)
alpha-lactalbumin
(4%)
Serum albumin,
Immunoglobulins
(5%)
Milk Protein Concentration
Not too variable
 Changes related to dietary energy
intake, not dietary protein

– little effect of dietary fat on milk protein
concentration unless perturb normal rumen
function
– fermentable carbohydrate intake is the key!

Maximize VFA and microbial protein production
1.5
60
50
40
30
20
10
0
1.0
0.5
0
0
10
20
30
40
hours since last milking
Secretion Rate
(kg/hr) (____)
Udder Pressure (mm
Hg) (----)
Udder Pressure & Secretion Rate
Secretion Rate
• Influence of udder pressure
• Milking intervals
– 12 hour vs. other
– 2X vs. 3X
– young vs. old cows
Yield and Milking Frequency
(Erdman & Varner, 1995 JDS)
Item
Milk yield
Fat%
Protein %
# studies
40
18
4
2X
42.9
3.65
3.10
3X
50.6
3.51
3.05
4
3
3
2X
54.6
4.71
3.49
4X
64.5
4.53
3.44
# studies
Milk Let Down
• Refractory period to oxytocin?
• Adrenaline interferes
Milk Ejection
brain
nerves
P.P.
Stimulus
Milk Ejection
nerves
P.P.
Milk Ejection
P.P.
Milk Ejection
Abdominal aorta
Oxytocin
Jugular
vein
heart
Milk Ejection
Oxytocin
Milk Ejection
Oxytocin
Milk Ejection
Milk Ejection
Milk Ejection
Contraction of an Alveolus
arteriole
venuole
Lumen
Myoepithelial Cell
Oxytocin
Duct
Contraction of an Alveolus
Oxytocin
Myoepithelial Cell
contracts
Contraction of an Alveolus
Releases milk into
duct system
Contraction of an Alveolus
Myoepithelial cell relaxes
Contraction of an Alveolus
Contraction of an Alveolus
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Milk Fat % as Milking
Progresses
foremilk
early
middle
late
leftover
Milk Fat % & Exercise
3.8
3.7
3.6
3.5
3.4
3.3
3.2
limted
moderate
considerable
Biosynthesis of Milk
components

Precursors from blood
 One liter of milk requires 500 L of blood
 Fat biosynthesis
 Protein biosynthesis
 Lactose biosynthesis
Milk Fat biosynthesis

Requires fatty acids and glycerol
 Two sources of fatty acids:
1- blood lipids (dietary lipids & adipose tissue)
2- De novo synthesis (within the MG)
Major Fatty Acids (wt%) of Bovine Milk
Polyunsaturated Fatty
Acids
18:2
Linoleic acid
18:3
Linolenic acid
2.9
0.3