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COLLECTION: GOAT HANDBOOK
ORIGIN: United States
DATE INCLUDED: June 1992
Extension Goat Handbook
This material was contributed from
collections at the National Agricultural Library.
However, users should direct all inquires about the
contents to authors or originating agencies.
DOCN 000000050
NO F-7
MILK SECRETION
G. F. W. Haenlein R. Caccese; U. of Delaware, Newark
P. E. Meckley; U. of Delaware, Newark
Anatomy and Physiology
1 Growth and Development The mammary glands of goats
are specialized cutaneous glands, related to the sebaceous
(oil producing glands of skin and hair) and sweat
glands. From a physiological viewpoint, they might
be classified as accessory reproductive organs, as
they are intrinsic to the reproductive function. Mammary
glands are present in both sexes. Functional activity
in the male is rare, although milk secreting glands
have been developed in both virgin does and bucks
by repeated gentle massage of the mammary area. Differentiation
in growth of mammary glands between the sexes is usually
not obvious until puberty. At that time, glandular
enlargement occurs in the female. Most of it is in
increased amount of connective tissue and fat deposition,
but not increased formation of secretory tissue. Estrus
periods bring on increases in the development of the
secretory and duct tissues, with a recession during
the anestrus period. The process of enlargement is
minimal however, and the glands will not approach
a functional state until the animal becomes pregnant.
2 Growth of lactating tissue is dependent
mainly upon two hormones, estrogen from the developing
follicles and progesterone from the corpus luteum.
The corpus luteum is the naturally regressed stage
of the follicle after it has ovulated, releasing ova
into the oviduct. Estrogen, which is cyclic, stimulates
the duct development of the mammary gland. Progesterone
is almost continuously secreted during pregnancy,
causing secretory tissue development. As the gestation
period nears its end, the mammary glands become capable
of producing milk. After parturition, the rate of
milk secretion increases for some time, reaches a
peak, and then gradually declines. A loss of secretory
epithelial cells occurs during involution, although
some new cells are being formed. The general activity
level of the individual cells declines also. At the
cessation of milk production, secretory epithelial
cells will totally disappear, leaving only myoepithelial
cells. The frequent release of oxytocin may slow down
the rate of the involution process and some goats
may secrete milk for years continuously.
3 Milk secretion during lactation
tends to inhibit the normal cycling of the estrus
periods, with some temporary suppression of ovulation.
With time, there will be a return to normal estrus
cycles. Milk production will decline, and the glands
will go into involution; although not complete, since
the glands increase in size with successive pregnancies.
4 Hormones other than estrogen and
progesterone that influence development of mammary
glands or lactation originate from the hypothalamus
and pituitary glands of the brain.
5 The anterior pituitary produces
in response to stimulation of the hypothalamus six
hormones, which exert either a direct or indirect
influence on milk secretion. Somatotropic or growth
hormone (STH); follicle stimulating hormone (FSH);
luteinizing hormone (LH); prolactin (most intimately
associated with initiating and maintaining lactation);
thyrotropic hormone (TTH); and adrenocorticotropic
hormone (ACTH).
6 The posterior pituitary stores
two hormones from the hypothalamus both related to
lactation. Vasopressin or antidiuretic hormone (ADH),
decreases the amount of water lost in the urine, retaining
it instead for use. Oxytocin is best known for its
ability to cause milk ''let down'' in mammals, although
it is also capable of stimulating contractions in
other smooth muscles such as the uterus during estrus
and parturition, as well as in the urinary bladder
and intestine.
7 The pars intermedia of the pituitary
secretes the hormone intermedin, which has some effect
on water metabolism.
8 In mature goats secretions of FSH
stimulate follicular development, which then becomes
a source of estrogen. LH, working in the presence
of FSH, produces ovulation; and from the corpus luteum
progesterone is released.
9 Estrogen by itself generally stimulates
duct development of the mammary glands, while estrogen
and progesterone together cause lobulo-alveolar growth.
In the goat, however, estrogen alone will initiate
abnormal development of the mammary, producing dilated
alveoli and even milk production in some cases. High
levels of estrogen in circulation have a negative
effect on feed consumption and milk production in
goats. Estrogen and progesterone are required both
for the complete development of the mammary system,
along with the other pituitary hormones. Large amounts
of prolactin are to be used just prior to parturition
in response to the higher levels of estrogen and progesterone
at this time. Prolactin matures the alveolar cells
to a functional condition. STH and thyroxine, also
exert an influence on the amount of milk produced.
Small injections of estradioltestosterone produce
a colostrum-like milk in goats, while large doses
produce a watery, clotted milk secretion.
10 The hormone of most interest is
oxytocin. It is responsible for milk let-down, causing
the myoepithelial cells that surround the alveoli
to contract, forcing the milk out into the ducts of
the udder. Oxytocin release is initiated in several
ways, the most natural being nursing. It will directly
stimulate the afferent nerve fibers of the teat, causing
the release of the hormone and transportation via
the bloodstream to the mammary glands, with subsequent
contraction of the myoepithelial cells. This milk
let-down reflex can also be initiated by other repetitive
occurrences, such as washing the udder, approaching
the milking area, the sounds of the milking machine
or milk buckets, or even the sight of the milker.
11 The process of milk let-down is
subject to interference if the goat should become
excited through some disturbance. A release of epinephrine
(adrenalin) follows such excitation. Epinephrine causes
constriction of the small arteries and capillaries
of the udder and may prevent the myoepithelial cells
from contracting. Therefore, anything that may cause
a disturbance or alarm near milking time should be
avoided.
12 After the initial stimulus for
let-down, it takes a period of about 20-60 seconds
for the response of oxytocin, which influence will
last about 5-6 minutes. It is important to milk goats
soon after their let-down and not a stimulate more
does than can be milked immediately. Due to an inability
to completely close off milk ducts, goats in contrast
to cows cannot hold back milk flow entirely.
13 Artificial induction of lactation
has been accomplished in goats with good results by
injections of hexestrol at a daily rate of 0.25 mg.
Pellet implantations of estrogen and progesterone
have also successfully created udder growth, with
final treatment of estrogens to initiate lactation.
Art ificial induction of lactation has experimental
value, but is not practical for several reasons. Injections
must be given over a period of time on a daily basis,
or tablet implants must be inserted. The methods are
costly, labor intensive, give a low yield of milk,
and do not produce income from the sale of the kids.
There may also be questions of safety of the milk
after using these injections.
14 Milk Composition Milk differs
in structure and composition from the blood from which
it originates. Milk is composed of protein, fats,
sugar and mineral salts. The osmotic pressure of milk
is essentially equal to that of blood. The pH at 6.5
is slightly acidic. Milk contains higher levels of
sugar, lipids, calcium, phosphorus and potassium than
blood, but lower amounts of protein, sodium and chlorine.
Proteins also differ, since milk protein is composed
primarily of casein, with smaller amounts of albumin,
globulins and others. Blood protein, is primarily
composed of albumin and globulins. Fat composition
differs, as milk lipids are predominantly triglycerides
and blood lipids consist of phospholipids and cholesterol.
Non-protein nitrogen compounds are also found in milk,
such as urea, uric acid, creatine, creatinine and
ammonia. Some of these are from the blood system,
while others are waste products of the mammary gland.
15 Lactose, the milk sugar, is a
disaccharide carbohydrate that is unique to the mammary
gland. Blood glucose is the primary precursor of lactose,
besides propionic acid, while acetate (another VFA)
is predominantly used in milk fat synthesis. Butyrate
is generally distributed fairly evenly between lactose,
casein and fat constituents of the mammary secretion.
16 Milk lipids (fat) consist primarily
of triglycerides, although there are small amounts
of phospholipids, cholesterol, fat-soluble vitamins,
free fatty acids, and monoglycerides. Fat exists in
small globules in the freshly secreted milk, averaging
under 4 microns in diameter in goats and being smaller
than for cow milk. The outer layer of the globule
contains phospholipids, cholesterol, protein, and
vitamin A and stems from the alveolus. Acetate is
the major precursor of milk fat in goats. Goat milk
fat does not rise or ''cream-line'' as easily as cow
milk fat since it lacks the coalescing factor besides
being of smaller average globule size. Rupturing the
milk fat globule membrane can add to off-flavor problems
of goat milk. Since milk fat synthesis depends on
the supply of acetate from the rumen, any feeding
regime, such as high grain feeding which lowers the
production of rumen acetate, will also lower the fat
content of goat milk. Supplementing the feed ration
with more crude fiber sources, such as hay, sunflower
seeds, peanuts in their shells, etc. will restore
milk fat levels to normal.
17 Most major vitamins are found
in goat milk, some in abundance. B-complex vitamins
are manufactured by the rumen flora. Vitamin K is
synthesized in the rumen and the intestine. Vitamins
A, D and C tend to be diet related; expecially vitamin
A and the precursor carotenoids. Milk levels of vitamin
D are usually increased during commercial milk processing.
Vitamin A in goat milk is related to blood levels;
and no carotenoids are found in goat milk.
18 Minerals in goat milk consist
mostly of calcium, phosphorous, sodium, potassium,
chlorine, magnesium and sulfur. Trace amounts of aluminum,
boron, bromine, cobalt, copper, fluorine, iodine,
iron, manganese, molybdenum, silicon, silver, strontium
and zinc are found in milk in less than 1 part per
million.
19 Colostrum The first milk after
a doe kids is of great significance to the young.
This colostrum has a high nutritional value and contains
antibodies essential for the survival of the newborn.
Goats like other ruminants have a 5-layer placenta
through which no antibodies can be transmitted from
the dam to the kid in utero. Thus the kids depend
on the colostrum as their source of antibodies, providing
passive immunity until they are developing their own
active immune system. The period in which the immunoglobulins
can be absorbed through the kid's GI tract lasts 3
to 4 days in goats. Beyond this time, the digestive
enzymes in the gastrointestinal tract of the kid will
break down the protein structures of the antibodies,
rendering them ineffective. The high percentage of
these immunoglobulins in colostrum, along with albumin,
gives it its thick, sticky consistency.
20 Globulin, having a high proline
content, is also important for the formation of hemoglobin
in the young kid. Normal milk is much lower in globulin
levels, having instead a higher level of casein. The
functional importance of casein is that it is the
only milk protein forming a curd upon coagulation
in the abomasum thereby creating a slower moving food
reserve for the young kid.
21 The dry matter content of colostrum
is much higher than in normal milk, primarily due
to the large amount of proteins, especially albumin
and globulins. The vitamin content of colostrum is
also higher than normal; the lactose content is low.
The colostral content of iron is generally about 15
times greater than in normal milk, while vitamin A
and vitamin D levels are about 10 and 3 times those
of normal milk.
22 Colostrum or milk bypasses the
reticulorumen area because of the formation of the
esophogeal groove, thus preventing the milk from laying
in the rumen and turning rancid, which would then
develop scours. The formation of this groove is in
response to the sucking reflex of the young kid. Actual
suckling need not occur however, as a kid can drink
from a bucket and still bypass the rumen.
23 Milk Production Milk production
consists of secretion and excretion. Secretion is
the formation of milk from its blood precursors within
the alveoli. Excretion is the discharge into the lumen,
ducts, cistern, teat and final harvest by the milking
person. As the collecting ducts begin to fill with
the accumulated excretion of the alveolar cells, they
experience difficulty in ridding themselves of milk.
When the pressure in the udder rises, the cells and
lumina of the alveoli become distended, compressing
the small collecting ducts. This prevents a pressure
overload on the teat sphincter and any leaking of
milk.
24 There is little milk production
going on while milking is being done. Udders can extend
themselves greatly to hold high volumes of milk. During
the first hour after milking, there is no discernible
mammary pressure; but a steady, gradual increase in
pressure occurs until the time of the next milking.
Milk let-down results in a dramatic increase in mammary
pressure which will gradually subside, even if no
milk is removed.
25 While much has been said about
the virtues of milking at equal intervals (2 x 12
hrs or 3 x 8 hrs) in order to keep up good production
levels, studies have shown that the drop in goat milk
production may be quite small. However, stress and
mastitis incidence can be reduced significantly by
equal milking intervals or by 3-times milking for
very high milkers.
26 Rapid removal of milk after stimulation
of letdown is essential for complete milking. There
is always some residual milk left in the udder after
milking, but it is normally less in goats than in
cows. Regular and complete milking is one of the requirements
for continuance of lactation. The stimulus of nursing
prevents mammary gland regression. The response is
due to release of prolactin from the anterior pituitary.
Lack of this hormone hastens mammary involution and
drying off of the doe.
27 Stage of Lactation Effects There
is a great difference in the composition of milk during
the various stages of lactation among does. Management
practices such as the length of the dry period, feeding
program and general health practices play a significant
role in the quality and quantity of milk that is produced
during lactation.
28 Colostrum appears to be a waste
product from the new development of secretory tissues;
and while essential to the kid, is not used for human
consumption normally. In some does, during the first
few weeks for milk production, there can be evidence
of some blood in the milk. This is more common in
the heavy producing, first-time freshner and likely
the result of rupturing some tiny blood vessels in
the udder. Milking the doe 3 or even 4 times a day
may alleviate the problem. A lack of calcium may also
be involved, and should be supplemented in the diet.
Forceful milking is another possibility.
29 The production level of goat milk
increases for about 20-30 days after kidding. During
this period of lactation, there is an inverse relationship
between levels of milk and fat content. The percentage
of total fat as well as the composition of milk fat
varies. Towards the end of lactation, fat and protein
contents rise while milk yields decrease.
30 Calcium and phosphorous levels
in milk are high in colostrum, then decrease constantly
until near the end, when they rise again. During this
period, the overall salt content of milk tends to
increase, affecting the taste of milk. Somatic cell
numbers are also very high normally in late lactation
as well as in colostrum.
31 Persistency of milk secretion
throughout lactation can be mathematically expressed
by determining the average percentage of decrease
in milk for each month, compared to the previous month.
Goats, given proper feed and not being rebred, will
continue to give milk with a high degree of persistency
for a long time. High production on a yearly basis
must combine high initial production and good persistency,
which is a heritable characteristic.
32 Age Effects Milk volume increases
with age up to the fourth or fifth year. After that,
the volume decreases with advancing age. The rate
at which production decreases is slower than the rate
at which it increased to maximum yield. The average
milking life of a doe maybe about 12 years.
33 Body Size The relationship of
size to milk production provides a misleading picture.
Large does are not necessarily more efficient producers.
Based on gross energetic efficiency, there is little
difference in milk production due to species. Although
a goat produces more milk per unit of bodyweight than
a cow, the actual net energy efficiency is close.
Goats have, however, a relatively high basal metabolic
rate and therefore tend to have among the single-purpose
dairy breeds a relatively high dairy merit and net
efficiency.
34 Estrus It appears that upon coming
into estrus, the doe goes through a decrease in milk
production. This is only temporary, and is usually
compensated for by a brief period of higher than normal
production after the estrus cycle. There may also
be an increase in the level of fat produced during
the estrus period, as is often the case when milk
production is lowered.
35 Disease Effects Most diseases,
including mastitis reduce milk yields. Fat content
will rise due to lower milk production. The solids
(and minerals), albumin, globulin and non-protein
nitrogen levels will increase, while casein and lactose
contents decrease.
36 Dry Period A dry period for the
doe is necessary to rebuild her body reserves, especially
minerals and to prepare for the period of heavy production
in the next lactation. Fattening during the dry period
can lead to ketosis or pregnancy toxemia problems.
A 60-day dry period is considered normal. Goats without
a dry period tend to produce less in the next lactation.
37 Season Effects Temperature, humidity,
management practices and feeds tend to vary with seasons,
thereby affecting milk and fat production. Does test
lower in the summer than in the winter, not necessarily
due to drop in milk production. Does which freshen
later in the spring or early summer will usually have
a higher test average for the year than does freshening
at other times.
38 There are many variables that
can affect quality and quantity of goat milk. Many
may be hard to control. Careful and efficient management
with a willingness to learn and try new ideas, is
certain to increase productivity of the goats and
the quality of their milk.
MILK SECRETION
COLLECTION;GOAT HANDBOOK
ORIGIN;United States
DATE_INCLUDED;June 1992
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