Agriculture Reference
In-Depth Information
tracting the alveolus and the letdown of milk. The smaller
ducts within the lobules are lined with a single layer
of epithelial cells and connect to the larger ducts, ending
in the gland and teat cisterns. Milk is secreted by the
alveolar cells through two secretory processes: synthesis
and diffusion. Milk removal is accomplished by passive
withdrawal of milk from teat and gland cisterns and the
major ducts during nursing, hand milking, or machine
milking. Active ejection of milk results from contraction
of myoepithelial cells around the alveoli in response to the
milk secretion hormone oxytocin elicited by the neural
stimuli of the suckling kid or the milking procedure
(Schmidt, 1971 ).
Following parturition, the increase in milk yield reaches
a maximum in 4-8 weeks in milking goats and cows. The
milk yield gradually declines toward the end of lactation.
After maximum yield is reached, a decline in milk produc-
tion can be calculated each month as a percentage of the
previous month. This percentage is a measure of lactation
persistency, which varies between breeds and individual
animals. Although milk volume is decreased at the end of
lactation, the solids content, such as protein, fat, and min-
erals increases, and lactose content decreases.
Milk yield declines after the onset of estrus (Peaker and
Linzell, 1974), and the variation in milk production may
be related to the different amounts of mammary blood fl ow
of individual animals. Goats are frequently used as experi-
mental animals in lactation and other physiological studies,
and considerable basic knowledge of the physiology of
mammals has thus been derived from goats.
mammary gland of most species is dependent upon the
neurohormonal refl ex process of milk ejection. Sensory
nerve endings in the teat are stimulated, and impulses are
carried via the spinal cord to the hypothalamus. This stim-
ulus reaches the central nervous system and causes the
posterior lobe of the pituitary to release oxytocin, a small
peptide hormone. Oxytocin travels via the blood supply to
the mammary gland, where it causes contraction of the
myoepithelial cells surrounding the alveoli. The contrac-
tion process forces the milk from the alveoli into the duct
system, from which it fl ows to the gland and teat cisterns.
Adrenalin is antagonistic to oxytocin. The neurohormonal
refl ex of the milk ejection process is inhibited by adrenalin,
resulting from fright and emotional disturbances of the
animal.
FACTORS AFFECTING MILK YIELD
AND COMPOSITION
Breed
There are six major dairy goat breeds in the U.S. The
Saanen, Alpine, Toggenburg, and Oberhasli of Swiss
origin are the most famous around the world for their
record high milk productivity; the LaMancha are of U.S.
origin; and the Nubian are of British origin. The Saanen is
best known as the Holstein of the goat world, producing
high milk yields with low fat levels (Haenlein and Caccese,
1984). The Nubian breed is similar to the Jersey breed of
cows, but is the most numerous in the U.S. Nubians
produce less milk with higher fat and protein content. The
Alpine, Toggenburg, LaMancha, and Oberhasli breeds
produce milk yields and composition between the levels
of the Saanen and Nubians in general but with some indi-
vidual exceptions (Table 14.2). There are at least 10 origi-
Initiation of Milk Ejection
The function of the mammary gland is under hormonal
control of the pituitary gland. Milk removal from the
Table 14.2
Individual U.S. goat breed leaders in milk production.
Lactation
Breed, goat registration no., year
of offi cial record
Age days
in milk
Fat, % (fat
yield kg)
4% FCM
1
,
yield, kg
(Yr - mo)
Milk kg
Milk/day, kg
Alpine #0177455, 1982
7 - 4
305
2,916
9.6
4.8 (140)
3,266
LaMancha #0618876, 1991
5 - 0
253
2,454
9.7
3.3 (81)
2,197
Nigerian Dwarf #1360584, 2006
3 - 10
305
782
2.6
5.4 (42)
943
Nubian #0904515, 1996
2 - 9
302
2,700
8.9
5.1 (138)
3,150
Oberhasli #0935588, 1997
3 - 0
304
2,120
7.0
5.0 (106)
2,438
Saanen #0894085, 1997
4 - 0
305
2,987
9.8
2.6 (76)
2,335
Toggenburg #0926741, 1997
3 - 1
305
3,620
11.9
3.9 (142)
3,578
1
4% Fat corrected milk = 0.4 × milk yield kg + 15 × fat yield kg
Source: ADGA, 2008
