Agriculture Reference
In-Depth Information
High-protein diets do not change the nitrogen content of
goat milk and have no effect on yield, while a slight
increase in nonprotein nitrogen content may occur. Goats
can recover normal lactation after extended periods of feed
restriction and low milk yield. This capacity is important
under rangeland grazing conditions with greatly fl uctuat-
ing feed quality and supply (Haenlein and Ramirez, 2007).
have no signifi cant benefi t in milk yield during the next
lactation period.
Environmental Temperature
The effect of environmental temperature on milk yield and
composition depends on the breed of goats, but generally
cold exposure reduces goat milk secretion, partially
because of reduced udder blood fl ow (Thompson and
Thompson, 1977 ). At
Stage of Lactation
The lactation period of goats is normally 10 months long
as in dairy cattle, but may be longer when a new pregnancy
has not been initiated. The lactation period of goats is
generally longer than that of dairy sheep.
Fat, protein, and mineral contents are high in the colos-
trum (fi rst secretion of mammary gland after parturition)
phase of early lactation, and their concentrations fall by
the fi fth day of lactation, reaching a low plateau by the
second to fourth months in milk but increase at the end of
lactation (Renner, 1982). There is a negative relationship
between milk yield and the percentage composition of fat,
protein, and minerals, while the relationship with lactose
is positive. Lactose content in goat and cow milk is low in
colostrum, rises in mid-lactation, and decreases again at
the end of the lactation.
3.9 ° C (25 ° F), mammary glucose
uptake, lactose secretion, and milk yield can be as low as
30% of the values at thermal neutrality (20°C; 68°F). Low
temperatures may have less effect on milk yield if extra
feed is provided to cover for the extra body energy required
to maintain body temperature. Within a relative humidity
range of 60-80%, milk yield may be unaffected by
temperature changes between 5 ° C (40 ° F) and 22 ° C (70 ° F).
Milk fat content increases with temperatures below 25°C
(75°F), and the SNF content follows the same pattern.
Above the range of thermal neutrality, a marked decrease
in milk production occurs, because feed consumption
decreases at increased temperatures while water consump-
tion generally increases. At about 40°C (105°F), feed con-
sumption and milk production approach zero (Figure
14.1). Milk production and feed consumption in cows are
−
Season
Goat milk composition and yield also vary with seasons
within the year. In temperate climates in late summer, milk
is often low in fat and solids-not-fat (SNF) content. A
Canadian study demonstrated that cheese yields varied
directly with seasonal variations in the levels of fat and
protein content of the milk (Irvine, 1974).
Since natural breeding of dairy goats is seasonal, in
autumn, in most dairy breeds in the Northern Hemisphere,
kidding will occur in spring. Thus, in autumn most
Northern dairy goats are in late lactation with decreased
yields and increased milk composition, unless goats are not
rebred. With artifi cial breeding methods and environmen-
tal light management, off-season kidding and year-round
or off-season milking can be achieved.
Milk
kg.
Feed
lbs.
40
80
70
60
50
40
30
20
35
Cold
Stress
Heat
Stress
30
25
20
15
10
Milk production
Feed consumption
°F
0
20
40
60
80
100
°C -20
-10 0
Environmental temperature
10
20
30
Length of Dry Period
This is a period when the dairy animal is pregnant but
not milking. Dairy goats need this period to replenish
mammary secretory tissues and body reserves and
to prepare the body for the next lactation. Animals
should be in good body condition but not fat. Dry periods
of less than 6 weeks can cause a decrease in milk yield
during the next lactation in comparison with a dry period
of more than 6 weeks. Dry periods greater than 8 weeks
Figure 14.1
Relationships between
environmental temperature and milk production/
feed consumption. When the environmental
temperature is 25°C (75°F) or above, reduced
feed intake is followed by a decrease in milk
production. At approximately
−
15°C (5°F), feed
consumption increases and milk production
decreases (adapted from Bath et al., 1985).
