Agriculture Reference
In-Depth Information
and help alleviate the demand for animal protein via human
consumption, it is essential to increase the reproduction
effi ciency (conception, fecundity, and kidding rate) also in
goats (Greyling, 1988).
The level of reproductive performance depends on the
interaction between genetic and environmental factors, but
it is particularly susceptible to the infl uence of the latter.
These factors include nutrition, ambient temperature,
stocking density, age groups, etc. Although goat breeds
have an excellent ability to accommodate and adapt to
fl uctuations in environment, this often involves some
degree of reproductive failure (Riera, 1982; Zarazaga
et al., 2005).
Reproductive effi ciency in does is determined by
many different processes. These processes include the
length of the breeding season, cyclic activity, ovulation
rate, fertilization rate, the postpartum anestrous period,
growth, and viability of the offspring. The reproductive
effi ciency as such is then measured and expressed in terms
of kidding rate, kidding interval, weight of kids born or
weaned, or length of the reproductive life of the dam
(Greyling, 1988). With the goat's general hardiness, high
fecundity, good mothering ability, and extended breeding
season, this specie is an ideal candidate for applied or
manipulated reproduction physiology to, among others,
increase the reproduction effi ciency (Devendra and Burns,
1983 ).
However, to manipulate the reproductive effi ciency of
the female goat using accelerated breeding techniques such
as artifi cial insemination (AI), synchronization, and super-
ovulation or multiple ovulation and embryo transfer
(MOET), a thorough knowledge of the hormonal status
and physiology of this animal is essential. The reproduc-
tive physiology must be seen in conjunction with the func-
tional anatomy for reproduction addressed in Chapter 6.
nisms leading to ovulation and fi rst estrus are capable of
operating long before they are called upon to function
(Greyling, 2000; Zarazaga et al., 2005).
Generally, breeding in does should be delayed until the
animal has attained 60-70% of its mature body weight.
Angora goats should weigh a minimum of 27 kg, while the
larger dairy goat breeds should weigh between 35 and 45 kg
before breeding. These weights must however not be con-
fused with the weight at puberty. There is good evidence
of inadequate nutrition (besides retarding growth rate)
having adverse effects on pituitary function and hence
gonadotrophin (sex hormone) secretion in the immature
animal. Once a critical body weight is reached, differences
in live weight have little infl uence on the time of onset at
puberty. In most goat breeds, the does are pubertal between
5 and 7 months of age (Smith, 1980; Jainudeen et al., 2000).
The potential for improved reproductive effi ciency
increases as the age at fi rst kidding decreases, but manage-
ment practices are often introduced to delay mating for
full development of the female, to increase the conception
rate, the frequency of multiple births, and to assure the
survivability of the offspring. It is important to realize that
climate, nutrition, and the presence of the male are factors
that could modify the age at puberty in the doe. There is
also ample evidence to indicate that seasonality is an
important factor in the attainment of puberty in goat
kids (male and female). The onset of cyclic activity is
usually stimulated during the season of shortening days
and its termination occurs during the lengthening (photo-
period) days. The presence of the buck, besides modifying
the age of puberty in the goat, also infl uences estrous
behavior, with some degree of synchronization of estrus.
There may be two distinct physiological actions involved
in the presence of males, namely a neurohormonal action,
which advances the preovulatory discharge of LH, and a
solely neural action, which gives rise to the “male effect”
(Riera, 1982; Chemineau et al., 1992; Gordon, 1997;
Bukar et al., 2006).
The mean body weight at fi rst estrus or puberty for Boer
goat female kids born in January (summer) and August
(late winter) is 31.3 kg and 27.4 kg, respectively, while that
in the Saanen is 30.0 kg for animals born in March/April
(spring). The mean age at puberty in Boer goat does born
during late winter and midsummer was recorded as 191.1
and 157.2 days, respectively. Kids weaned in autumn
(during the natural breeding season) generally exhibited
estrus signifi cantly earlier than animals weaned outside the
natural breeding season. It would thus seem as if season is
one of the main cues for initiating the onset of puberty.
The age at the onset of puberty in the Boer goat compares
PUBERTY
Female
One of the fi rst factors important in determining lifetime
production performance in the goat is puberty. Puberty
occurs as a consequence of the activation of the gonado-
trophin surge mechanism by the positive (stimulatory)
feedback action of oestradiol on the hypothalamus. In
the female, puberty is defi ned as the stage when the doe
starts and exhibits regular cyclic estrous activity. So for
example, some of the information on puberty relates to
body weight and age of the doe at fi rst standing estrus. It
must be kept in mind that many of the endocrine mecha-
