Agriculture Reference
In-Depth Information
well and in fact is sooner than in most other goat breeds,
for example the Saanen (217.9 days), Angora (240 days),
Black Bengal (196.5 days), and Barbari Nannies (213
days). From the estrous response (percentage does exhibit-
ing estrus) obtained, it is evident that the permanent pres-
ence of the male has a marked benefi cial effect on animals
exhibiting estrus or puberty in- and outside the natural
breeding season. Research regarding puberty in the buck
however is generally very sparse (Amoah and Bryant,
1984; Greyling and Van Niekerk, 1990b; Papachristoforou
et al., 2000 ).
sonally polyestrous, though periods of complete anestrus
are not observed. The pattern of the seasonal occurrence
of the animal sexual activity for the adult Boer goat doe
and the female Boer goat kid (aged 5-17 months) is very
similar. The peak of sexual activity occurs during autumn
or the period of short daylight length, while the period of
lowest sexual activity occurs from late spring to midsum-
mer (see Figure 7.1). The mean number of estrous periods
per annum per doe recorded for the Boer goat is quoted as
being 11.7
4.4 (approximately 280 days). One buck per
35-40 does is recommended when fl ock mating is imple-
mented, and it is important to endeavor to mate the young
maiden does separately from the mature does (Devendra
and Burns, 1983; Restall, 1992; Ahmed et al., 1997; Rivera
et al., 2003 ).
Generally the male does not exhibit such a pronounced
mating season, although there is a tendency for decreasing
photoperiod (daylight length) to stimulate follicle stimulat-
ing hormone (FSH), luteinizing hormone (LH) secretion,
and testosterone production. Certain buck breeds tend to
be more sensitive to daylight length and thus be more
seasonal, for example, the Angora (Greyling and
Grobbelaar, 1983; Barkawi et al., 2006; Todini et al.,
2007 ).
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Male
In the buck, the onset of puberty is associated with a sig-
nifi cant increase in testosterone production and subsequent
spermatogenesis. This phenomenon occurs at 4-6 months
of age. Factors that affect the onset of puberty include
photoperiod and nutrition. Sexual maturity (when bucks
can be used for breeding) is generally at approximately 18
months of age (60-80 kg, depending on the breed).
Both internal and external cues affect time of puberty,
and it is diffi cult to isolate a single factor involved in the
attainment of puberty. Generally a sequence of events is
involved in the process of sexual maturation and an inter-
action possibly exists between the male effect, seasonality,
and nutrition (Greyling, 1996; Nishimura et al., 2000;
Todini et al., 2007 ).
PHYSIOLOGY OF THE
FEMALE REPRODUCTION
SEASONALITY AND THE BREEDING SEASON
Regarding the breeding season, different climatic factors
(such as temperature and photoperiod) regulate the physi-
ological response. The length of the breeding season as
such is primarily the result of genetic and environmental
interactions. The breeding season commences as the day-
light length becomes shorter. There is evidence to indicate
that in some species the pineal gland, through its secretion
of melatonin, is involved in mediating the effects of pho-
toperiod on gonadal function. Melatonin levels are high
during the dark periods and low during light periods. There
is also evidence to suggest that the premaxillary area of
the hypothalamus is an important target for melatonin to
regulate the reproductive activity (Hunter, 1980; Zarazaga
et al., 2005).
Breeds differ in their length of the breeding season. For
example, the Angora doe has a short breeding season,
varying between 94 days (or 4-7 estrous cycles) and
117
The Estrous Cycle
The normal cyclic doe undergoes a repeated secretion of
steroid sex hormones that infl uences both the reproductive
tract and the animals' sexual behavior. The period of
estrous behavior, when the female shows characteristic
sexual behavior in the presence of the male, is the only
period when physical mating is allowed. Ovulation or the
shedding of the ovum occurs at the end or shortly prior to
the end of the period of estrus and is usually taken as a
reference point for the description of the estrous cycle. In
any event, it is the period of estrus itself that is of greatest
concern to the farmer (especially when implementing arti-
fi cial insemination [AI]). Recognition of this receptive
period relies on overt signs in the presence of other animals
(Hunter, 1980; Greyling and Van Niekerk, 1987).
Currently little is known of the sequence of endocrine
events during follicular and luteal development in the goat
during the estrous cycle. The mean plasma progesterone
concentration on the day of estrus is extremely low (0.2
nanograms per milliliter [ng/ml]), after which the concen-
tration increases to a maximum of approximately 4 ng/ml
at about day 10 of the 21-day estrous cycle, and decreases
3.5 days. Proper management of the introduction of
the buck can be used to slightly hasten the onset of the
breeding season and also synchronize estrus among the
does involved. It would appear as if the Boer goat is sea-
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