Page 163 - Agriculture_Form_Three

Agriculture for Secondary Schools

Table 7.1: Age at puberty for some farm animals
Species Age at puberty (months)
Cattle 10 - 18
Sheep 5 - 7
Goat 5 - 7
Pig 6 - 7

Rabbit 4 - 6
Chicken 5 - 6 (varies with breed)

Reproductive cycle
The reproductive cycle is a natural mechanism by which a mature animal undergoes
through reproductive phase and a non-reproductive phase. In mammals, the

reproductive cycle is also termed as oestrus cycle and it is divided into four stages.
Proestrus: This is a stage marked by the animal coming in heat. The graafian
follicle within the ovary is growing by increasing secretions of follicular fluid which
surrounds the ovum. This fluid contains the hormone oestradiol, which influences

sexual desire and is manifested by changes in the animal behaviour (behavioural
heat signs).

Oestrus: This is a period of sexual desire when graafian follicle is mature, very
turgid and the ovum is undergoing developmental changes. This stage is marked
by rupture of the follicle (ovulation) to release the ovum. It is also termed as heat
period.

Metestrus: This is a period when the graafian follicle cavity from which the ovum
has been ejected becomes reorganised to form a structure called Corpus Luteum
(CL). The corpus luteum is an endocrine gland, which secretes a hormone called
progesterone. The hormone serves the following functions:

(i) Prevents maturation of graafian follicles, thus preventing the occurrence of
oestrus period.
(ii) Maintains the fertilised egg (pregnancy).

(iii) Nourishes the foetus during the first half of pregnancy.
(iv) Plays the role in the initial development of the mammary gland.

Dioestrus: This is a phase when the corpus luteum is fully grown and its effect in

the uterine wall is fully observed. It is the longest part of the cycle. When pregnancy
follows, the corpus-luteum persists throughout the gestation period. In absence

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