novel approaches to expression and detection of oestrus in dairy cows

1.2 REPRODUCTIVE PHYSIOLOGY
1.2.1 The Bovine Oestrous Cycle
Heifers reach puberty at 6-12 months of age, at a weight of approximately
200-250kg, at which they commence their oestrous cycles (Forde et al.,
2011), throughout the whole of the adult cow’s life, at intervals of
approximately 21 days. Oestrus marks the beginning of the oestrous cycle
which is followed by first ovulation, ending at the next episode of oestrus.
This occurs every 18-24 days as the cow is polyoestrous (as opposed to
the ewe and mare which can only reproduce at certain times of the year),
and continues indefinitely (Forde et al., 2011), only interrupted by
pregnancy or periods of anoestrus, caused by inadequate nutrition or other
factors affecting the delicate hormonal balance (Peters and Lamming,
1983).
The oestrous cycle is the result of cyclical changes in the ovaries controlled
by hormone interactions (see Figure 1.1) of the hypothalamus (GnRH;
gonadotrophin releasing hormone), the anterior pituitary gland (FSH;
follicle stimulating hormone, LH; luteinising hormone), the ovaries
(progesterone, oestradiol, inhibin) and the uterus (prostaglandins). These
exert their actions by negative and positive feedback mechanisms (Webb
et al., 1992;Forde et al., 2011). Figure 1.1 shows the hormonal changes
that occur throughout the bovine oestrous cycle and key structures. FSH
stimulates follicle recruitment in waves of 2 to 4 per cycle. LH pulses
continue to stimulate the growth and development of the dominant follicle,
influencing oestradiol secretion by the dominant follicle. Positive feedback
of increasing oestradiol levels results in oestrus and increasing LH pulse
frequency to a peak which results in ovulation. Progesterone is produced
by the corpus luteum (CL) during the luteal phase of the oestrous cycle and
increases post ovulation. In the non-pregnant cow prostaglandin-F 2α
(PGF 2α ) from the uterus causes corpus luteum regression and decreasing
progesterone concentration, which allows increasing oestradiol
concentrations, due to increased basal LH, resulting in oestrus prior to the
LH peak for ovulation (Webb et al., 1992;Webb and Campbell,
2007;Garnsworthy et al., 2008). The oestrous cycle of the cow is best
described in 2 phases; the follicular phase and the luteal phase, describing
the key structures present during that time. During the shorter follicular
phase (4-6 days) oestradiol is the dominant hormone, produced by the
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