novel approaches to expression and detection of oestrus in dairy cows

on average in Holsteins (Dransfield et al., 1998). The percentage of cows
actually standing to be mounted has also declined from 80% to 50%
(Dobson et al., 2008). This results in only 50% of cows being observed in
oestrus (Van Eerdenburg et al., 2002). Hence detection of oestrus is
increasingly more difficult and to improve the decline in fertility oestrous
detection rates must be improved.
1.3.1.1 Endocrine, Neural and Genomic Changes Associated with Oestrous
Behaviour
Oestradiol is the key regulator that synchronizes the endocrinological and
behavioural events to drive oestrus; resulting from the action of ovarian
steroids on behavioural centres in the brain (Roelofs et al., 2010). The
production of oestradiol from the ovary synchronises mating and ovulation;
rising above its threshold in an all or nothing response (Allrich, 1994).
During follicle development increasing concentrations of oestradiol are
produced and secreted mainly from the dominant follicle (Staigmiller et al.,
1982). This increases follicular oestradiol 3-4 days before oestrus, causes
circulating concentrations of oestradiol to increase (Roelofs et al., 2010),
acting at the level of the hypothalamus to trigger a series of programmed
neurological events that result in behavioural oestrus (Reames et al.,
2010). Other centres in the brain also trigger the closely related LH surge
which is required for ovulation and occurs 28-32 hours after oestrus
(Walker et al., 1996). In contrast progesterone from the CL also controls
oestrus by inhibiting GnRH and LH pulses which reduces oestradiol
concentration (Smith and Jennes, 2001).
Often in the post partum period there is a silent oestrus which involves
ovulation without overt oestrous expression. This is thought to be caused
by high oestradiol levels following gestation inducing a refractory period
(Allrich, 1994). Oestrus can be affected through the duration of
progesterone and progesterone amplitude during the luteal phase which
can influence the increase in levels of oestradiol. It is suggested that this
occurs by the influence of progesterone on the neural mechanisms
controlling release of GnRH, influencing the elements targetted by
oestradiol to induce the preovulatory LH surge in the ewe (Skinner et al.,
2000). It has also been reported in the ewe that previous progesterone
exposure can affect the intensity of oestrous expression (Fabre-Nys and
Martin, 1991). However, in the cow these mechanisms differ and oestrus
can occur without progesterone exposure. Even with low levels of
16