novel approaches to expression and detection of oestrus in dairy cows

oestrous expression with increasing lactation is that a greater cumulative
yield of milk has been produced. However these cows are moving into
positive EBAL so it is less likely that the effects of lactation yield are seen
on oestrous expression at this stage. Lower activity levels in later lactation
could be related to the larger proportion of problem cows more than 80
days in milk (Peralta et al., 2005). In conclusion there may be many
underlying causes for decreased oestrous expression and it is possible that
problem cows do not survive to get in calf due to culling for infertility.
2.4.1.3 Time of Year
There was a larger increase in activity at oestrus, in periods 2 and 3
compared to periods 1 and 4. Periods 2 and 3 are normally associated with
hotter temperatures, and it is suggested that heat stress and factors
associated with heat stress, affecting follicle development and
steroidogenesis, can influence oestrous expression (Roche, 2006). Reports
in this area are inconsistent; some authors report increased expression of
oestrus in the hotter, summer months (Peralta et al., 2005); others report
the opposite, with increased expression in the colder, winter months (Nebel
et al., 1997). However, the patterns associated with temperature are
mostly from outside the UK, and few reports from studies in the UK have
found any association between temperature and oestrous expression. This
relationship applies more to countries with hot climates with large
fluctuations in temperature. Some components of the reproductive system
are susceptible to extreme temperatures compromising the steroidogenic
capabilities of the theca and granulosa cells (Wolfenson et al., 2000;De
Rensis and Scaramuzzi, 2003).
The results of the current study, however were recorded in the UK
temperate climate and therefore could be affected by other causal effects
of seasonal variation on reproduction; day length, photoperiod, humidity,
level of nutrition, management or combinations of these factors (Critser et
al., 1987). Cattle are not seasonal breeders in the strictest sense, as they
now breed and cycle all year round, but seasonal influences can have an
effect. This is more subtle than in the sheep, in which reproduction can
only occur at certain times of year and is strictly controlled by photoperiod
influencing the ability of oestrogens to inhibit LH (Legan et al., 1977). The
influence of season on cattle reproduction has been linked to a number of
events associated with reproduction; return to cyclicity is longer if calving
is in winter compared to summer (Hansen, 1985) and season of birth and
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