novel approaches to expression and detection of oestrus in dairy cows

season of attainment of puberty can influence age at puberty in heifers
(Schillo et al., 1983).
Seasonal changes in peripheral concentrations of gonadotrophins have
been reported (Critser et al., 1987), which could explain the influence of
season and photoperiodic variations in activity levels at oestrus. An
increase in LH release in cows has been reported in summer compared to
winter (Hansen et al 1982), perhaps related to the effects of oestradiol, as
described for the sheep (Legan et al., 1977). This might explain how
photoperiod can affect reproductive behaviour, especially oestrous
expression.
2.4.2 Effect of Milk Yield on Increase in Activity
There was an association between increasing milk yield and declining
activity and oestrous expression. This trend follows the widely recognised
change in cattle reproductive physiology recorded over the past 50+ years,
coupled with rapidly increasing milk production since the 1950s (Lucy,
2001). The average increase in yield from 2010/11 to 2011/12 is 241 litres
per cow with the average yield at 7617 litres per cow per annum
(provisional 2012 data; DairyCo, 2012a). However poor conception rates
still persist (Royal et al., 2000a;Butler, 2003).
High milk yields have been reported to affect oestrous expression. Harrison
et al., (1990) reported that low yielders showed stronger oestrous
expression than high yielders. When comparing the duration of oestrus in
high and low yielders, low yielders had a longer duration of oestrus, 10.9
vs. 6.2 hours. Total standing events was also increased, 8.8 vs. 6.3, as was
total standing time, 28.2 vs. 21.7 seconds (Lopez et al., 2004). Studies of
activity also concur with the results of the current study where there was a
clear pattern between high milk production and lower activity at oestrus
(Yániz et al., 2006). Lopez-Gatius et al. (2005) also reported that for each
1kg increase in milk yield walking activity at oestrus decreased by 1.6%.
Possible explanation for the effects of milk yield on reduced activity at
oestrus is through the interlinking reproductive and somatotropic axes
which can be influenced by metabolite levels and influence hormone
production (Chagas et al., 2007). High yielding animals require a high
plane of nutrition, which increases the rate of metabolic clearance by the
liver, rapidly removing steroid hormones, oestradiol and progesterone from
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