novel approaches to expression and detection of oestrus in dairy cows

loss of body condition, due to energy balance regulation, which has been
related to poor oestrous expression (Mayne et al., 2002). NEBAL and loss
of body condition has been related to attenuation of LH pulse frequency
and low levels of blood glucose, insulin and IGF-1 which together impair
the production of oestradiol by the dominant follicle (Butler, 2003).
However, Macmillan et al., (1996) also found that the effect of milk yield
and stage of lactation was not an absolute indicator of NEBAL. Lower
yielding cows could also have lower feed intake and thus have a more
severe energy deficit which could affect oestrus. However NEBAL is highly
correlated with genetic improvement for milk yield (Veerkamp and Beerda,
2007).
A further possibility is that as cows get older their activity decreases due to
age (Lovendahl and Chagunda, 2009). It is also possible that cows are
culled for infertility and only fertile cows survive to the next lactation; so a
larger proportion of the cows in later lactations display oestrous behaviour
and oestrus is more easily detected.
2.4.1.2 Oestrous Number
As oestrous cycles progressed without conception the activity at each
oestrus decreased post partum. Typically the first oestrus post partum is
silent (Ferguson, 1996) and it is possible that the first oestrus was
removed from analysis in this study. All oestruses before 25 days post
partum were removed, due to increased activity levels because of re-entry
of cows into the herd after solitary calving, due to increased activity
associated with the establishment of hierarchies. The current results
demonstrate that activity is greater at the 2 nd oestrus, although there was
no significant difference between activity at 1 st and 2 nd oestrus. Therefore
activity at oestrus decreased from the 3 rd oestrus onwards. Previous
reports have suggested a similar pattern, that oestrous expression
increases up to the third oestrous cycle post partum (Ferguson,
1996;Thatcher and Wilcox, 1973).
Peralta et al., (2005) reported a significant increase in number of standing
events in cows less than 79 days in milk, compared to those more than 80
days in milk, where there was also a larger proportion of problem cows
(Peralta et al., 2005). This is in agreement with results of the current study
and provides an explanation for increased activity in the first oestrous
cycles after calving. Another possible explanation for the decrease in
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