novel approaches to expression and detection of oestrus in dairy cows

Table 2.1 Definitions of cow factors analysed for their effects on the activity
increase at oestrus
Variable Definition
Days in Milk Number of days post partum until each individual oestrus event
(DIM)
were investigated for the association between DIM and activity at
oestrus.
Milk Yield at The average milk yield was calculated over each oestrus period
Oestrus
to investigate the effect of level of production at the time of
oestrus on the activity increase. Milk yield at oestrus was
calculated as the average of 5 days before, the day of oestrus
and 5 days post oestrus. This accounted for daily variation and
decrease in yield that is reported at oestrus.
Average Daily The total milk yield to each oestrus event was divided by the
Milk Yield number of DIM to calculate a representative daily average. This
variable was used to assess whether the overall level of daily
production within each lactation had an effect on oestrous
expression.
Cumulative The total milk yield produced per lactation up to each individual
Lactation Yield oestrus event was used to assess the effect of overall level of
production on activity.
Parity
Cows were grouped into parity 1, 2 and 3 corresponding to cows
in their 1 st parity, 2 nd parity and cows in their 3 rd parity and
above. These were analysed for the effect of age and lactation
number on oestrous expression.
Oestrous Successive oestruses per lactation were analysed for the effect of
Number
multiple oestrous cycles on the expression of oestrus. The whole
data set was analysed with oestrus events per lactation included
in statistical analysis as variates, n=930. However this analysis
may include poor cows or cows that were later culled for
infertility. The average number of oestrous cycles per lactation in
the total data set was 3.12, therefore cows that conceived at
their 3 rd oestrous cycle were analysed separately for the effect of
oestrous number on expression. Further investigation then
looked at the difference between the 1 st and 2 nd oestrus event.
Time of Year This variable takes into account the time of year in which each
oestrus event occurred; 1 - Jan-Mar, 2 – Apr-June, 3 – July-Sept
and 4 – Oct-Dec. This was to account for environmental variables
such as day length which could physiologically affect the
expression of oestrus.
Successful Activity increase at oestrus was analysed for its effect on whether
Conception successful conception was related to oestrous expression.
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