novel approaches to expression and detection of oestrus in dairy cows
novel approaches to expression and detection of oestrus in dairy cows novel approaches to expression and detection of oestrus in dairy cows
2.3.1 Effect of Days in Milk, Parity, Oestrous Number and Time of Year of Oestrus on Increase in Activity The effect of parity, oestrous number and time of year on activity at oestrus are reported in Table 2.2. Maximum activity at oestrus was significantly associated with parity, oestrous number and time of year. DIM was not significantly associated with the activity increase at oestrus. As parity increased the activity increase at oestrus became smaller. First parity cows had an average activity count 10 units higher at oestrus than those in parity 3 and above. Percentage increase in activity at oestrus was also significantly related to parity, P=0.044, where 2 nd parity cows had larger increases in activity from baseline compared to 1 st parity and 3 rd parity or greater cows. When analysing all cows with multiple oestrous cycles per lactation it was reported that oestrous number was significantly related to the activity increase at oestrus, P=0.009. Activity increases at oestrus were smaller further through the lactation as the number of oestrus events without conception occurred. However, this may be inclusive of poor cows that were later culled for infertility. The average number of oestrous cycles per lactation from the complete data set was 3.12. When analysing cows with 3 consecutive oestrous cycles to conception, n=151, activity increase at oestrus was significantly associated with oestrous number, P=0.01. The activity increase at oestrus was greater for the 2 nd oestrus post partum compared to the 1 st and 3 rd . Further investigation into the 1 st and 2 nd oestrus post partum, n=215, revealed no significant difference in the activity increase. Time of year of oestrus was significantly associated with activity at oestrus, P=0.004. Activity increases were greater in periods 2 and 3, coinciding with longer day length, compared to periods 1 and 4. 2.3.2 Effect of Milk Yield on Increase in Activity Three different measurements of milk yield are reported to investigate their relationship with activity at oestrus (Table 2.1). The significance of these parameters are reported in Table 2.3. They all showed the same pattern that as yield increased activity at oestrus decreased. Milk yield at oestrus (P=0.002) and average daily yield to oestrus (P=0.002) significantly affect the increase in activity at oestrus. Cumulative lactation yield to each 56
individual oestrus was not significantly related to activity at oestrus although followed the same inverse relationship of increasing yield and decreasing oestrous expression. 2.3.3 Effect of the Interaction between Milk Yield and Parity on Activity at Oestrus The interaction between milk yield (at oestrus, daily average to oestrus and cumulative lactation yield) and parity was analysed for the combined effect on the increase in activity at oestrus. Results are reported in Table 2.4. When correcting for the effect of milk yield at oestrus on activity, and then including parity in the model, the result was significant (P
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<strong>in</strong>dividual <strong>oestrus</strong> was not significantly related <strong>to</strong> activity at <strong>oestrus</strong><br />
although followed the same <strong>in</strong>verse relationship <strong>of</strong> <strong>in</strong>creas<strong>in</strong>g yield <strong>and</strong><br />
decreas<strong>in</strong>g oestrous <strong>expression</strong>.<br />
2.3.3 Effect <strong>of</strong> the Interaction between Milk Yield <strong>and</strong> Parity on Activity at<br />
Oestrus<br />
The <strong>in</strong>teraction between milk yield (at <strong>oestrus</strong>, daily average <strong>to</strong> <strong>oestrus</strong> <strong>and</strong><br />
cumulative lactation yield) <strong>and</strong> parity was analysed for the comb<strong>in</strong>ed effect<br />
on the <strong>in</strong>crease <strong>in</strong> activity at <strong>oestrus</strong>. Results are reported <strong>in</strong> Table 2.4.<br />
When correct<strong>in</strong>g for the effect <strong>of</strong> milk yield at <strong>oestrus</strong> on activity, <strong>and</strong> then<br />
<strong>in</strong>clud<strong>in</strong>g parity <strong>in</strong> the model, the result was significant (P
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