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Accuracy of predicted calving dates in Holstein-Friesian dairy cows based on fetal ages estimated using manual rectal
1School of Veterinary Science, The University of Queensland, St Lucia, Queensland, Australia. beren@countryvet.com.au
Aim:
To assess potential risk factors for differences between predicted and actual calving dates in Holstein-Friesian cows in seasonal calving dairy herds where predicted dates are based on fetal ages estimated using manual rectal palpation.
Methods:
Accuracy of predicted calving dates was assessed in 10,487 Holstein-Friesian dairy cows that were diagnosed as pregnant and that subsequently calved in 108 seasonal calving dairy herds in Victoria and Tasmania, Australia. Of these cows, 9,214 (88%) were between approximately 7 and 18 weeks of gestation when diagnosed as pregnant. Potential risk factors for differences >10 days between predicted and actual calving dates were analysed using a multinomial logistic regression model.
Results:
For all 10,487 cows, 8,444 (80.5%) calved within 10 days of the predicted calving date, 1,329 (12.7%) calved >10 days earlier than predicted and 714 (6.8%) calved >10 days later than predicted. Estimation of fetal age was less accurate where cows had received no compared with one artificial insemination (AI), when cows were <8 weeks of gestation or >14 weeks of gestation, and when cows were examined ≥15 compared with <15 weeks after their most recent AI. The risk of calving >10 days earlier than predicted was reduced in cows with an interval of ≥3 weeks, compared with <1 week, between their most recent AI and the end of the herd's AI period.
Conclusions:
Important proportions of Holstein-Friesian dairy cows will calve >10 days from predicted dates based on manual rectal palpation assisted by AI dates. Risk factors for such differences include number of prior AI, fetal age at pregnancy diagnosis, and times from the cow's most recent AI to both pregnancy diagnosis and end of the herd's AI period.
Clinical Relevance:
Herd managers should be made aware of likely proportions of Holstein-Friesian dairy cows that will calve >10 days from predicted dates. If accurate predictions of calving dates are required, manual rectal pregnancy diagnosis in seasonal calving dairy herds should be scheduled to ensure that most cows are examined within 15 weeks after their most recent AI. Accuracy of estimates is reduced for cows with no AI, and for cows most recently inseminated close to the end of the herd's AI period.
