Effect of time of artificial insemination after oestrus detection on non-return rate of lactating dairy cows on pasture.
OBJECTIVE
To determine the difference in reproductive performance between cows detected in oestrus during morning or afternoon milking when artificial insemination (AI) was carried out once daily, either after the morning or afternoon milking.


METHODS
The study used 20,816 records for cows in 30 spring calving dairy herds that used a camera-based system for oestrus detection during both morning and afternoon milkings. This system automatically determined whether a pressure-sensitive oestrus detection device had been activated or not, or was missing. In all study herds cows were milked twice daily and AI was performed by professional AI technicians once daily, after either morning or afternoon milking. The time of recorded oestrus was classified into AM or PM oestrus groups using 12:00 noon as the cut-off point and time of AI was classified into AM or PM AI groups using 13:00 as the cut-off point. Reproductive performance was determined using non-return to oestrus (NRR) between 2 and 24 days after AI, and was compared between oestrus and AI groups using generalised linear mixed models.


RESULTS
Most AI (18,621/20,816; 89%) were carried out after morning milking, and more cows were recorded in oestrus during morning milking (12,328/20,816; 59%) than during afternoon milking (8,488/20,816; 41%). The NRR of cows inseminated after morning milking did not differ (OR=1.04; 95% CI=0.97-1.10) for cows recorded in oestrus in the morning (LSM=68.6 (95% CI=66.3-70.9)%) or afternoon (LSM=67.9 (95% CI=65.4-70.2)%); and did not differ for cows inseminated after the afternoon milking (OR=1.10; 95% CI=0.92-1.32) for cows recorded in oestrus in the morning (LSM=63.4 (95% CI=58.2-68.2)%) or afternoon (LSM=61.2 (95% CI=55.8-66.3)%). There were no differences in NRR between cows detected in oestrus during the morning or the afternoon milking after AI with either liquid (OR=1.04; 95% CI=0.97-1.12) or frozen-thawed (OR=1.02; 95% CI=0.88-1.18) semen.


CONCLUSIONS
Results from the present study show that, under New Zealand conditions, the difference in NRR between cows first detected in oestrus during morning milking and those first detected in oestrus during afternoon milking is likely small when AI is carried out once daily after either morning or afternoon milking, regardless of whether the semen used for AI is liquid or frozen. Because twice daily AI is more costly than once daily AI, these results do not support a change to the current once daily AI practice in New Zealand.
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D000822:Animal Husbandry; D000818:Animals; D002417:Cattle / Q000502:physiology*; D004971:Estrus / Q000502:physiology*; D005260:Female; D007315:Insemination, Artificial / Q000662:veterinary*; D007774:Lactation; D011247:Pregnancy; D013997:Time Factors
Artificial insemination; dairy cow; non-return rate; oestrus detection