Return intervals to first insemination and conception rates to second insemination in New Zealand Dairy Cattle

Abstract

Analyses of the return intervals to first insemination for cows serviced by Auckland and Taranaki Herd Improvement Associations in 1968 showed that 18.0 and 16.3 percent respectively were of 17 days duration or less. More than 25 percent of these short returns were recorded on the 8th and 9th day after first insemination. Conception rates to second insemination were lower than the respective average for the Association if the interval from first to second insemination was from 2 to 7, 10 to 17, 25 to 28, or 32 to 37 days. The increased incidence of 8- and 9-day returns to first insemination was associated with normal conception rates when these cows were re-inseminated. A similar "midoestrus" peak in conception rates was noted at the 29- to 31-day interval. Many of the cows apparently returning after 8 or 9 days may be exhibiting oestrous symptoms because of previously reported increases in urinary oestrogen levels and blood levels of luteinising hormone at this stage of a normal oestrous cycle.

The lower conception rates to second insemination are considered to be partly due to the re-insemination of a cow not in oestrus after a first insemination at the correct time.