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Abstract
Average conception rate (C.R. = 49-day non-return rate) from semen of 12 bulls in one region was 68.3% compared with 59.8%, for the same in another region. C.R. differences between the regions were less for sires of above-average fertility than for below-average sires. C.R. increased by 0.14% per day in one region and by 0.17% per day in another region from early September to mid December, accounting for over 40% of variation in C.R.s for individual collections. Sire differences in fertility accounted for only 9% of variation in one region and 17% in the other. Records for 51 208 inseminations using semen from 19 sires were classified into: cows diagnosed in oestrus before or during an evening milking on the day before insemination (p.m. cows); and cows diagnosed before or during the morning milking on the day of insemination (a.m. cows). Almost all cows were inseminated between 8.00 a.m. and 12 noon. C.R.s were highest when a.m. cows were inseminated with semen from sires of above-average fertility, but below-average sires gave higher C.R.s with p.m. than with a.m. cows. A split-ejaculate technique, with dose rates of 2.5 and 0.5 million total sperm per insemination, gave C.R.s for p.m. cows of 65.9% and 60.5% respectively, and for a.m. cows of 62.0% and 53.2%. These results support the concept that fertility differences between sires arise through differences in sperm survival after insemination. Laboratory tests designed to simulate post-insemination sperm survival may enable more accurate prediction of fertility.
