IX. Pregnancy rates to first insemination: Mating with a herd sire compared with artificial insemination

Abstract

Two field studies with a total of 959 cows showed that the computed conception rate (C.R. = 49-day non-return rate to first insemination) of 64.0% was 2.9% higher than the pregnancy rate determined by rectal palpation. Cows returning to service more than 49 days after their first insemination was the main reason for this discrepancy. In a small sample of herds in which sires fitted with a chin-ball harness ran freely with cows at pasture, C.R. for first and second mating was 63.0% and 71.1% respectively. A total of 4.7% of first matings by a herd sire was followed by a second mating 6–10 days later. The fertility of artificially inseminated dairy cows in New Zealand is high. It was estimated that in the Wellington-Hawke's Bay Livestock Improvement Association, 8% of the cows submitted for a first insemination will have been incorrecty diagnosed in oestrus. A further 2% will have a genuine short oestrous cycle and be re-inseminated 8–10 days later, and 3.5% will not be re-inseminated until approximately 6 weeks after their first insemination.