Validation of non-return rate as a parameter for stallion fertility

The non-return rate is evaluated as a parameter for assessment of reproduct ive performance of stallions. This parameter gives the percentage of mares that did not return (NR) for breeding within a specific time after last ins emination. Breeding data were recorded in a computerised registration syste m, from six studfarms on 18 Dutch Warmblood, seven Friesian and 11 Shetland pony stallions used for artificial insemination or natural mating. Mares ( n = 3731) were inseminated or served in a total of 6361 cycles. Non-return rates at 21, 28, 35, 42 and 56 days and at the end of the season were, resp ectively, 84, 72, 68, 64, 59 and 54%. The average NR rate after first cycle at 28 days was 73, 67 and 72% for the Dutch Warmblood, the Friesian, and t he Shetland pony stallions, respectively, and gradually decreased towards 5 0% for all the breeds by the end of the season. The NR rate differed remark ably between stallions. The ranking of the stallions according to NR at 28 days did not change when the length of this NR period increased. A live bor n foal was registered for 67% of the mares. Correlation between non-return rates at 21, 28, 35, 42 and 56 days and at the end of the season were, resp ectively, 0.77, 0.86, 0.89, 0.92, 0.92 and 0.95. The probability of a Live foal increased from 50% at NR 21 days to 84% at NR for the end of season ir respective of stallion. The non-return rate at 28 days proved to be a valua ble parameter for early assessment of fertility of stallions. (C) 1999 Else vier Science B.V. All rights reserved.