Observations have been made on the reproductive performance of 530 Yorkshire and Landrace gilts mated with forty-four Yorkshire and Landrace boars. The four breed-mating classes (two breeds and two reciprocal crosses) differed significantly in the proportion of gilts which returned to service (P<0·05). Landrace boars mated to Yorkshire gilts yielded the highest return rate while the Y × L, Y × Y and L × L mating classes did not differ significantly from one another.
The same data have been examined by dividing the matings into nine mating classes based on the transferrin types of the boar and gilt. Differences in returns to service between the nine transferrin-mating classes were significant (P<0·01). The matings of bb boars to ab sows resulted in a return rate of 46·2% while the overall return rate was 25·3%. Differences among the remaining eight transferrin-mating classes were not significant.
The nine transferrin-mating classes did not differ significantly with respect to interval between mating and return to service, average gestation length, number born, number born alive or number born dead. The mean interval between mating and return to service was 25·5 days or 4·5 days longer than the normal oestrous cycle. The frequency distribution of days to return was distinctly bimodal, return being delayed in approximately half the gilts.
These data support the hypothesis that early embryonic mortality is a major cause of failure of gilts to conceive and that the high return rate in the bb × ab transferrin-mating class is due to higher embryonic mortality rather than failure of fertilization in this class.