Service Sire Influences on Fertility Rates of Repeat Breedings

Christian and Casida (1) have shown previously from a study of 1,274 second services by ten bulls that some first service bulls influence the conception rates of other bulls used on their repeat service cows.This lowering of fertility was greatest for repeat services when the first service was from semen of a low fertility bull.Salisbury et al. (8,9), using thousands of breedings, have demonstrated that variation among bulls is extensive with respect to differences in delayed returns to service, with age of semen and season of the year also showing measurable differences.Corley et al.(3) could demonstrate no real differences in fertility between intrabreed and interbreed inseminations.The possibility that bulls below average in fertility lower the fertility of their repeat service cows is of considerable economic importance.It is the purpose of this report to present further information on this subject. DATA AND DISCUSSIONData from 6,232 inseminations by one technician in one area (Whatcom County) from August 1949 through Octobe.r1951 were used for this study.Most of the breedings were made with semen less than 60 hours old, and 90 per cent of the inseminations were known to have been made into the uterus.Penicillin and streptomycin were added irregularly to the semen throughout the period covered.All repeat services were considered, since the inseminator field notes were available for each breeding.Nonreturn estimates were made at successive 30-day intervals in order to determine rates of nonreturn decline.Statistical procedures used were as outlined by Snedecor (11).The 39 bulls involved in this study were divided into three groups according to 180-day nonreturn rates.Nine bulls were in the low, ten in the high, and 20 in the medium fertility groups.Nonreturn rates were calculated for first services and for repeat services by the same bull and for all other bulls on each bull's repeats.A few instances of crossbreeding occurred, but these were too few to yield significant findings and were deemed unimportant in view of the findings of Christian et al. (2).As shown in Table 1, there were no important breed differences in fertility levels or average length of return service interval.The low fertility group (nonreturn rate of 60 per cent or less) consisted of three Guernsey, two Holstein and four Jersey bulls.The range in average fertility between fertility groups was 11 per cent for first service and 13 per cent for repeat services by other sires (Table 1).This difference