COMPARISON AMONG SIRE GROUPS IN CONCEPTION RATE, GESTATION LENGTH AND CALVING EASE OF THEIR MATES

Data on conception rate, gestation length, incidence of calving difficulty and retained placenta were obtained from 269 heifers and 525 cows as mates of the pureline foundation phase sires of the National Cooperative Dairy Cattle Breeding Project. The data were analyzed by least squares separately for heifers and cows using a model containing fixed effects for station, year of calving, season of calving, sex of calf, line, sire groups within line, sires within sire group and line, birth weight of the calf as a covariate and residual. Effect of age at first calving was used as a covariate in the heifer data while the effect of parity was included in the cow data. The effect of station, year of calving, season of calving, line, sire groups within line and sires within sire group and line were significant (P < 0.05) for gestation length of heifers and cows. Other significant (P < 0.05) effects in heifers were age at calving on conception rate, sex and birth weight of the calf on calving difficulty. In addition the effects of sex and birth weight of the calf on gestation length, station and parity on calving difficulty and station and line on retained placenta were significant in cows. Key words: Reproductive performance, dairy cattle