Factors associated with first service conception in artificially inseminated nulliparous Holstein heifers.

Animal and management factors associated with first service conception in nulliparous dairy heifers were determined in 601 Holstein heifers from a dairy farm in north central Florida. Animal data collected included body weight, height at the withers and tail head, body condition score at 6 months of age and just prior to first artificial insemination (AI), and pelvimetry measurements taken just prior to first AI. Management data included season of first AI, inseminator, service sire, method of estrus detection, whether the estrus of first insemination was induced using prostaglandin F(2alpha) (PGF(2alpha)), and whether the heifer received a modified live virus (MLV) vaccine within 21 days of first insemination. Data were analyzed using multivariable logistic regression. Heifers inseminated in the summer were more than four times less likely to become pregnant to first insemination than heifers bred during the rest of the year (odds ratio (OR)=0.24; 95% CI=0.14, 0.41). Using secondary signs for estrus detection instead of standing estrus resulted in significantly reduced odds of conception to first service (OR=0.37; 95% CI 0.13, 1.02). Also, heifers inseminated at estrus induced by PGF(2alpha) were approximately one-third less likely to conceive than those heifers inseminated to a naturally occurring estrus (OR=0.66; 95% CI 0.46, 0.95). An interaction between pelvic size and breeding season was found indicating that large pelvic size had a significant positive effect on fertility in the summer, but was not associated with conception to first service in the winter.