Factors affecting heifer fertility in U.S. Holsteins M. T. Kuhn* and J. L. Hutchison Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD Abstract M INTRODUCTION ◆ AIPL began receiving data on individual breedings, including heifer breedings, in  Oct ◆ Literature on U.S. heifer fertility appears to be sparse ◆ Information on factors affecting heifer fertility, as well as genetic and phenotypic parameters, could be useful for: Heifer management (e.g., age at breeding) Mating programs (e.g., inbreeding effects) Feasibility of combining heifer and cow breedings OBJECTIVES ◆ Assess impact of various factors on heifer conception rate (CR) ◆ Estimate heritability (h 2 ) of heifer fertility and correlation with first lactation cow fertility and milk yield MATERIALS & METHODS Factors Affecting Heifer CR ◆ Fixed, linear model: y= 1 (conception), 0 (no conception) ◆ Holstein heifers with confirmed outcomes only ◆ 383,972 AI breedings on 258,890 heifers ◆ Factors (all fit as categorical variables): Herd AI month AI year AI age Service sire (SSR) age at breeding and birth year Inbreeding: heifer, SSR, mating (embryo) SSR Inbreeding x SSR age Stud Parent average breeding value for DPR Parameter Estimation ◆ Linear animal and threshold sire models to estimate h 2 ◆ Linear model to estimate correlation of number of services to conception (NS) for heifers and cows ◆ In addition to the genetic term, models included only Herd-Yr and Yr-State-Month of breeding RESULTS Factors Affecting CR Importance of factors (based on mean square) 1) AI yr 2) Age 3) Parent avg. DPR 4) AI mo 5) Herd 6) Stud 7) SSR Inbreeding x SSR age 8) SSR birth yr 9) Inbreeding: Mating, SSR, Heifer 10) SSR age Overall Mean CR:58.2% Year effects: Only 3 years included; no general trend Age (of heifer) at Breeding ◆ Mean breeding age = 15.7 mo ◆ Breeding heifers at less than 8 mo of age lowers CR 20% ◆ Best CR when heifers are bred between 14 and 16 mo ◆ CR was also lower for heifers bred at ages > 16 mo, e.g., 7% lower for heifers bred at 26 mo of age Similar result was obtained using first service only Parent Average DPR ◆ Difference of 3.2% in CR from lowest DPR group to highest ◆ Linear regression indicated.5% increase in heifer CR with every 1% increase in DPR Month of Breeding ◆ Maximum difference in CR among months = 3.3% ◆ Best CR in July and August ◆ Lowest CRs were in April and May ◆ Better CR in warmer months is in contrast to cow fertility. Perhaps more heat expression or better heat detection for heifers in summer months Stud ◆ Although 9% difference in heifer CR amongst all NAAB stud codes included, there was only a 2.6% maximum difference among the 5 major US studs Factors Affecting CR SSR Inbreeding and Interaction with SSR age Main Effect of SSR Inbreeding ◆ Main effect indicated consistently lower CR with higher inbreeding except at the highest level of inbreeding ◆ However, for breedings at 1.5 yr of age for SSRs, the mean CR (%) for the 4 levels of SSR inbreeding were 56.9, 50.5, 49.4, and 47.1 ◆ Perhaps the low CR for the higher inbred bulls is noticed early on by the AI organization and is compensated for with higher sperm concentrations Inbreeding of the Heifer and of the Mating (embryo) ◆ CR steadily decreased, with increased inbreeding, for both ◆ Most highly inbred heifers had 5.1% lower CR than least inbred heifers ◆ 2.7% lower CR for highest inbred matings vs lowest Parameter Estimates ◆ h 2 of heifer CR =.8%, for both the linear and threshold models ◆ Genetic correlation between NS for heifers and NS for cows = 92% ◆ Genetic correlation of NS in heifers with 1st lactation milk yield = 10% CONCLUSIONS ◆ Average CR for U.S. Holstein heifers is about 58% ◆ Herd, year, month, and age at breeding are all factors affecting heifer CR ◆ Breeding at 14 to 16 mo appears to maximize CR ◆ Heifer CR decreases with inbreeding in the animal itself, in the embryo and with inbreeding in the SSR, although the reduction in SSR fertility caused by inbreeding can apparently be compensated for with higher sperm dosages ◆ Heritability of heifer CR is quite low and is not genetically identical to cow fertility. Nonetheless, selection for DPR will improve CR in heifers as well. Table caption GroupInbreeding %Heifer CR 1  to to to