Genetic aspects of heifer fertility in Italian Holstein population

Fertility is fundamental to enhance the production efficiency of the dairy herd and thus it is a contributor to annual farm profitability. Cow fertility has been included in Italian Holstein breeding objectives since 2009. Heifer fertility is another key trait that deserves attention as it has direct connection with overall efficiency. In general, the main goal is to improve conception and daughter pregnancy rates, favour shorter calving interval in lactating cows, and reduce failure of conceiving in heifers. The advantages of heifer over cow fertility traits are the early availability in life and, overall, the moderate to strong genetic correlations with fertility of lactating cows. The aims of the present study are to assess genetic parameters of Italian Holstein heifers and develop an aggregate selection index to improve heifer fertility. Data (ANAFIBJ, Cremona, Italy) included information on insemination, calving, and pregnancy diagnosis dates of Italian Holstein heifers. The investigated traits (mean ± standard deviation) were age at first insemination (AFI, mo; 17.25 ± 2.89), nonreturn rate at 56 d from the first insemination (NRR56, binary; 0.78 ± 0.41), conception rate at first insemination (CR, binary; 0.61 ± 0.49), and interval from first to last insemination (IFL, d; 26.09 ± 51.85). Genetic parameters were estimated using a 4-trait animal model that included the fixed effects of herd-year of birth and month of birth for AFI, and herd-year-season of birth and month-year of insemination for IFL, NRR56, and CR. The animal additive genetic effect was included as random term. An aggregate index was developed from the estimated additive genetic (co)variance matrix by considering CR as the breeding goal and AFI, NRR56, and IFL as selection criteria. Heritability ranged from 0.012 (CR) to 0.015 (IFL), except for AFI (0.071). Conception rate at first insemination was strongly correlated with both IFL (−0.730) and NRR56 (0.668), and weakly to AFI (−0.065). The relative emphasis placed on each selection criteria in the aggregate index was 10%, 47%, and 43% for AFI, IFL, and NRR56, respectively. Results of the present study suggest that heifer fertility should be considered as an additional trait in the breeding objectives of Italian Holstein.