Revision of the aggregate fertility index for Italian Holstein-Friesian dairy cattle

The aggregate fertility index was added to the national Holstein breeding objective in 2009, and is a linear combination of five selection criteria: angularity, milk yield (MY), interval calving-first service (DTFS), 56 days non-return rate (NR56), and calving interval. The objective of the present study was to include new traits, measured in both productive cows and heifers, to such aggregate index, by exploiting information from linear scoring and fertility-related events. Six subsets of ~15,000 animals calving since 1994 were extracted from the national database. Traits identified were: age at first insemination in heifers (AFI), interval first-last insemination (IFL), NR56, and conception rate (CR) in heifers and cows, and DTFS, MY, and body condition score (BCS) in cows. A multiple-trait animal model was employed to estimate (co)variances of these phenotypes, separately for cows and heifers. Fixed effects per trait were: herd-year of birth (AFI), month of birth (AFI), herd-year-season of birth (IFL, NR56, CR in heifers), month-year of insemination (IFL, NR56, CR in heif-ers), herd-year-season of calving (MY, DTFS, IFL, NR56, CR in cows), month of calving (DTFS, IFL, CR in cows) or insemination (NR56 in cows), age-year of calving (DTFS, IFL, NR56, CR in cows), herd-year-season of classification (BCS), age-stage of lactation at linear scoring (BCS), and year of calving (BCS). Animal additive genetic and residual were treated as random terms. The pedigree was traced back 5 generations. Estimates of G and R matrixes of all the six subsets were averaged to apply selection index methodology for deriving appropriate index weights, by fixing CR in cows and heifers as the only breeding goals. Heritability of cow fertility traits ranged from 0.013 (NR56) to 0.076 (DTFS), whereby estimates of heifer traits were between 0.012 (CR) and 0.071 (AFI). Relative emphasis placed in the selection criteria of the aggregate index for cows were: 0.55 (IFL), 0.16 (DTFS), 0.13 (NR56), 0.09 (MY), and 0.07 (BCS). Index weights for heifer traits were 0.47 (IFL), 0.43 (NR56), and 0.10 (AFI). Both indexes were scaled to have mean 100 and standard deviation 5. The aggregate fertility index published in the official genetic evaluation release is a combined index which includes both indexes by placing 90% emphasis on cow aggregate index and 10% emphasis on heifer aggregate index.