Changes on female fertility aggregate index in Italian Holstein dairy cattle

The aggregate index for female fertility was introduced into the national Holstein cattle breeding objectives in 2005, and included five selection criteria, namely days from calving to first service (DTFS), non-return rate at 56 d from first insemination (NR56), calving interval (CI), angularity (ANG), and equivalent-mature milk yield (MY). Conception rate at first service (CR) was the only breeding goal. The objective of the present study was to revise this aggregate index by including new information from both linear scoring (body condition score, BCS), services and pregnancy testing (interval from first to last insemination, IFL). After edits, information was available on 5,466,546 primiparae calving since 1994; to reduce computational time, six subsets of approximately 15,000 cows were extracted. (Co)variance components of DTFS, NR56, IFL, MY, BCS, and CR were estimated simultaneously with a multi-trait animal model, which included the fixed effects of herd-year-season of calving (DTFS, NR56, IFL, MY, and CR), age-year of calving (DTFS, NR56, IFL, CR), month of calving (DTFS, IFL, CR), month of insemination (NR56), herd-year-season of linear scoring (BCS), age-stage of lactation at linear scoring (BCS), and year of calving (BCS). Random terms were animal additive genetic and the residual. At a national level, means of DTFS, NR56, IFL, BCS, MY, and CR were 88 d, 59%, 71 d, 2.98, 10,483 kg, and 35%, respectively. (Co)variances estimated in each subset were averaged to derive G and R matrixes. Heritability estimates of fertility traits ranged from 0.013 (NR56) to 0.076 (DTFS); heritability estimates of BCS and MY were 0.18 and 0.29, respectively. Selection index methodology was employed to derive appropriate index weights from (co)variances between breeding goal (CR) and (among) selection criteria. Results from the present study indicated that the genetic response of the breeding goal using information on BCS and IFL, substituting ANG and CI, respectively, increases by 5.6% after one selection round.