Age at first calving in Italian Holstein Friesian population: a preliminary study

Age at first calving (AFC) is the period from birth to first calving that a female calf needs to reach puberty and to reproduce for the first time. The AFC plays a relevant role in the cost of rearing replacements in dairy herds. For this reason, identifying the optimal AFC is an important management aspect of the herd. Therefore, the aim of the present preliminary study was to investigate environmental factors affecting AFC as a prelude for their inclusion in the genetic evaluation of AFC in Italian Holstein population. National data on AFC of 5,434,428 heifers born between 1993 and 2020, and calving from 1996 to 2020 were used. Animals with AFC less than 18 months or greater than 36 months were removed from the dataset. Moreover, only heifers with gestation length between 240 and 305 days were kept. To evaluate possible sources of variation of AFC, a forward stepwise approach based on Akaike Information Criterion was used and herd-year-season of first calving, year-season of first insemination and class of length of gestation were selected as fixed effects. The mean AFC decreased by 2.36 months from 1996 (28.45 ± 3.2) to 2020 (26.09 ± 3.07). The coefficient of determination of the model explained 49% of the variation of AFC. Genetic parameters estimation was carried out with the Gibbs sampler THRGIBBS1F90, which supports linear traits. The post-Gibbs analysis was carried out using the software POSTGIBBSF90. A posterior mean of 0.11 was found for heritability, with a posterior standard deviation of 0.006. Further investigations are needed in order to set up a genetic and genomic evaluation for this trait