Associations between age at first calving and subsequent lactation performance in UK Holstein and Holstein-Friesian dairy cows.

Lactation records from 396,534 pedigree Holstein and Holstein-Friesian primiparous cows from 6,985 UK milk recorded herds, calving for the first time during the period between the 1st of January 2006 and the 31st of December 2008, were examined in order to determine the associations between age at first calving (AFC) and subsequent production, udder health, fertility and survivability parameters. Heifers were grouped by AFC into single month classes ranging from 21 to 42 months. Mixed effects multivariable regression modelling was used for data analysis.