Associations between age at first calving and subsequent performance in Irish spring calving Holstein-Friesian dairy cows

Little is known about the association between age at first calving and subsequent performance in seasonal calving dairy production systems where grazed grass constitutes the majority of the basal diet. The objective of this study was to quantify, using the Irish national database, the association between age at first calving and subsequent milk production, calving performance, fertility and survival in grazing, seasonal-calving Holstein-Friesian dairy cows. Median age at first calving from a population of 196,120 animals varied from 751 to 764 days across the years 2000 to 2006, inclusive while age at first calving ranged from 660 days to 1156 days. A frequency distribution of age at first calving revealed a large peak coinciding with approximately 24 and a smaller peak at approximately 36 months of age. Associations with age at first calving were undertaken on between 6338 and 50,786 animals. First lactation 305-day yield of milk. fat and protein decreased almost linearly by 55.5 kg. 0.6 kg and 2.3 kg, respectively for each month earlier at first calving. Although heifers calving younger for the first time had a greater likelihood of requiring some assistance at calving there was no association between age at first calving and risk of severe assistance at first calving. Age at first calving was associated with survival to second, third, fourth and fifth parity with no association with survival to sixth parity; heifers calving at 24 months of age had generally the greatest odds of survival to older parities with heifers calving at approximately 3 years of age having lower odds. (C) 2008 Elsevier B.V. All rights reserved.