Effects of calving age, breed fraction and month of calving on calving interval and survival across parities in Irish spring-calving dairy cows

The objective of this study was to investigate the effect of parity, age at calving, percentage North American Holstein-Friesian and calving date on subsequent calving interval and survival to facilitate the estimation of transition probabilities for month of calving. The economic value of traits that influence calving date, age distribution and survival can be assessed in models using a transition probability matrix. Such a matrix contains the probabilities that a cow of a particular age or breed calving in a particular month will calve in the same, an earlier or later month next year, or be culled. Following editing 1,046,855 calving records in spring-calving herds between the years 1990 and 2004 were analysed. Shorter calving intervals were associated with cows calving later in the calendar year. Age at first calving of < 24 months resulted in longer calving intervals to second calving across all levels of Holstein percentage with cows calving for the first time at 25–26 months of age having the shortest subsequent calving interval. Age at second calving of 37–38 months and third calving of 49–50 months were optimum for shorter subsequent calving intervals. Calving interval increased with Holstein percentage across the first 5 parities. Survival rate decreased with later month of calving and with older parities. When survival rate was measured as the ability of the cow to re-calve within 500 days, the highest survival rate was found in cows calving at 25–26 months of age whereas there was a noticeable reduction in survival across all parities in the 88–100% Holstein percent category.