Data from: 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. Mean and
median AFC were 29.1 and 28 months respectively. Within the study, only
48,567 heifers (12.3% of the studied population) calved for the first time
at 24 months of age or younger. 162,157 heifers (40.9%) were 30 months or
older at their first calving. An increased AFC was associated with
increased first lactation milk, fat and protein yields. The lowest
predicted mean 305-day yield (6,617kgs; 95% confidence interval (CI):
6,546–6,687 kgs) was recorded for the 21 month AFC class, significantly
lower than any other class. The 36 month AFC class had the highest
predicted mean (7,774 kgs; 95% CI: 7,737–7,811 kgs). However, an increased
AFC was also associated with increased calving interval and increased
first lactation somatic cell count (SCC). Animals calving at 21 months had
a predicted mean lactation SCC of 72,765 (95% CI: 68427–77378). Animals
calving at 36 months of age had a predicted mean lactation SCC of 86,648
(95% CI: 84,499–88,853). Importantly, an increased AFC was also associated
with decreased lifetime daily milk yield and decreased likelihood of
calving for a second successive time. Animals calving at 22 months of age
had a predicted mean daily lifetime milk yield of 15.24 kgs (95% CI:
15.06–15.35); animals calving at 36 months of age had a predicted mean
daily lifetime milk yield of 12.83 kgs (95% CI: 12.76–12.91). Our results
highlight the importance of achieving a lower age at first calving which
was here associated with improved udder health, increased lifetime daily
milk yield, improved reproductive performance and increased likelihood of
calving for a second time.