Risk factors associated with multiple ovulation and twin birth rate in Irish dairy and beef cattle.

The objective of this study was to quantify the phenotypic and genetic risk factors associated with multiple ovulations and twin births in cattle. Prevalence and cow- and herd-level risk factors associated with ovulation rate were determined using 40,617 ultrasonographic records of the reproductive tract from 27,907 dairy and beef cows from 738 commercial herds. Prevalence of twin births was estimated from the Irish national database containing 23,658,351 calving events from 8,546,695 cows from 125,251 dairy and beef herds; factors associated with twin births were determined using a random subsample of 505,200 calving events from 280,638 cows in 81,329 herds. The mean prevalence of multiple ovulations was 6.83% while the prevalence of twin births was 1.74%. Occurrence of both multiple ovulations and twin births was associated with the month of scan (P < 0.0001) and month of calving (P < 0.0001), respectively, and peaked in November for multiple ovulations and October for twin births. The likelihood of multiple ovulations increased with interval postpartum and peaked between 45 to 185 d postcalving, after which the likelihood declined. The likelihood of both multiple ovulations (P < 0.0001) and twin births (P < 0.0001) increased with increasing cow parity. A greater proportion of Holstein, Friesian, Simmental, Hereford, and Charolais breed fractions were associated with a greater likelihood of multiple ovulations. There was no difference between breed proportion of the cow and incidence of twin births, where all breed proportions examined, litter difference existed among breeds in their association with risk of twin births. Although multiple ovulations were lowly heritable (0.028 ± 0.003), their occurrence was repeatable (0.326 ± 0.342) while twin birth rate in cattle was lowly heritable (0.017 ± 0.004) and repeatable (0.018 ± 0.011). The genetic SD of the presence of multiple ovulations and twin births was 0.04 and 0.02, respectively, indicating considerable genetic variation, especially for multiple ovulations. A moderate genetic correlation (0.66 ± 0.16) existed between multiple ovulations and twin births.