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.

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The objective of this study was to identify detailed fertility traits in dairy and beef cattle from transrectal ultrasonography records and quantify the associated risk factors. Data were available on 148 947 ultrasound observations of the reproductive tract from 75 949 cows in 843 Irish dairy and beef herds between March 2008 and October 2012. Traits generated included (1) cycling at time of examination, (2) cystic structures, (3) early ovulation, (4) embryo death and (5) uterine score; the latter was measured on a scale of 1 (good) to 4 (poor) characterising the tone of the uterine wall and fluid present in the uterus. After editing, 72 773 records from 44 415 dairy and beef cows in 643 herds remained. Factors associated with the logit of the probability of a positive outcome for each of the binary fertility traits were determined using generalised estimating equations; linear mixed model analysis was used for the analysis of uterine score. The prevalence of cycling, cystic structures, early ovulation and embryo death was 84.75%, 3.87%, 7.47% and 3.84%, respectively. The occurrence of the uterine heath score of 1, 2, 3 and 4 was 70.63%, 19.75%, 8.36% and 1.26%, respectively. Cows in beef herds had a 0.51 odds (95% CI = 0.41 to 0.63, P < 0.001) of cycling at the time of examination compared with cows in dairy herds; stage of lactation at the time of examination was the same in both herd types. Furthermore, cows in dairy herds had an inferior uterine score (indicating poorer tone and a greater quantity of uterine fluid present) compared with cows in beef herds. The likelihood of cycling at the time of examination increased with parity and stage of lactation, but was reduced in cows that had experienced dystocia in the previous calving. The presence of cystic structures on the ovaries increased with parity and stage of lactation. The likelihood of embryo/foetal death increased with parity and stage of lactation. Dystocia was not associated with the presence of cystic structures or embryo death. Uterine score improved with parity and stage of lactation, while cows that experienced dystocia in the previous calving had an inferior uterine score. Heterosis was the only factor associated with increased likelihood of early ovulation. The fertility traits identified, and the associated risk factors, provide useful information on the reproductive status of dairy and beef cows.