Date of this Version
Data were obtained from 23,978 calving records collected by the Eastern Artificial Insemination Cooperative's Calving Survey. The survey information included breed, parity and size of dam, date of breeding and birth, identification of sire and maternal grandsire, sex, size, and vigor of calf, and difficulty of birth. Due to small numbers of records for non-Holstein breeds, only Holstein records were used in the analyses. As parity increased, the incidence of twinning increased (P<.05). Month of birth (conception) and the size of dam measured within parity did not affect the twinning rate. The heritability of twinning, estimated using the genetic covariance between paternal half-sibs was .05 over all parities and within parity was .06, .03, and .03 for the first, second and third parities, respectively. Length of gestation for twins was 5 days less (P<.05) than for singles. In addition, twins were smaller and weaker, and their birth was accompanied by more dystocia than monoparous calves (P<.05). Neonatal mortality was four times higher (P<.05) for twins as compared to single births. In view of these facts, it does not appear that selection programs designed to increase twinning rates would be desirable for Holstein dairy cattle.