Service bulls have essentially no influence on the incidence of twinning in cattle. Most twins are the result of a multiple ovulation (fraternal twins). A much smaller number of twins are the result of a fertilized egg cleaving into two embryos (identical twins). Multiple ovulation is a lowly heritable trait and is expressed as a trait of the dam. Incidence of identical twins is not heritable. Because it is heritable, the sires of the cows do contribute to the incidence of twinning. Some bulls' daughters do have a higher percentage of multiple births. A research project at the U.S. Meat Animal Research Center at Clay Center, Neb., has been able to greatly increase the incidence of twinning in cattle by: • the use of bulls whose daughters have a history of twinning, and • the use of ultrasound to identify heifers that have had a multiple ovulation.

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