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The economic benefits of increasing reproductive rate indicate a need to determine the feasibility of increasing the frequency of twinning in cattle, either by selection or by an artificial method. Rate of reproduction has a major impact on life cycle costs of production for different meat animal species and, thus, upon the production resources for which different species are competitive. For example, the average beef cow is capable of producing about .7 of her body weight per year in progeny market weight, but the comparable multiple is 8 in pigs and more than 70 in meat chickens. The objectives of this project focus on gaining the understanding needed to develop a technology for increasing the frequency of twinning in cattle. A comprehensive physiological examination of cows that produce a high frequency of twins may establish the biological and/or environmental requirements for multiple births in cattle and the feasibility of increasing twinning frequency by selection, artificial induction, or by both. This experiment is being conducted to provide understanding relating to these considerations, Specific objectives of the experiment are: (1) determine the effectiveness of selection for multiple births in cattle; (2) develop and evaluate selection criteria for multiple births in cattle; (3) accumulate data that will contribute to an economic assessment of multiple births in cattle for varying resource situations; (4) establish husbandry requirements for herds of cattle that have a high twinning frequency; (5) determine the relative importance of multiple ovulation and embryo survival in contributing to multiple births in cattle in both spring and fall breeding; and (6) determine the usefulness of cows with high twinning frequency as :models: to gain understanding of biological factors that relate to embryo survival for both single and multiple births in cattle.