Selection for components of litter size are reviewed. Although ovulation rate responded to direct selection, half of the advantage was lost by day 30 and fetal mortality continued thereafter until about 20% of the increased ovulation rate was realized as greater litter size. Selection for litter size in high ovulating gilts was effective due to increases in ovulation rate and uterine capacity. Emphasis on ovulation rate and prenatal survival to day 50 of gestation was optimized by index selection, producing changes in components that were less hostile than direct selection for ovulation rate! Measurement of uterine capacity by use of a surgical procedure was validated. A conceptual model of litter size partitioned prenatal mortality into embryonic and fetal phases. Fetal mortality occurs due to the interaction of uterine capacity with ovulation rate and potential embryonic viability. Selection for litter size is equivalent to selection for components by fixed independent culling levels. Genetic information is accruing on control of ovulation rate, prenatal survival, and uterine capacity.

K. A Leymaster, R. K Johnson

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 17. Genetics and breeding of dairy and beef cattle, swine and horses, , 307–314, 1994
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