Influencing prolificacy of sows by selection for physiological factors.

The potential to improve prolificacy, with major emphasis on selection for components of litter size, ovulation rate and prenatal survival, and selection for physiological factors are reviewed. Response to selection for physiological factors is superior to direct selection if physiological factors have moderate heritabilities and moderate to high genetic correlations with the trait to be improved. There are very few estimates of the genetic parameters needed to calculate the relative efficiency of direct and indirect selection. Testis size is highly heritable and positively correlated genetically with ovulation rate in both mice and pigs and may be potentially useful in an index with litter size to improve ovulation rate and prenatal survival. Selection for growth, ovulation rate or litter size has increased ovulation rate in mice, but the physiological explanations are different. Selection for litter size in a line of pigs previously selected for ovulation rate was effective; the realized heritability was 0.18 +/- 0.06. Potential improvements in litter size from index selection for ovulation rate and prenatal survival are discussed and compared to direct selection for litter size.