Abstract
Improvement of litter size in pigs through selection was considered a difficult task in the past. Low heritability, negative correlation between direct additive genetic and maternal effect, limited size of the nucleus population, expression only in females, and relatively late age of measurements, were the main reasons for slow improvement. Many genetic and environmental factors, as well as complex interactions between them, influence litter size in pigs. The present work describes and reviews the most important effects influencing litter size: (1) genetic effects, such as the effect of genotype, direct additive, maternal and sire effect; (2) environmental effects such as common litter and permanent effect; and (3) other main factors recorded by commercial producers. Further improvements in selection of litter size depend on new selection criteria related to overall efficiency of pig production. Use of quantitative genetics based on a multiple trait evaluation system together with genomic selection will provide further improvement in litter size.