Effect of sex ratio of the birth litter on subsequent reproductive performance of gilts.

Records on age at puberty from 1,555 gilts and total number of pigs born in litters of 1,187 gilts from the Nebraska gene pool population were used to evaluate the effects of uterine environment on subsequent reproductive performance. Independent variables were line, year, line x year, proportion of males in the birth litter (sex ratio), number born in the birth litter (fraternity size) and sex ratio x fraternity size. Sex ratio, fraternity size and their interaction influenced age at puberty (P less than .01) but not number born (P greater than .2). Partial regression coefficients indicated that age at puberty tended to decrease as sex ratio increased, particularly in small litters. Although the regression coefficients were relatively large, sex ratio, fraternity size and their interaction accounted for only 1.3% of the variation in age at puberty within line x year subclass. These results offer little encouragement for the use of sex ratio as a phenotypic selection criterion for improvement of reproductive performance in gilts. Results suggest that female swine are similar to rodents in response to uterine environmental effects.