The sow’s offspring sex ratio is a subject of considerable interest from both the theoretical and the practical point of view. In domestic animals, including pigs, the offspring sex ratio is also a feature of economic value. The gilts from litters with a higher proportion of females can deliver and feed more piglets as they have more teats, a higher fertility rate, and a better reproductive performance. But we already knew that, right?
A study conducted to assess whether litter size, maternal age and parity, paternal breed, maternal birth year and month, and litter birth year and month influenced the offspring sex ratio in domestic pigs. A total of 436 litters on 21st day of life were considered. It was found that paternal breed and litter size significantly affected the offspring sex ratio (fewer males in larger litters). Also maternal month of birth had a significant influence on offspring sex ratio (sows born in September-February delivered litters with a higher male proportion than those born in March-August). There was also a correlation at a marginally significant level between the offspring sex ratio on 21st day of life and proportion of stillborn piglets (more stillborn ones in the litters with the future higher male proportion).
Another study found that females with abundant resources would produce more females, as sows are territorial and would benefit from these resources. Conversely, if resources are poor, sows would favour male offspring, which disperse upon reaching maturity and move into new ranges. This theory has been supported by demonstrating that under normal conditions perinatal (immediately before or after birth) mortality is female biased. In support of this suggestion, it was reported that reduced embryo development and decreased female embryo survival were associated with differences in the variance of epigenetic (non-genetic influences) traits in the surviving litters at Day 30.
Different factors have been shown to be associated with birth sex ratio in mammals, such as nutrition, season of birth, diseases, stress, female’s age and parity, social status, levels of different hormones, type and time of insemination, oestrus synchronisation before insemination, habitat quality, population demography and sex of adjacent embryos in sows own birth litter.
So there seems to be no definitive conclusion other than knowing when your sow/gilts date of birth and monitoring if she was born from September to February or March to August and of course did she come from a litter that was higher in number of males and females. Pretty much similar to us humans i guess. As i know of one couple who in 6 generations from the male side have never had a daughter and all three brothers married and whilst the wives had sisters not one of them had a daughter but all sons!
The Oxford Sandy and Black Pig Group Foundation Charity are conducting a three year research to establish the sex ratio in seasonal farrowings, with quarterly reports being posted on their facebook forum here