Causes of variation in litter size in Swedish piglet production

Abstract: Litter size is influenced by a number of different factors including parity number and age at first mating as well as genetic potential and environmental factors such as management and feeding. The aim of this study was to evaluate the effect of age at first farrowing, lactation period and weaning to service interval on subsequent litter size, as well as to obtain descriptive statistics on the variation in litter size over the last few years. Records from 2012 to 2017 including observations from two commercial Swedish piglet producing herds were collected from the pig herd monitoring program WinPig. Sows included in the study were Landrace x Yorkshire crossbreeds. Information available for the study were, herd, sow identity number, date of the birth of the sow, date of insemination, date of farrowing, date of weaning, number of piglets born alive and number of stillborn piglets. Sows were for the analyses divided into four groups based on age at first farrowing; 330-359 days, 360-369 days, 370-379 days and 380-409 days. Four groups were also created based on length of weaning to service interval; 0-5 days, 6-20 days, 21-41 days and 42-80 days. The results show that litter size increased during the studied time period, with an increase in number of piglets born alive and no significant changes in number of stillborn piglets. Litter size also increased with increasing parity number, reaching the highest total number of born piglets in parity 5 and 4 for Herd A and B respectively. This increase in number of piglets born alive can be due to genetic improvement, however, breed was not included in the present study and can thereby not be evaluated. Litter size in first parity was lowest for sows with age 330-359 days at first farrowing. From second parity and onwards litter size differed less between the groups, which implies no long lasting effect of age at first farrowing on litter size. Therefore, it could be beneficial to inseminate gilts at an earlier age if they have reached the right body weight, as the differences in litter size are not continuous after first parity. The lactation period had an impact on subsequent litter size. Total number of born piglets and piglets born alive significantly (P<0.001) increased with increased lactation period, which means that the null hypothesis could be rejected. The number of stillborn piglets also increased with increased lactation period, although this increase was not significant. However, a longer lactation period would increase the pressure on sows, as piglets are heavier and more demanding later on in the lactation period. Weaning to service interval also significantly influenced subsequent litter size, which means that the null hypothesis could be rejected. Sows that were inseminated early (0-5 days) had a higher subsequent litter size compared with later inseminated sows (6-20 days). The highest subsequent litter size was found amongst sows that were bred at second estrus (21-41 days). Sows that are bred at second estrus have more time to establish a good body condition after weaning, although the number of non-productive days will increase. All factors analyzed in the present study influence litter size and each producer has to consider their own conditions in order to make the right decisions.