Effect of birth weight and litter size on the performance of landrace gilts until puberty

Background: The selection for larger litter size has resulted in reduction of average birth weight and in increase of withinlitter birth weight variation in pigs. Birth weight is one of the most important factors affecting the survival and performance of piglets until weaning. The aim of this study was to evaluate the effect of litter size into which gilts were born and their individual birth weight on mortality and culling rate until the time of selection, as well as on puberty onset. Materials, Methods & Results: Female Landrace piglets (n = 1525) were identifi ed and weighed within 18 h after birth. They were also weighed at weaning (n = 1379), and at the end of the nursery (n = 1198) and rearing phases (n = 940). Three classes of litter size were created: Small LS (7-11 piglets), Intermediate LS (12-13 piglets) and Large LS (14-19 piglets). Gilts were also separated in three classes according to their birth weight: Low BW (530-1200 g), Intermediate BW (1205-1600 g) and High BW (1605-2535 g). When compared to High BW gilts, the risk of death until weaning was greater (P < 0.05) in Low BW gilts from Intermediate LS and Large LS litters, but not in Low BW gilts from Small LS litters. Within the Low BW class, the risk of mortality until weaning was greater or tended to be greater if gilts were born in Intermediate LS (P < 0.05) or Large LS (P = 0.079) litters compared to those from Small LS litters. Average daily weight gain (ADWG), and weights at 22, 75 and 155 days of age, increased according to the increase in birth weight (P < 0.05). There was no effect of birth weight or litter size on mortality and culling rate, during nursery and rearing phases, and on percentages of gilts in anoestrous until 30 days after boar stimulation (P > 0.05). The risk of not being selected for breeding was greater (P < 0.09) in Light BW gilts than in High BW gilts in all litter size classes. Low BW gilts were older at the beginning of boar exposure and had a lower boar exposure-puberty interval compared to High BW gilts (P < 0.05), but there was no effect of birth weight on puberty age (P > 0.05). Discussion: The importance of birth weight for survival until weaning and for development until the selection time was confi rmed in this study. Since birth weight was associated with ADWG, the expectation was that puberty onset would happen earlier in gilts with greater ADWG. In other studies, gilts with greater growth rates reached puberty earlier if stimulated at an early age (~140 days of age). In the present study, however, puberty onset was not affected by birth weight, probably because gilts were exposed to boars at 165 days of age. This aspect may indicate that, after a certain age, puberty is less dependent on growth rate. The greater risk of mortality until weaning and of not being selected for reproduction, in Light BW gilts, contribute to economic losses in replacement gilt units. Segregation of gilts based on their birth weight can be performed at birth or in subsequent phases. Early selection may help to reduce costs for maintaining gilts that will probably not be selected, hence increasing the profi tability of pig production. In addition, it provides the opportunity to pay special attention to gilts with greater potential to be future breeders. The results show that birth weight is more important than litter size into which gilts were born for their survival until weaning, as well as for an adequate weight gain, which will ensure their retention in the herd until the selection time. When the gilts are stimulated close to 165 days of age, age at puberty and anoestrous rate are not affected by birth weight or litter size into which they were born.