Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/30/2006
Publication Date: 12/20/2006
Citation: Vallet, J.L., Freking, B.A., Miles, J.R., Nienaber, J.A., Brown Brandl, T.M. 2006. Factors affecting litter size in pigs. Proceedings National Swine Improvement Federation (NSIF) Conference and Annual Meeting (7-8 Dec 2006; Nashville, TN). NSIF Vol. 31:15-21.
Technical Abstract: From the producer’s perspective, the best measure of litter size in pigs is the number of piglets weaned per sow. The number of piglets weaned is influenced by the number of fully formed piglets at the end of gestation, the stillbirth rate, and the rate of preweaning death. In turn, the number of fully formed pigs is influenced by the number of ovulations, the fertilization failure rate, the embryonic mortality rate, and uterine capacity, but in today’s swine, uterine capacity represents the most limiting factor of the number of fully formed piglets. Uterine capacity is influenced by uterine factors (blood flow, uterine protein secretion), placental factors (placental size and efficiency of placental transport), and fetal factors (fetal erythropoiesis, fetal nutrient distribution among various organs). Uterine blood flow reaches maximum at a litter size of about 10 fetuses. Uterine protein secretion during early pregnancy appears to affect uterine capacity by altering the course of the elongation phase of the porcine blastocyst, reducing the eventual size of both the placenta and fetus during the rest of pregnancy. An improved understanding of how the structure of the placenta affects placental efficiency, and how that structure may be altered to improve efficiency, is needed. Our research over the last decade has demonstrated a role for fetal erythropoiesis in uterine capacity, and this work has resulted in a genetic marker associated with the number of piglets born alive that is based on the erythropoietin receptor gene. Turning to stillbirth rate, both size of the piglets at birth and timing of the farrowing process are associated with stillbirth in pigs. Size of piglets at birth is another manifestation of uterine capacity. We recently began a study of the farrowing process in pigs and its relationship to stillbirth rate. The stillbirth rate in our BX population is 5%, and 3% of this appears to be related to aspects of the farrowing process. Early in the litter, delivery of each piglet occurs more slowly, and speeds up as farrowing progresses. Our results indicate a significant delay in the birth of the last piglet in each litter. There was a significant association between farrowing interval and stillbirth rate, and the slope of the relationship indicates that an increase in average farrowing interval of 10 min corresponds to an extra 1% in stillbirth. Preweaning death is also associated with low birth weights. However, breeds and lines of pigs vary in this association. Both Meishan piglets, and piglets from a line of pigs selected for fatness, display improved survival of low birth weight piglets. The commonality appears to be piglet energy stores (glycogen and fat) and simple, easily obtainable measures are needed to begin to assess piglet fat stores in lines of pigs, to enhance selection for preweaning survival.