Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2007
Publication Date: 8/13/2007
Citation: Freking, B.A., Leymaster, K.A., Vallet, J.L., Christenson, R. 2007. Number of fetuses and conceptus growth throughout gestation in lines of pigs selected for either ovulation rate or uterine capacity. Journal of Animal Science. 85(9):2093-2103. Interpretive Summary: Increased ovulation rates have resulted in less than optimal piglet survival rates during both prenatal and early postnatal periods and have hindered the desired outcome of increased numbers of pigs weaned. Understanding the biological basis for improved uterine capacity as a consequence of genetic selection would allow exploitation of higher levels of reproductive potential. Unique fetal survival curves resulted from selection of pigs for ovulation rate or uterine capacity. Embryonic losses from ovulation to d 25 were proportional to ova shed in all three lines. Direct selection for uterine capacity has increased survival of fetuses to term, primarily by increasing survival between d 25 and 45 of gestation without altering fetal or placental weights. Losses continued to occur from d 45 to 105, but line differences in number of fetuses at d 45 were essentially maintained to d 105. The critical period of time identified coincides with rapid development of the placenta and focuses attention on metabolic pathways that can create and develop a placental environment capable of maintaining more fetuses of a similar birth weight to term.
Technical Abstract: Selection for 11 generations in swine for ovulation rate (OR) or uterine capacity (UC) resulted in 19.6% greater prenatal survival at term in UC compared to OR. Our objective was to characterize number of fetuses throughout gestation in each line, including an unselected control line (CO). Five hundred ninety three gilts produced over four farrowing seasons were subjected to unilateral-hysterectomy-ovariectomy at 160 d of age and mated within line at 280 d of age. Gilts were assigned within sire family to be slaughtered (± 2 d) at d 25, 45, 65, 85, or 105 of gestation. Ovulation rate and number of live and dead fetuses were recorded for each pregnant gilt (n = 402). Fetal and placental weights were also recorded. A mixed model analysis of variance and non-linear curve fitting procedures were applied to the data. Fixed effects of farrowing season, line, gestation age, and two-way interactions were fitted. Random effect of sire (n = 116) of the gilt within farrowing season and line was used to test line differences. Ovulation rate of OR line gilts (18.0 ± 0.3 ova) exceeded (P < 0.001) both CO and UC lines, (15.0 ± 0.3 and 14.0 ± 0.3 ova, respectively). Line and gestation age interacted to affect number of live fetuses (P < 0.001). Least squares means for CO were 10.1, 8.3, 7.2, 6.7, and 7.3 live fetuses for d 25, 45, 65, 85, and 105, respectively (average SE = 0.46 fetuses). Corresponding means for OR were 13.4, 8.3, 7.9, 6.5, and 6.7 live fetuses, respectively (average SE = 0.44 fetuses). Means for UC were 10.2, 9.0, 8.5, 7.5, and 8.0 live fetuses, respectively (average SE = 0.47 fetuses). In each line, number of live fetuses at d 25 was about 72% of ovulation rate. Mortality to d 45 was highest in OR, intermediate in CO, and lowest in UC. Reductions in live fetuses continued to occur from d 45 to d 105 but line differences at d 45 were essentially maintained to d 105. Number of live fetuses in gilts at d 114 was estimated from each of the survival curves and predicted values of 7.0, 5.9, and 7.8 per uterine horn for CO, OR, and UC lines, respectively. Selection for uterine capacity improved fetal survival primarily during the critical time period between d 25 and d 45. Relative to CO and OR, the growth rate coefficient of placental tissue was greatest for the UC line at d 65, without altering fetal or placental weights near term, implicating metabolic pathway targets as the likely means to improve placental efficiency or uterine capacity.