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1 USDA, ARS, U.S. Meat Animal Research Center, Clay Center, NE 68933-0166
* To whom correspondence should be addressed. E-mail: Brad.Freking{at}ars.usda.gov.
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Abstract |
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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 (avg SE = 0.46 fetuses). Corresponding means for OR were 13.4, 8.3, 7.9, 6.5, and 6.7 live fetuses, respectively (avg SE = 0.44 fetuses). Means for UC were 10.2, 9.0, 8.5, 7.5, and 8.0 live fetuses, respectively (avg 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 45. Relative growth rate coefficients throughout gestation for placental tissue indicated a change in rank of the line means implicating a relative later growth pattern of placental tissue in the UC line.
Key Words: fetal survival, placental growth, pigs, uterine capacity
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