1337 1 The authors gratefully acknowledge J. C. Hulin, Y. Lebreton, M. Massard, V. Beaumal, and C. David for their efficient technical assistance. 2 This study was carried off within the framework of a research program on “Genetic variability of embryonic survival,” which was planned and partly financed by INRA between 1991 and 1993. Received July 22, 1996. Accepted December 15, 1996. Effect of Number of Pig Embryos in the Uterus on Their Survival and Development and on Maternal Metabolism 1,2 Marie-Christine Pe ` re, Jean-Yves Dourmad, and Michel Etienne Station de Recherches Porcines, Institut National de la Recherche Agronomique, 35590 Saint-Gilles, France ABSTRACT: The effects of pig embryo number on fetal survival and growth and maternal metabolism were evaluated with 114 Large White gilts. Gilts were assigned at 38 kg to three treatments: control (CTR), ligature of the left oviduct (LIG), or right hemi- hysteroovariectomy (HHO). Insemination occurred at 311 ± 18 d of age. A laparotomy was performed at d 35 of gestation, and gilts were slaughtered at d 112. Ovulation rate per uterine horn was 4.30, 8.70, and 17.12 in the LIG, CTR, and HHO groups, respectively. The hierarchy was the same for litter size at d 35 of gestation, but the relative differences were reduced (3.24, 5.98, and 8.40 fetuses/uterine horn, respective- ly). Litter size per uterine horn was similar in the CTR and HHO groups at d 112 of pregnancy (2.93, 4.69, and 4.76 fetuses in the LIG, CTR, and HHO groups, respectively). Early (before d 35 of gestation), late, and total fetal mortality increased with embryo potential per uterine horn. There was a compensation between early and late fetal mortality in the CTR and HHO groups. Fetal weight at d 112 was related to litter size in early pregnancy (1.50, 1.38, and 1.27 kg in the LIG, CTR, and HHO groups, respectively). Uterine capacity limits litter size and fetal develop- ment, even in sows with a conventional potential of embryos. Availability of energetic and gluconeogenic substrates was higher at 110 than at 60 d of gestation in the three groups. Blood substrate levels suggested that lipid mobilization and glucose uptake were higher in the gilts with a larger litter weight. Key Words: Sow Pregnancy, Metabolism, Uterus, Fetal Death, Fetal Growth J. Anim. Sci. 1997. 75:1337–1342 Introduction Ovulation rate in pigs is often 30 to 40% greater than litter size at farrowing (Wrathall, 1971). Be- cause 90 to 95% of the ova are fertilized (Perry and Rowlands, 1962; Polge, 1982), most of the difference is due to prenatal mortality, which occurs mainly during the embryonic phase, before 30 d of gestation. Later mortality has been determined in very few experi- ments, but it is considered to be low (3% according to Lambert et al., 1991). In fact, sows seem to be able to ensure the development until term of a limited number of fetuses only. The concept of uterine capacity in swine has been researched by different authors and methods (Bazer et al., 1969a; Webel and Dziuk, 1974; Knight et al., 1977; Christenson et al., 1987; Bennett and Leymaster, 1989). Limitation of uterine space available to fetuses (Rathnasabapathy et al., 1956) and competition among embryos for biochemical factors or nutrients (Bazer et al., 1969b) were suggested as mechanisms. The aims of this study were to determine on the same sows the consequences of initial litter size on early and late fetal mortality and on fetal development, and the extent to which the pregnant sow adapts its metabolism to litter size. Materials and Methods Animals and Diets The experiment used 114 Large White gilts as- signed in three replicates at 93 ± 6 d of age and 38 ± 6 kg live weight (mean ± SD). Gilts were fed a diet containing 3.1 Mcal of DE/kg, 17.6% CP, and .85% lysine during the growth period and housed in groups of five and fed individually according to a feeding scale. The feed allowance progressively increased from 2 kg/d between 35 and 40 kg live weight to a maximum of 2.7 kg/d between 65 and 100 kg live weight. The gilts above 100 kg live weight were fed 2.5