INFLUENCE OF INITIAL LENGTH OF UTERUS PER EMBRYO AND GESTATION STAGE ON PRENATAL SURVIVAL, DEVELOPMENT, AND SEX-RATIO IN THE PIG

The effects of uterine length per embryo and stage of gestation on pre natal survival, development, and sex ratio were determined by systemat ically restricting embryos to 5, 10, 15, 20, 25, 30, 35, 40, 45, or > 50 cm of initial uterine length per potential embryo and examining the reproductive tracts at d 17, 23, 29, 35, or 41 of gestation. At d 3 t he mean length of a uterine horn was 169 +/- 35 cm, with a range of 80 to 255 cm. The mean total number of corpora lutea (CL) was 12.4 +/- 2 .34. Ovulation rate tended to be correlated with the length of uterine horns (r = .38; P > .1). Prenatal survival was highly correlated with initial assigned uterine space from 5 to 25 cm/CL (r = .95; P < .05). Prenatal survival was not affected by space (r = .53; P < .1) when sp ace was > 25 cm/CL. In all groups, regardless of space available, some prenatal loss occurred before d 17. In sections with < 25 cm/CL, a se cond loss took place between d 29 and 35. Before d 35, there was no si gnificant effect of restriction on either fetal length or weight, but after d 35, fetuses from sections with > 25 cm/CL were longer and heav ier than those from sections with less space (P < .05). Greater uterin e space was occupied by larger fetuses (P < .05). Male fetuses occupie d more uterine space than female fetuses did in both crowded and roomy sections (P < .05). In crowded sections with assigned space < 25 cm/C L at d 41, males were lighter than females (male 10.17 g, female 11.56 g). In sections with assigned space > 25 cm/CL, males were heavier th an females (male 12.91 g, female 12.05 g) (P < .05). Sex of embryos at very early stages of gestation was determined by using a probe of a D NA segment of the Y-chromosome. When embryos were restricted to < 10 c m initial uterine length, 42% were male (P > .1), and when embryos wer e restricted to greater-than-or-equal-to 15 cm, sex ratio of male:fema le was 49:51.