Examination of ovulation rate, uterine and fetal interactions, and reproductive age in Chinese Meishan, Yorkshire, and reciprocal cross gilts: effects of fetal and maternal genotypes

Two studies were designed to examine ovulation rate, uterine and fetal interactions, and reproductive age in Chinese Meishan (Ms) and Yorkshire (Y) gilts. In Experiment 1, ten Ms, ten Ms × Y, ten Y × Ms and ten Y females were mated at third estrus to Y, Ms × Y, Y × Ms and Ms sires, respectively, so that fetuses were 1 2 Ms and 1 2 Y. Gilts were slaughtered at 51 days of gestation (SD = 2) and reproductive tracts examined. Crossbred gilts had more corpora lutea (CL; 17.3 for both crosses) and fetuses (14.7 for Y × Ms and 12.9 for Ms × Y) than Y (12.5 CL and 10.9 fetuses) or Ms (14.2 CL and 9.2 fetuses; P < 0.10) gilts. Uterine lengths did not differ among groups (P > 0.05). Fetal weight, crown-rump length and amniotic fluid volume per fetus were highest for fetuses from Y females, intermediate for fetuses from Ms and Y × Ms females, and lowest for fetuses from Ms × Y females (P < 0.05). Fetuses in Ms gilts had more uterine space than the other groups (P < 0.05). Space per fetus was intermediate in Y and Ms × Y females and lower in Y × Ms females (P < 0.05). Volumes of allantoic fluid were greatest for fetuses from Ms females (P < 0.05). In Experiment 2, ten Ms and ten Y gilts were mated at third estrus to Ms and Y sires, respectively. These gilts carrying purebred fetuses (Ms-P and Y-P) were compared with Ms and Y gilts carrying crossbred fetuses (Ms-X and Y-X) from Experiment 1. Breed of gilt effects were detected for number of ovulations (P < 0.05) but not number of fetuses (P > 0.05). The highest fetal survival occurred in Y-X females and the shortest uterine lengths were present in Ms-P females (P < 0.05). Fetuses in Ms-X and Y-P females occupied the most uterine space, fetuses from Y-X females were intermediate and fetuses from Ms-P females occupied the least uterine space (P < 0.05). Mean allantoic fluid volumes per fetus from Ms-X, Y-X, Ms-P and Y-P were 177 ml, 122 ml, 99 ml and 69 ml, respectively (P < 0.05). Fetuses from Y-X females were the longest, heaviest and had the most amniotic fluid followed by fetuses from Ms-X, Y-P and Ms-P females, respectively. Fetuses from Ms-P gilts weighed less than fetuses from other groups (P < 0.05).