Effect of increased ovulation rate on embryo and foetal survival as a model for selection by ovulation rate in rabbits

Selection for ovulation rate in prolific species has not improved litter size, due to an increase in prenatal mortality, with most mortality observed in the foetal period. The aim of this study was to investigate the magnitude and timing of embryo and early foetal survival in females with high ovulation rate using hormonal treatment as a model for selection by ovulation rate. Two groups of females (treated and untreated) were used. Treated females were injected with 50 IU equine chorionic gonadotropin 48 h before mating. Females were slaughtered at 18 d of gestation. Ovulation rate (OR), number of implanted embryos (IE), number of live foetuses at 12 and 18 d (LF12 and LF18, respectively) were recorded. In addition, embryo survival (ES=IE/OR), foetal survival at 18 d of gestation (FSLF18=LF18/IE), foetal survival between 12 and 18 d of gestation (FSLF18/LF12=LF18/LF12) and prenatal survival (PSLF18=LF18/OR) were estimated. For each female, the mean and variability of the weight for live foetuses (LFWm and LFWv, respectively) and their placentas (LFPWm and LFPWv, respectively) were calculated. Treated females had a higher ovulation rate (+3.02 ova) than untreated females, with a probability of 0.99. An increase in the differences (D) between treated and untreated females was observed from implantation to 18 d of gestation (D=–0.33, –0.70 and –1.28 for IE, LF12 and LF18, respectively). These differences had a low accuracy and the probability that treated females would have a lower number of foetuses also increased throughout gestation (0.60, 0.70 and 0.86 for IE, LF12 and LF18, respectively). According to the previous results for OR and LF18, treated females showed a lower survival rate from ovulation to 18 d of gestation (D=–0.12, P=0.98 for PSLF18). Treated females also had lower embryo and foetal survival (D=–0.10 and P=0.94 for ES and D=–0.08 and P=0.93 for FSLF18). Main differences in foetal survival appeared from 12 to 18 d of gestation (D=–0.09 and P=0.98 for FSLF18/LF12). Unexpectedly, treated females showed similar foetus weight and higher foetal placenta weight than untreated females (D=0.25 g, P=0.98) and lower variability for these traits (D=–0.02 g, P=0.72 for LFWv and D=–0.05 g, P=0.83 for LFPWv). These results are not related to a lower number of IE or LF18. Thus, the effect of increasing by three ova in rabbits leads to a lower embryo and early foetal survival. There seems to be no relationship between foetal mortality and foetus weight.