EFFECTS OF PREGNANT MARE SERUM GONADOTROPIN ON REPRODUCTION IN 4 GENOTYPES OF GILTS

Effects of PMSG and genotype on various measures of reproductive effic iency were investigated. Prenatal data were obtained at 40 d of gestat ion from 96 gilts representing four genotypes. Data on Duroc (D), York shire (Y), Synthetic (Large White x Landrace) (SYN), and Crossbred Dur oc x Yorkshire (XB) gilts were collected from January, 1990 through Ma y, 1991. Litter size (LS) data were collected from 482 farrowings of s iblings. Treatment with exogenous hormones significantly increased num ber of corpora lutea (CL), number of embyros (EN), ovum wastage, (OVWS ) and embryo length (ELG). Breed group differences (P < .05) were dete cted for natural ovulation rate, hormone-induced ovulation rate, CL, O VWS, ELG, embryo weight, ovum success, uterine length, ovary weight, r ange and variance of within-litter embyro weight (RWT and VWT), and li tter size born alive. Natural ovulation rates for D, Y, SYN and XB wer e 10.46 +/- 1.61, 12.64 +/- 1.41; 14.10 +/- .99 and 10.90 +/- 1.47, an d hormone-induced ovulation rates were 15.00 +/- 1.53, 17.69 +/- 1.40, 19.43 +/- 1.17 and 12.19 +/- 1.43, respectively. Range and variance o f within-litter embryo length were not affected by either treatment or genotype. Increases in RWT and VWT observed in D and XB gilts after P MSG treatment did not adversely affect embyro survival to 40 d gestati on. Significant genetic differences existed for litter size at birth. The PMSG treatment and interactions with PMSG were not significant for litter size born alive. Breed groups seem to differ for CL and EN in response to PMSG but only Yorkshire showed any response in LS (P < .10 ). Although PMSG increased ovulation rate in siblings by 4.06 ova and number of embyros at 40 d gestation by 1.87 compared with control gilt s, there were no differences in litter size born alive due to PMSG tre atment. The increase in ovulation rate and number of embryos generated by PMSG seems to negated by fetal losses occurring both before and af ter 40 d of gestation.