Effects of a single injection of hCG or GnRH agonist on day 12 post mating on fetal growth and reproductive performance of sheep.

Reproductive performance and fetal growth were determined when hCG (150 i.u. Pregnyl; n=44), GnRH (4 microg synthetic GnRH agonist, buserelin, Receptal; n=43) or saline (control, n=45) was administered (i.m.) to ewes on day 12 post mating during the breeding season. A total of 12 ewes was slaughtered on day 45 of pregnancy (four from each treatment group). Non-return rate and lambing rate were higher for ewes in the hCG (0.89 and 84%) and GnRH treated groups (0.86 and 79%) than for ewes in the control (0.69 and 62%) group (P<0.05). The ewes in the hCG and GnRH groups also had more twins (P<0.05). Birth weights of these twin lambs in the hCG and GnRH groups were heavier than those in the control group (P<0.05), but this difference had disappeared at weaning 60 days later. Lamb mortality was similar among treatment groups resulting in a higher number of lambs weaned in the hCG and GnRH groups. The ovarian weights and the number of corpora lutea (CL) and luteal weights of ewes slaughtered on day 45 of pregnancy were greater (P<0.05) in the hCG and GnRH treated groups than those measured in the control group. Administration of hCG on day 12 post mating increased gravid uterus weight, crown-rump-length (CRL), chorioallantois weight and total cotyledon weight (P<0.05) of conceptuses recovered on day 45 of pregnancy compared to the control group. The weights of caruncules (P<0.05) and placenta (P<0.01) were higher in conceptuses of both the hCG and GnRH groups. The weights of fetuses in the hCG group were higher than those in both the GnRH and control groups (P<0.05). In conclusion, both hCG and GnRH administration improved reproductive performance of ewes when administered on day 12 post mating. However, hCG and GnRH appeared to act differently on embryo survival because only hCG administration increased fetal growth.