The effect of hCG treatment on Day 12 post-mating on ovarian function and reproductive performance of ewes and ewe lambs.

The objectives of this study were to determine the effect of a single intramuscular injection of 200IU hCG (Chorulon) on Day 12 post-mating on ovarian function and subsequent lambing performance in ewes and ewe lambs bred at synchronised oestrus during the breeding season and on the lambing performance of ewes induced to breed during late anoestrus. All animals were mated to rams at synchronised oestrus and on Day 12 post-mating given normal saline or 200IU hCG. In Experiment 1, laparoscopic results showed that hCG treatment induced accessory corpora lutea in ewes (control=0/7; hCG=5/7) but not in ewe lambs (control=0/7; hCG=0/7). In Experiment 2, hCG treatment did not improve the lambing rate (control=50; hCG=57) or the litter size (control=1.80; hCG=1.96) in ewes (control=100; hCG=91). However, hCG treatment significantly (P>0.05) improved the lambing rate (control=29; hCG=58; P<0.05) in ewes conceiving at the first oestrus after treatment. hCG treatment (control=42; hCG=42) also failed to improve the lambing rate in ewe lambs (control=48; hCG=41). In Experiment 3, hCG treatment had no significant (P>0.05) effect on the lambing rate (control=72; hCG=62) or the litter size (control=1.59; hCG=1.58) in ewes (control=111; hCG=115) induced to breed during anoestrus or on ewes returning to oestrus and conceiving after treatment (lambing rate: control=86; hCG=72; litter size: control=1.44; hCG=1.35). In conclusion, the data obtained in this study suggest that during the breeding season hCG may, by stimulating ovarian function, improve embryo survival in ewes conceiving at the first post-treatment oestrus. This effect, however was not observed in ewe lambs.