Effect of replacing the second gonadotropin releasing hormone of Ovsynch and Co-synch protocols with human chorionic gonadotropin on the reproductive performance of primiparous lactating cows during the cool months of the year

Abstract

The study objective was to evaluate the effect of replacing the second gonadotropin releasing hormone (GnRH2) of the Ovsynch and Co-Synch protocols with human chorionic gonadotropin (hCG) on ovulation, conception, pregnancy, and embryo loss rates in primiparous lactating dairy cows. Forty-nine primiparous Holstein cows 53 to 65 days in milk were allocated to the following groups: (1) OVS (Ovsynch; n=12): GnRH, 7 days, PGF2α, 56 hours, GnRH, 16-18 hours, timed artificial in-semination (TAI); (2) COS (Co-Synch; n = 12): GnRH, 7 days, PGF2α, 72 hours, GnRH + TAI; (3) OVS-hCG (Ovsynch + hCG; n = 12): as OVS except that GnRH2 was replaced with 1500 IU hCG; (4) COS-hCG (Co-Synch + hCG; n = 13): as COS except that GnRH2 was replaced with 1500 IU hCG. Ovaries of all animals were scanned by transrectal ultrasonography to determine the incidence of ovulation on days 9, 10 and 11 at 24-hour intervals after the initiation of the treatments (day 0). Ovulation was defined as the disappearance from one scanning session to the next of a previously identified follicle ≥ 8 mm in diameter. Pregnancy diagnosis was performed by transrectal ultrasonography on days 30 ± 1 and 42 ± 1 post TAI to determine conception and pregnancy status, respectively. The results showed that the animals in OVS tended to have higher ovulation and conception rates (P=0.08). In addition, pregnancy rate was higher and embryo loss rate was lower for OVS than the other groups, but the differences were not significant (P>0.05). It was concluded that replacing GnRH2 with hCG in the Ovsynch and Co-Synch programmes has no beneficial effect on the reproductive performance of primiparous Holstein cows. However, conducting the experiment at a larger scale may provide more precise results.

Keywords

• co-synch
• human chorionic gonadotropin
• ovsynch
• primiparous holstein cow
• reproductive performance