A modified Cosynch protocol for timed artificial insemination in beef cattle
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The Ovsynch protocol for timed artificial insemination (TAI) is widely used. In beef cattle the Cosynch protocol is often preferred, as TAI and the second GnRH injection may be conducted simultaneously, thus reducing the number of handlings. Pregnancy rates achieved with either protocol are not fully satisfactory. Critical points influencing the success rates are the proportion of animals ovulating after the first GnRH injection, the progesterone producing capacity of the corpus luteum and the incidence of short cycles after TAI. Possibly, an insufficient LH peak following induction with exogenous GnRH may be the reason why these problems occur. Human chorionic gonadotrophin (hCG) may serve as an alternative to GnRH, due to its longer half-life. The aim of this study was to determine the effectiveness of the Cosynch program using GnRH (Buserelin), hCG (Chorulon) or a combination of GnRH and hCG in beef cows. The trial was conducted on 240 beef cows (120 Simmental (SIM), 120 German Angus (DA)) at the experimental farm Rudlos of Giessen University. At the onset of the breeding season about 70 days after the previous calving all animals were subjected to the Cosynch program, involving a single TAI. Four variations of the Cosynch program were randomly applied taking breed, age and days post partum into account: hCG-PG-hCG (Group 1), GnRH-PG-GnRH (Group2 = standard Cosynch), hCG-PG-GnRH (Group3), GnRH-PG-hCG (Group4). All inseminations were carried out by the same experienced person. Semen of the different sires (5 Charolais, 5 Limousin) was distributed systematically among treatments. Blood samples taken on days -9, 0, 7 (day of PG treatment) and 9 were analysed for progesterone to determine the stage of the estrous cycle prior to treatment and to evaluate the response to the respective hormone treatment. Breeding sires were introduced into the herds to impregnate return cows. In cows treated with hCG on day 0, progesterone concentrations prior to PG treatment (day 7) were higher than in GnRH-treated cows (P<0.05). On the basis of parturition date the calving rates for Groups 1 to 4 were: 37, 54, 48 and 66%, respectively, with fertility of Group 1 being significantly lower than that observed in Groups 2, 3 and 4 (P<0.05). Calving rates for cows cycling prior to treatment as compared to those non-cycling were 53 and 41%, respectively (P>0.05). It can be concluded from this study that, whereas inclusion of hCG at the beginning of the Ovsynch protocol failed to improve fertility, replacement of GnRH by hCG at the end might be beneficial.