Pregnancy rate after fixed-time artificial insemination of suckled beef cows subjected to a cosynch protocol with either buserelin or hCG as ovulation inducing agent

The aim of this study was to document the effectiveness of the implementation of a cosynch protocol and fixed time artificial insemination in suckled beef cows kept under range conditions in Germany. In addition, the suitability of hCG administration as an alternative to the GnRH analog buserelin (Receptal®, Intervet, Germany) as ovulation inducing agent was investigated. The 120 Simmental and 120 German Angus cows were randomly divided into four groups. Cows of Group 1 were subjected to the classical cosynch protocol, constituting administration of GnRH, followed, 7 days later, by prostaglandin F2α and, after 2 days, another GnRH administration and simultaneous AI. In cows of Group 2, hCG was administered instead of GnRH at the beginning of the protocol, in Group 3 at the end and in Group 4 at the beginning and at the end. Inseminations with frozen-thawed semen from Limousin, Charolais, Simmental and German Angus sires were carried out by the same skilled inseminator. Blood samples collected 11 days before and on days 0, 7 and 9 of the cosynch protocol were analyzed for progesterone concentration. The study extended over three consecutive years. Of the total of 489 inseminations, 53% resulted in pregnancies (Simmental: 60%; German Angus: 48%). When GnRH was administered at the beginning and at the end of the cosynch protocol (Group 1), pregnancy rate across breeds was 65% (Simmental: 61%; German Angus: 68%); likewise when hCG was substituted for GnRH at the end of the protocol (Group 2). Pregnancy rate across breeds reached 48% when hCG was given at the beginning (Group 3) and was lowest (37%) when hCG was given at the beginning and at the end. German Angus cows responded particularly poorly to hCG treatment. In cows that were cycling, as indicated by serum progesterone levels 11 and 0 days before treatment, pregnancy rate across breeds and treatments was highest (57%) if the cosynch protocol was initiated at a stage of the cycle when peripheral progesterone levels were on the rise. In non-cycling cows, representing only 13% of the herd, contrary to expectation pregnancy rate reached 65%. In conclusion, implementation of fixed-time artificial insemination in suckled beef cows kept under local range conditions led to satisfactory pregnancy results, especially if the cosynch protocol was initiated at a stage when progesterone levels were on the rise. Substitution of hCG for GnRH proved to be of no advantage; in fact, when administered at the beginning of the protocol it resulted in a detrimental effect on fertility.