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Conception rates after Ovsynch have been higher in primiparous than in multiparous cows. The objective of this study was to investigate whether this difference might be due to differences in ovulation rate or follicular size. The experiment was conducted with 136 Holstein Frisian cows from a commercial herd in Brandenburg, Germany. All cows were synchronized using Buserelin (GnRH analogue) at day -10, Tiaprost (PGF2alpha analogue) at day -3 and again GnRH at day -1. Timed artificial insemination (TAI) was carried out 16-20 h after the second dose of GnRH on day 0. Milk samples for analysis of milk progesterone were obtained on days -17, -10, -3 and at TAI. Progesterone concentrations were used to determine the stage of oestrus cycle at the start of the synchronization protocol and to investigate the presence of functional luteal tissue before treatment with PGF2alpha and TAI. All animals were examined by ultrasound at the second treatment with GnRH, at AI, 8 and 24 h after AI. Overall synchronization rate (proportion of cows with an ovulation within 40 h after GnRH) was 86.8% in primiparous and 88.2% in multiparous cows, respectively. Ovulation occurred earlier in primparous than in multiparous cows (p < 0.05) and ovulatory follicles were smaller. Conception rates were numerically higher in primiparous cows but the difference was not significant. Cows that displayed signs of oestrus on day -1 and received an additional AI on this day were more likely to conceive than cows that only received TAI 16 to 20 h after GnRH2. It is concluded that ovulation occurs earlier in primiparous than in multiparous cows after Ovsynch. However, a significant relationship between these differences and the probability of conception could not be established.

Systematic breeding programs are used to induce estrus, or may even allow timed artificial insemination (AI) in dairy cows. We conducted a trial in a commercial dairy herd to compare an Ovsynch protocol to a protocol using Prostaglandin F2α (PGF2α) in cows with a corpus luteum (C.L.) diagnosed by rectal palpation. Further objectives of the study were to analyze the timing of ovulation in the Ovsynch group by ultrasound, and effect of the stage of estrus cycle at the start of an Ovsynch protocol on the conception rate using timed AI.

A reproductive management protocol has been developed to synchronize time of ovulation in lactating dairy cows (Ovsynch). This protocol allows an effective management of reproduction in dairy cows without continuous detection of estrus. We evaluated whether initiation of an Ovsynch protocol at different stages of the estrous cycle altered the conception rate after timed artificial insemination (AI). A further objective of this study was to analyse the time of ovulation within an Ovsynch protocol by ultrasound. Cows in the Ovsynch protocol were treated with GnRH (d 0), PGF 2α (d 7) and GnRH (d 9) and bred 16 -20 hrs later. Milk samples were collected three times: 7 d prior to 1 st GnRH-treatment, at the 1 st GnRH-injection and at PGF 2α -injection. The first two milk samples were collected in 186 cows to determine stage of estrous cycle at the start of the Ovsynch protocol by milk progesterone concentration. The 3 rd milk sample was collected in 218 cows to determine the presence of a corpus luteum (C.l.) at the time of PGF 2α -injection. Ovaries were scanned in 138 cows via ultrasonography four times: at the second GnRH injection, at AI, as well as 26 hrs and 40 hrs after the second GnRH injection. Milk progesterone levels indicated stages of estrous cycle: 37.6% cows were around the estrus, 26.7% on the beginning of diestrous, 31.7% on the end of diestrous and 4.3% were anestrous cows. First service conception rate did not differ significantly between the different stages of cycle (37.2, 30.6, 33.9, 25.0%, resp.). Synchronization rates did not differ for the different stages of cycle (89.2, 78.8, 88.9 and 80.0%, resp.). Maximum diameter of the ovulatory follicle did not vary by the stage of estrous cycle. The overall percentage of cows ovulating during a 40 hrs period following the second injection of GnRH was 87% (synchronization rate). The percentage of follicles ovulating between 0 to 17 hrs, 17 to 26 hrs and 26 to 40 hrs was 9.4, 20.3 and 57.2%, respectively. First service conception rate did not differ significantly between the ovulation intervals (38.5%, 39.3% and 31.7 %). Maximum diameter of the ovulatory follicle was 15.6 mm without differing significantly between the ovulation intervals (16.4, 16.3 and 15.3 mm). A C.l. was demonstrated in 60.6% of cows at the time of PGF 2α -injection. First service conception rate did not differ significantly between cows with and without a C.l. at PGF 2α -injection (38.6% and 34.9% resp.). In conclusion, overall synchronization rate for the Ovsynch protocol was high. Conception rate did not differ between the ovulation intervals (p>0.05). The stage of estrous cycle at the initiation of an Ovsynch protocol had no significant effect on the size of the ovulatory follicle and conception rate. Abstracts -Oral presentations at 11 th ICPD 197Acta vet. scand. Suppl. 98 -2003