A method for synchronization of ovulation (OVSYNCH) was evaluated for its practical use in seven North German dairy cattle herds. The original procedure of PURSLEY et at. (1995) was applied, i. e. cows were injected Gonadotropin Releasing Hormone (GnRH) at a random stage of the estrous cycle, followed by an injection of a prostaglandin F-2 alpha (PGF(2 alpha))-analogon seven days later and another injection of GnRH two days after PGF(2 alpha). Cows were inseminated 24 h after the last injection by artificial insemination. Control animals were assigned by corresponding lactation number and stage of lactation (month of partus). Cows from the OVSYNCH group returned to the conventional reproductive management after the OVSYNCH treatment, which was the same management as that for the control group. The average milk yield in 1997 and 1998 was 8128.6 +/- 637.0 kg and 8688.3 +/- 665.0 kg per year, respectively. First service conception rate (FSC) in the OVSYNCH group (n = 187) (39.77 %) was lower than in the controls (n = 175) (54.34 %; P < 0.01). In 1997, FSC for the OVSYNCH group did not differ from controls (48.94 % vs. 55.45 %; P > 0.05), whereas in 1998, FSC for the OVSYNCH group (29.27 %) was significantly lower than in the control group (29.27 % vs. 52.78 %) and the OVSYNCH group in 1997 (each time P < 0.01). First service conception rate of first lactation cows in the OVSYNCH group was significantly lower than in the control group (37.84 % vs. 71.05 %; P < 0.01), whereas no difference was found for the other lactation groups. The mean interval calving to first service was shortened in the OVSYNCH group by about 14 d as compared with the controls (P < 0.01). However, mean days open only differed by 2.92 d (P > 0.05). Proportion of cows in each class of lactation number (1., 2. and > 2. lactation number) with days open less than or equal to 115 d in the OVSYNCH group were similar to controls (P > 0.05). There was a great influence of body condition on the success of OVSYNCH. Cows with a body condition score (BCS) of 3.00 had a significant better FSC than cows with a BCS < 3.00 (P < 0.05). First service conception rate was lower for cows with a BCS > 3.25 and < 2.75 (cows in good and poor condition taken together) than for cows with a BCS of 3.00 (P < 0.05). A positive effect of OVSYNCH was found in acyclic cows as determined by progesterone concentration in blood plasma. Nine out of eleven cows reacted with the resumption of ovarian activity. Two cows became pregnant after first insemination.
