Acta Veterinaria Scandinavica 2009, 51:48 http://www.actavetscand.com/content/51/1/48 Eighteen days after the AI, a bull was introduced to the herd in order to serve cows that returned to oestrus. Preg- nancy examinations were performed by palpation per rec- tum six to eight weeks after the AI, when it was easy to detect pregnancies conducted from AI. All examinations were made by one and the same experienced veterinarian. Data were analysed using the SPSS 13.0 for Windows soft- ware. To study the effect of season, suckling, parity and interval between calving and entering the program among cows on the rejections and pregnancy rates, the data were analysed using logistic regression for binomially distrib- uted data. Before analyses, parity was classified to three groups, first parity cows, middle-aged cows (parity 2-5) and old cows (parity 6-), and interval between calving and entering the program to three groups, short (50-70 days), medium (71-100 days) and long (101-days) interval. For parity and interval, the last and first category, respectively, served as a reference category. Initially, all the variables were included in the analysis. In the backward stepwise analysis procedure, non-significant variables were omit- ted. Chi-square test was used to analyse the differences in rejections and pregnancy rates between cows and heifers. P-values less than 0.05 were considered significant. Results Of 164 animal cases, 34 (20.7%) were rejected from the program since no CL were detected on the day of clopros- tenol/dexcloprostenol administration. During summer and winter seasons, 20.2% and 21.4% were rejected, respectively. The rejection rates in parity * interval between calving and entering the program (later "time interval") groups are presented in Table 1. Differences in rejections in cows between parity (p = 0.132), suckling (p = 0.160) and time interval groups (p = 0.214) and seasons (p = 0.932) were not statistically significant. The rejections in heifers were not significantly different (p = 0.416) from those in cows. However, heifers and old cows were rejected numerically 10 to 20 percentage points more than younger, parity 1-5, cows. During the experiment 130 AIs were performed, of which 67 (51.5%) led to pregnancy. Distributions of the AIs into parity and time interval classes are presented in Table 2. In summer and winter seasons, the pregnancy rates were 53.3% and 49.1%, respectively. Effects of season and suckling were not statistically significant (p = 0.964 and p = 0.776, respectively). Thus, these variables were omitted form the final analysis. The pregnancy rates in parity * time interval groups are presented in Table 2. Time inter- val from calving to the start of the program affected con- ception. The differences in pregnancy rates in cows between the three time interval groups were statistically significant (p = 0.029). In particular, pregnancy rate in the group having medium time interval (71-100 days, p = Table 2: Pregnancy rates (%) after fixed-time artificial insemination in different parity * interval from calving to entering the program groups. Parity 0 1 2-5 6- Total Interval from calving 50-70 71-100 101- Total 83.3 (6) 100.0 (9) 0.0 (3) 77.7 (18) 56.0 (25) 49.0 (49) 33.3 (6) 50.0 (80) 16.7 (6) 55.6.(9) 0.0 (1) 37.5 (16) 43.8 (16) 54.1 (37) 56.7 (67) 20.0 (10) Numbers of artificial inseminations in different groups are shown in parentheses. 0.019) as well as in the group having long time interval (100-days, p = 0.010) were significantly lower when com- pared to the reference group with short interval (50-70 days). The effect of parity on pregnancy rate was not as clear as the effect of time interval. The overall differences between the three parity groups of cows tended to be statistically significant (p = 0.063). However, the pregnancy rates of the first parity and middle-aged cows, when contrasted to that of the old cows (the reference group), were signifi- cantly (p = 0.032 and p = 0.019, respectively) higher. The pregnancy rates in heifers were not significantly different (p = 0.506) from those in cows. However, the pregnancy rates in heifers seemed to be numerically somewhat lower than in parity 1-5 cows. Discussion The average pregnancy rate of 51.5% was reached with a slightly modified Ovsynch protocol in this Charolais beef herd. During the preceding years, the approximate preg- nancy rate after AI based on oestrus detection had varied around 20% when calculated according to the herd book- keeping. No data about overall pregnancy results after AI in beef cattle in Finland are available, but in dairy cattle, the 60-days non-return rate is about 63%, and thus, the real pregnancy rate can be expected to be very close to 50%. Originally the Ovsynch protocol for synchronisation of ovulation was developed for reproductive management in dairy herds [10]. Since this paper, many studies have eval- uated the fertility of lactating dairy cows following the Ovsynch protocol, and pregnancy rates per AI have varied from 27% to 39% [11-13,15,18,19]. These pregnancy rates have been similar to [11,12] or only slightly lower [18,20] than the pregnancy rates with AI after oestrus detection or after oestrus detection following synchroni- sation of oestrus with PGF2α in the control cows. The Page 3 of 6 (page number not for citation purposes)