Acta Veterinaria Scandinavica 2009, 51:48 http://www.actavetscand.com/content/51/1/48 programs demanding double inseminations are generally out of the question. A protocol using gonadotropin-releasing hormone or its agonists (GnRH) and prostaglandin F2α or its agonists (PGF2α), called the Ovsynch protocol, was developed to synchronise ovulation in dairy cattle [10-14]. The objec- tive of the Ovsynch protocol is to synchronise ovulation within an 8 hour period, enabling good fertility to fixed- time AI without oestrus detection [10]. The Ovsynch pro- tocol has also been applied to postpartum beef cows [1]. The protocol consists of three hormonal treatments: the first one, GnRH, is intended to synchronise follicular waves, the second one, PGF2α, given 7 days later, induces luteolysis, and the third one, GnRH, given 36 to 48 hours after the PGF2α administration, induces ovulation at a pre- determined time. Artificial insemination is performed 16 to 24 hours after the second GnRH administration. Some variations exist in time periods between the treatments. The Ovsynch protocol and its modifications are generally aimed to obtain the best possible conception rate with minimal need of labour. In circumstances explained above, one of the goals is to optimize the number of off- spring per AI dose, since the target is to import new and valuable genetic material into the herd rather than con- ceive all animals per AI. In practice, the conception rate could be improved by selecting the involved animals according to the known facts that affect fertility. In addi- tion, the selection method should be easy enough to apply in field conditions. Vasconcelos et al. [15] have shown that cows having low progesterone concentrations at the time of the administration of PGF2α in the Ovsynch protocol had significantly lower conception rate when compared to that of those cows having high progesterone concentration. The probability that included animals have high progesterone concentration at the time of the treatment can be increased in field conditions by detec- tion of a corpus luteum by palpation of ovaries per rectum [16]. Instead Gümen et al. [17] have shown that anovula- tory cows react well to Ovsynch protocol. Thus, on the day of the first GnRH injection of the Ovsynch protocol, there are not any clear criteria to select the cows into the pro- gram. The aim of the study was to test the usefulness of fixed- time AI following the modified Ovsynch protocol, and its effect on the pregnancy rate in beef cattle managed in Finnish field conditions. The modification is aimed to optimize the number of offsprings per AI dose. In addi- tion, the effect of season, parity, suckling and interval from calving to insemination on the pregnancy results are studied. Methods The experiment was performed in a commercial Charolais elite herd having some 90 cows. In the herd, calvings are divided into two seasons, from March to April and from the end of October to the beginning of December. Thus, breeding periods are June - July and January - February. During the breeding periods the animals involved were loose housed in barns. The experiment lasted three years, from the summer of 1999 to the summer of 2002. Altogether, seven breeding seasons, four in summer and three in winter, were included in the experiment. Before each season, 11 to 27 animals (mean 19) with a high breeding value were selected into the AI program. In all, 90 animals were entered into the program an average of 1.8 times (a maximum of 4 times for 5 animals). Thus, 164 animal cases were included in the experiment, of which 94 were on summer and 70 on winter seasons. Dis- tributions of the animal cases into parity and interval between calving and entering the program classes are pre- sented in Table 1. The majority of the cows were suckling; 19 cows did not lactate. In the beginning of the Ovsynch protocol, all animals selected to the program were administered 10 to 12 μg of buserelin (ReceptalR 4 μg/ml, Intervet International B.V., Boxmeer, The Netherlands). Seven days later, the animals were manually examined per rectum by an experienced operator in order to determine the status of ovarian func- tion. Animals not having a clear palpable corpus luteum (CL) were rejected from the program, while animals with a CL were administered 0.5 mg of cloprostenol (EstrumatR 0.25 mg/ml, Schering-Plough A/S, Ballerup, Denmark) or 0.15 mg of dexcloprostenol (GenestranR 0.075 mg/ml, Vetcare, Salo, Finland). Forty-eight hours after the treat- ment, the animals were administered 8 to 10 μg of buser- elin. Fixed-time AI was performed 16 to 20 hours after the last injection with semen of bulls of known normal fertil- ity. Table 1: Rejection rates (%) due to missing CL of animal cases in different parity * interval from calving to entering the program groups. Interval from calving 50-70 71-100 101- Total Parity 0 1 2-5 6- Total 25.0 (8) 18.2 (11) 25.0 (4) 21.7 (23) 10.7 (28) 16.9 (59) 40.0 (10) 17.5 (97) 25.0 (8) 25.0 (12) 50.0 (2) 27.3 (22) 27.3 (22) 15.9 (44) 18.3 (82) 37.5 (16) Numbers of animal cases in different groups are shown in parentheses. Page 2 of 6 (page number not for citation purposes)