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The objective of the study was to investigate factors that may influence conception rate (CR) after synchronization of ovulation and timed artificial insemination (TAI). A total of 172 lactating Holstein dairy cows were submitted 63±3 days postpartum to a synchronization protocol consisting of two treatments with GnRH (0.02 mg buserelin) 9 days apart and one treatment with PGF2a (0.15 mg D-cloprostenol) 7 days after the first administration of GnRH. TAI was carried out either 6 or 16 hours after the second treatment with GnRH. Primiparous cows were kept in the same barn but in a group separated from multiparous cows. Blood samples were collected from all cows 7 days after TAI to determine the presence of a functional corpus luteum and to assess the metabolic situation at arrival of the conceptus in the uterus. Furthermore, body condition, milk production during the week prior to PGF2a and apparent clinical diseases were recorded. Data were compared using Chi-square test and Mann-Whitney-U-test.
In accordance with other reports (Pursley et al. 1998), timing of AI 6 or 16 hours after the second GnRH treatment had no significant impact on CR (23.3% vs. 27.9 %) and was higher in primiparous than in older cows (45.1 vs. 17.4 %, p<0.01, Tenhagen et al. 2004). A body condition < 2.75 was associated with poorer conception rates compared to cows in better condition (17.0 vs. 35.9 %, p<0.01). Cows with at least one other health disorder had lower conception rates than those without disorders (17.8 vs. 37.3 %, P<0.05). Both parameters were significantly worse in multiparous than in primiparous cows. Therefore, low BCS and presence of health disorders may have contributed to the lower CR in multiparous cows. Differences between age groups may have been biased by separate housing of primiparous cows. However, both age groups were housed opposite of each other in identical facilities and received the same total mixed ration in the same feeding trough.
Activity of AST and GLDH and serum levels of bilirubin did not differ significantly between cows that conceived and those that did not. This indicates that the liver metabolism one week after TAI is of limited importance for the risk of conception. Serum urea, on the other hand, was higher in cows that conceived (338 vs. 313 mg/l, p<0.05). This is in contrast to reports on the negative effect of high serum urea on fertility (Butler et al. 1996). The reason for this finding remains unclear. There was no association of the metabolic parameters with the two parity classes.
Milk production level did not affect conception rates in cows of second and greater lactation (39.7 vs. 39.3 kg/day). In primiparous cows, production was lower in cows that conceived than in those that did not conceive (28.4±4.1 vs. 31.0±4.3, p<0.05). In a recent study, milk production level had no significant effect on CR after TAI using the same synchronization protocol (Tenhagen et al. 2003).
It has been proposed, that insufficient luteal activity may contribute to conception failure in high yielding cows. In our study, progesterone levels 7 days after TAI did not differ significantly between cows that conceived and non pregnant cows.
Overall, results underline the importance of the general condition of cows for the success of reproductive management programs. More research is needed to analyze the differences between primiparous and older cows in this and in other studies and their possible relations to separate housing of primiparous cows.