Use of a split or single prostaglandin F2α treatment in a 6-day synchronization protocol for nonlactating dairy cows

The 6-d timed artificial insemination protocol has been designed to advance luteolysis after the first administration of GnRH so that the preovulatory follicular diameter at second GnRH is reduced and thereby pregnancy outcome may be improved. To achieve an earlier and complete luteolysis (5 to 6 d after the first GnRH treatment), an extra PGF2α treatment must be administered to cows 24 h after the initial PGF2α treatment. Although the use of 2 PGF2α treatments increases labor costs resulting from the increased handling of cows, no alternative and efficient protocol with a single PGF2α treatment has been found to date. The objective of this study was to compare the effect of a modified 6-d synchronization protocol on the luteolytic response and final preovulatory follicle diameter. The study followed a crossover design: 14 nonlactating dairy cows were included in 2 treatment doses. All cows received a presynchronization treatment consisting of 2 administrations of a PGF2α analog (PGF) 14 d apart followed by treatment with GnRH 11 d later. After the first GnRH administration, one treatment consisted of 150 µg of d-cloprostenol 5 and 6 d later (split dose) and the other treatment consisted of 375 µg of d-cloprostenol as a single dose 6 d after the first GnRH (single large dose). All cows were then treated with a second GnRH 8 d after the first. The luteolytic response to treatment was evaluated by blood progesterone concentration and CL area regression -1 to 3 d relative to the last PGF treatment obtained by ELISA and ultrasonography, respectively. Fewer cows of the split dose tended to have complete luteolysis 3 d after the last PGF treatment compared with the cows of the single large dose (35.7 and 64.3%, respectively). The final preovulatory diameter of the dominant follicle was similar in cows from the split dose and single large dose (13.7 ± 0.3 and 13.1 ± 0.5 mm, respectively). Our results support the modification of the 6-d synchronization protocol by administering a single high dose of PGF 6 d after GnRH (with the subsequent reduction in labor resulting from reduced handling of animals) without detrimental effects on the luteolytic response of dairy cows and preovulatory diameter of the dominant follicle compared with the original protocol. However, this modification of the 6-d synchronization protocol should be tested in a large field study involving fertility data with lactating cows before its use can be recommended.