Effects of presynchronization and bovine somatotropin on pregnancy rates to a timed artificial insemination protocol in lactating dairy cows

The objective was to examine the effects of presynchronization and bovine s omatotropin (bST) on pregnancy rates to a timed artificial insemination pro tocol in lactating dairy cows. Lactating Holstein cows (n = 543) were assig ned randomly in a 2 x 3 factorial experiment in which cows received a presy nchronization treatment or not, and were treated with bST (500 mg) at 63 +/ - 3, 73 +/- 3, or 147 +/- 3 d postpartum. The latter group was used as a co ntrol. Presynchronization treatment consisted of two injections of PGF(2 al pha) (25 mg) given 14 d apart, with the second injection of PGF(2 alpha) be ing administered 12 d before initiation of the timed artificial inseminatio n protocol. All cows received GnRH(100 mug) at 63 +/- 3 d postpartum, an in jection of PGF2 alpha (40 mg) 7 d later, a GnRH injection at 48 h after PGF (2 infinity) and were inseminated 16 to 20 h later. Cows were resynchronize d if determined to be nonpregnant at ultrasonography at 32 d after insemina tion with a GnRH injection (100 mug), an injection of PGF(2 alpha) (40 mg) 7 d later, and a GnRH injection at 48 h after PGF(2 alpha) and were insemin ated 16 to 20 h later. Cows were examined for pregnancy at 32 d and reexami ned at 74 d after insemination. No differences in pregnancy rates were obse rved between cows receiving bST treatment at 63 +/- 3 d postpartum or at 73 +/- 3 d postpartum An interaction between presynchronization and bST treat ment indicated that pregnancy rates were increased for cows treated with bS T when cows were presynchronized. When anestrous cows were excluded from th e analyses, both an effect of bST and of presynchronization were observed, indicating that bST increased pregnancy rates regardless of presynchronizat ion treatment and that presynchronization also increased pregnancy rates in dependently of bST treatment. Presynchronization and bST treatment may be u sed to increase first-service pregnancy rates to a timed artificial insemin ation protocol.