Use of Bovine Somatotropin in Lactating Dairy Cows Receiving Timed Artificial Insemination

¹ Department of Dairy and Poultry Sciences, University of Florida, Gainesville, 32611
² Large Animal Clinical Sciences, University of Florida, Gainesville, 32610
³ CNPGL-EMBRAPA, Coronel Pacheco, MG 36155-000, Brazil
⁴ Alberta Agriculture, 6909-116 Street, Edmonton, Alberta T6H 4P2, Canada

Objectives of the research were to examine the effect of bovine somatotropin (bST) on pregnancy rates to a timed artificial insemination protocol and to test a re-synchronization system with two consecutive synchronized services. Lactating Holstein cows (n = 403) were assigned to the following treatments: bST treatment (500 mg) was initiated at 63 ± 3 d postpartum concomitantly with initiation of the timed artificial insemination protocol or bST treatment was initiated at 105 ± 3 d postpartum. At 63 ± 3 d postpartum, all cows received GnRH (100 µg), an injection of PGF₂ (25 mg) 7 d later, and a GnRH injection at 48 h after PGF₂ and were inseminated 16 to 20 h later. Cows were reinseminated at detected estrus or resynchronized with a GnRHinjection at 20 d after insemination. At 27 d after insemination, cows were examined for pregnancy. Resynchronized cows diagnosed nonpregnant received an injection of PGF₂ and were inseminated at detected estrus or received an injection of GnRH at 48 h after PGF₂ and inseminated 16 to 20 h later. Cows pregnant at d 27 were reexamined for pregnancy at 45 d after insemination. First-service pregnancy rates at d 45 were increased in cows not resynchronized that initiated bST treatment at 63 ± 3 d postpartum, compared with cows initiating bST treatment at 105 ± 3 d postpartum (37.7 ± 5.8% and 22.1 ± 4.2%, respectively), but the effect of bST treatment was not observed when cows were resynchronized (25.6 ± 4.3% and 25.8 ± 5.5%, respectively). Thus, bST increased pregnancy rates to a timed artificial insemination protocol.

Key Words: timed artificial insemination • bovine somatotropin • lactating cows

Submitted on August 13, 1999
Accepted on January 5, 2000

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