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Effect of bST and Reproductive Management on Reproductive Performance of Holstein Dairy Cows^*

¹ Veterinary Medicine Teaching and Research Center, University of California-Davis
² Department of Animal Science, University of Florida, Gainesville
³ Monsanto Co., St. Louis, MO

Corresponding author: J. E. P. Santos; e-mail: Jsantos{at}vmtrc.ucdavis.edu).

The objective was to determine the effects of bovine somatotropin (bST) and two artificial insemination (AI) protocols on reproductive performance of Holstein cows. Lactating cows (n = 840) were assigned at 37 d in milk (DIM) to one of four treatments in a 2 x 2 factorial arrangement. Treatments consisted of either bST (500 mg/14 d) starting at 63 ± 3 DIM or no bST (control), with cows either submitted for timed AI following a synchronized ovulation (Ovsynch) protocol or assigned to receive AI based on estrus detection (ED). Two injections of PGF₂ at 37 ± 3 and 51 ± 3 DIM were used to presynchronize estrous cycles. Cows then received an injection of GnRH at 63 ± 3 DIM, followed 7.5 d later by PGF₂. Cows assigned to ED treatments were inseminated after observed estrus during a 7-d period. Cows in Ovsynch treatments received a second GnRH injection 48 h after the last PGF₂ and received timed AI 16 to 18 h later. Pregnancy was diagnosed by ultrasound at 31 d after AI and confirmed 14 d later. Frequency of anovulation (18.5%) at 63 DIM was similar across treatments, but proportions of anovulatory cows decreased quadratically as body condition at 70 DIM increased from 2.25 to 3.75. Estrus detection rate after PGF₂ tended to be lower in multiparous cows receiving bST, and bST reduced returns to estrus in nonpregnant cows. Conception rates were higher in cows receiving AI after ED and bST improved conception rates to first AI in cyclic cows by reducing embryonic mortality. Pregnancy loss was similar for cows inseminated following ED or the Ovsynch protocol. There was a positive impact of bST on fertility of cyclic cows inseminated at fixed time or at detected estrus, but effective resynchronization protocols are needed to optimize reinsemination of nonpregnant bST-treated cows.

Key Words: bovine somatotropin • reproduction • timed AI • dairy cow

Abbreviation key: CL = corpus luteum, ED = group assigned to be inseminated after detection of estrus, GH = growth hormone, Ovsynch = synchronized ovulation using sequential injections of GnRH and PGF₂, TAI = timed artificial insemination

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