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The Effect of Presynchronization and Bovine Somatotropin on Timed AI in Lactating Dairy Cattle
by Teresa L. Steckler and Darrel J. Kesler


  • Pregnancy rates for cows treated with bST were greater when the cows were presynchronizationed
  • bST increased pregnancy rates in estrus-cycling cows regardless if cows were presynchonization
  • Presynchronization increased pregnancy rates in estrus-cycling cows regardless if cows were administered bST


Bovine somatotropin (bST) results in increased milk yields and improvement in efficiency in dairy cattle. bST is involved in orchestrating many physiological processes, including metabolism of carbohydrates, lipids, proteins, and minerals and alterations in key enzymes. Commercial use began in 1994 in the U.S. and has been extensively adopted. Since 1994 several synchronization programs utilizing gonadotropin releasing hormone (GnRH) and prostaglandin F2" (PGF) have been introduced and discussed for use in dairy cattle. The purpose of this article is to report research conducted that describes the effects of presynchronization and bST on pregnancy rates to a timed artificial insemination protocol in lactating dairy cattle (Moreira et al., 2001).


This experiment included 543 lactating dairy cows from a commercial dairy farm in Florida kept in free-stall facilities and milked three times daily. The experiment was designed as a 2X3 factorial experiment (6 treatment groups) with 2 main effects: presence or absence of presynchronization and the day that bST treatment was begun. Cows were assigned to treatment when they were 37±3 days post partum (PP).

Presynchronization consisted of 2 injections of PGF (25 mg), given 14 d apart (Figure 1), then 12 days later a modified OvSynch protocol was begun. The modified Ovsynch protocol consisted of an initial injection of 100 :g of GnRH, followed 7 days later by 40 mg of PGF, then 2 days later a second 100 :g injection of GnRH. (The original Ovsynch protocol required a 25 mg injection of PGF versus the 40 mg employed in this trial). Cows were bred 16 to 20 hours after the last GnRH injection. All cows received their first service insemination 73±3 days post partum. BST injections were begun either on 63±3, 73±3 or 147±3 days postpartum and injections were administered every 14 days until 30 d before the dry period. Pregnancy was determined 32 days after AI by ultrasonography and by rectal palpation 74 days after AI.


Pregnancy Rates

Table 1 shows pregnancy rates (PR) for all treatment groups for All cows or Cycling cows. Cows were considered to be cycling if one or both blood samples collected 51 and 63 d PP had >1.0 ng/mL of plasma progesterone.

The administration of bST during synchronization with Ovsynch did not decrease pregnancy rates in All cows. However, presynchronization and the administration of bST improved PR over the Control/bST-treated cows in All cows.

Cycling cows also shows similar trends. Those cows that had been treated with presynch had greater PR at both 32 and 74 days after AI. Again bST did not negatively affect PR in Ovsynch-treated cows.

Pregnancy rates in cycling cows were greater when treated with bST than for all cows.

Pregnancy Losses

Pregnancy loss overall was 14.6% between 32 and 74 days after AI. A trend (P<0.01) was observed between presynchronized and cows not presynchronized. Pregnancy loss was less for those cows presynchronized.

Table 1. First service pregnancy rates at 32 and 74 days after timed AI for all cows and cycling cows (least square means).

  All Cows Cycling Cows
Treatment Groups PR (%) 32d after AI PR (%) 74d after AI PR (%) 32d after AI PR (%) 74d after AI
Control/bST-63 32.5a 29.4a 39.1cx 34.2cw
Control/bST-73 41.5a 28.1a 50.6cx 33.7cw
Control/bST-147 36.0a 30.1a 34.4cy 25.3cz
Presynch/bST-63 52.8b 45.4b 67.8dx 58.2dw
Presynch/bST-73 55.5b 48.6b 63.5dx 56.1dw
Presynch/bST-147 36.9a 33.6a 46.9dy 42.6dz

a,bDifferent superscripts within columns of all cows indicate presynch by bST interaction (P<0.01). c,dDifferent superscripts within columns of cycling cows indicate presynch effect (P<0.01). x,yDifferent superscripts within columns of cycling cows indicate bST effect (P<0.01). w,zDifferent superscripts within columns of cycling cows indicate bST effect (P<0.04).


Moreira, F., C. Orlandi, C.A. Risco, R. Mattos, F. Lopes, and W.W. Thatcher. 2001. Effects of presynchronization and bovine somatotropin on pregnancy rates to a timed artificial insemination protocol in lactating dairy cattle. J. Dairy Sci. 84:1646-1659.

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