Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2001
Publication Date: 6/1/2001
Citation: FOSTER, H.A., WHITTIER, J.C., BURNS, P.D., BRUEMMER, J.E., FIELD, T.G., GEARY, T.W. HALF DOSE GNRH DOES NOT AFFECT PREGNANCY RATES WITH THE CO-SYNCH ESTRUS SYNCHRONIZATION PROTOCOL. WESTERN SECTION OF ANIMAL SCIENCE PROCEEDINGS. 2001. v. 52. p. 374-376. Interpretive Summary: The CO-Synch and Select Synch protocols are ovulation and estrous synchronization protocols, respectively that use injections of gonadotropin releasing hormone (GnRH) and prostaglandin (PGF). Their acceptance by beef producers has been limited in part because of the cost of GnRH. Dairy trials have shown that reduced dosages (50 instead of 100 mcg of GnRH; 1 cc verses 2 cc injections) do not affect estrous and pregnancy rates. These data agree with the dairy data and report that using half dose GnRH does not affect estrous and pregnancy rates in beef cattle. These data suggest that producers could use half dosages of GnRH with the Select Synch and CO-Synch protocol; however, caution should be performed during administration. During this study, the injection of GnRH was performed carefully to ensure that cows were receiving their entire 1 cc or 2 cc dose. In an applied production scenario, where beef cows routinely receive injections in alleyways, injectables sometimes leak out of the injection site. As long as producers still use the labeled dosage (2 cc), up to half of the GnRH could leak from the injection site before any decrease in response might be expected.
Technical Abstract: Primiparous (n = 76) and multiparous (n = 328) lactating Angus cows were randomly assigned to receive either a 50 mcg or 100 mcg dose of GnRH at either the first or second GnRH injection of the CO-Synch estrus synchronization protocol to evaluate the efficacy of a reduced dosage in inducing a fertile ovulation. The result was four separate treatment groups: 50:50, 50:100, 100:50, and 100:100. The CO-Synch protocol traditionally consists of GnRH on d 0, followed by an injection of PGF on d 7. On d 9 cows were injected with another GnRH injection and mass inseminated. On d -10, and again on d 0, blood samples were obtained to examine progesterone levels and determine the proportion of anestrous cows. Calves were removed for a 48 h period on d 7 and were put back with the cows immediately after breeding. Clean- up bulls were turned in with the cows on 14 d following AI. Cows were pregnancy checked via transrectal ultrasonography 68 d after AI. Serum progesterone levels indicated that 34.9% of the cows were estrual at the start of breeding. All treatments were equally effective (P > 0.05) in inducing anestrus cows to ovulate and become pregnant. There was no difference in pregnancy rates among the treatment groups (50:50 = 50.7%; 50:100 = 49.3%; 100:50 = 50.3%; and 100:100 = 44.6%, P > 0.05), thus indicating that a reduced dosage of GnRH is effective in inducing fertile ovulation in the CO-Synch timed insemination protocol. We conclude that using a lower dose of GnRH is an effective method of reducing cost of synchronization with the CO-Synch protocol.