Abstract
We evaluated if an additional GnRH injection 7 days before pre-synchronization with simultaneous PGF2α and GnRH (PG+G) would improve responses to presynchronization, synchronization, and pregnancy per AI (P/AI). We hypothesized that administering GnRH 7 days before PG+G would increase ovulation and corpus luteum (CL) presence at the PG+G, improve response to OvSynch treatments and P/AI. Holstein cows were blocked by parity and randomly assigned to either a PG+G (Control, n = 205); or to GnRH followed 7 days later by PG+G (ExtG, n = 201). At enrollment, Control was left untreated, whereas ExtG received GnRH. Seven days after enrollment, Control and ExtG received PG+G followed by OvSynch 7 days later (GnRH, 7 days PGF2α, 56 h GnRH, 16 h timed AI). Ovarian dynamics were assessed using ultrasonography in a subset of cows (n = 53 for Control; and n = 50 for ExtG) at each treatment, except the 2nd GnRH of OvSynch. Pregnancy diagnosed at 32- and 67-days post AI. Ovulation at enrollment tended (P = 0.06) to be higher for ExtG, but ovulation was not different at PG+G (P = 0.41) and first GnRH of the OvSynch (P = 0.25). There was a tendency (P = 0.08) for ExtG to have larger CL than Control at PGF2α of the OvSynch. There were no differences in CL and follicle sizes in any other treatment point assessed. There were no differences (P = 0.12) in luteolysis between treatments after PG+G. Overall P/AI was similar between treatments on Day 32 (Control = 33.0% vs. ExtG = 34.6%, P = 0.75) and 67 (Control = 31.8% vs. ExtG = 32.5%, P = 0.29) post AI. There was a tendency for an interaction between treatment and parity (P = 0.09) for P/AI at day 67 post-AI. In multiparous cows, ExtG tended to have greater P/AI than Control, whereas, in primiparous cows Control tended to have greater P/AI than ExtG at day 67 post-AI. In conclusion, the effects of GnRH 7 days before PG+G presynchronization lead to positive and negative tendencies, respectively, in multiparous and primiparous cows for P/AI at day 67 post-AI and needs further investigation.
Highlights
There is a body of evidence indicating that 25 to 30% of lactating dairy cows remain anovular at 50 to 70 days in milk (DIM) [1,2,3,4,5,6]
There was no difference in the percent of cows with an ovulatory follicle at enrollment (P = 0.65), at PGF2α and GnRH (PG+G) (P = 0.12), at first GnRH of Ovsynch (P = 0.34), and PGF2α of the OvSynch (P = 0.29) between Control and ExtG (Table 1)
Cows receiving the extra GnRH 7 days before the PG+G tended to have greater ovulation at the time of GnRH (P = 0.06) when compared with the Control cows not receiving the extra dose of GnRH (Table 1)
Summary
There is a body of evidence indicating that 25 to 30% of lactating dairy cows remain anovular (defined by the absence of a functional CL) at 50 to 70 days in milk (DIM) [1,2,3,4,5,6]. Double-OvSynch demonstrated improved outcomes when compared to the classic Presynch (two PGF2α treatments 14 days apart) followed by OvSynch 12 days later [11]. A presynchronization that helps anovular cows such as the DoubleOvSynch, with an additional injection of PGF2α at the end of the program for improved luteolysis, requires 4 days per week for treatments [13]. The G6G presynchronization (PGF2α followed 2 days later by GnRH and 6 days later first GnRH of the OvSynch) has fewer overall treatments than the Double-OvSynch, but still requires treatments 4 days per week to complete all of the injections when combined with OvSynch [8]. Possible improvements of the presynchronization program with GnRH and PGF2α may be possible by reducing the randomness in regard to which day of the estrous cycle cows receive these treatments. Any additional steps to improve the PG+G presynchronization may only be attractive if it does not increase the number of days per week on which treatments are administered
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