Pregnancy per AI results at days post
Pregnancy per AI results at 32 days post TAI tended to differ among GnRH products and was greater for FER and CYS than FAC according to the Tukey-grouping analysis statistical approach (Table 1). In addition, P/AI was greater for cows treated with gonadorelin diacetate products than for cows treated with gonadorelin hydrochloride (Table 1). Similarly, at 60 d after TAI, P/AI differed among groups and was greater for cows treated with FER and CYS than FAC, and was also greater for cows treated with gonadorelin diacetate than cows treated with gonadorelin hydrochloride (Table 1). Pregnancy loss between 32 and 60 d after TAI did not differ as a result of treatment with different GnRH products or GnRH salts (Table 1).
Discussion A GnRH treatment induces a surge release of LH and FSH from the anterior pituitary and ovulation from a Sulforaphane follicle (Vasconcelos et al., 1999; Giordano et al., 2012, 2013). Interestingly, there were indications from results in a previous study that there was a lesser amplitude of the induced LH surge and ovulatory response in cows treated with gonadorelin hydrochloride compared to cows treated with gonadorelin diacetate (Martínez et al., 2003). These previous findings may indicate the lesser ovulatory responses as well as lesser P/AI in the current trial occurred because of a lesser amplitude of the induced preovulatory LH release when there was administration of gonadorelin hydrochloride as compared with what occurred as a result of the other GnRH salt treatments. The exact mechanism by which gonadorelin diacetate induces a greater GnRH-induced surge release of LH and subsequent ovulatory response and possibly fertility is unknown, but results of the present study indicate the lesser ovulation response to gonadorelin hydrochloride is independent of circulating P4 concentrations at the time of GnRH administration. Perhaps the hydrochloride salt has a lesser capacity to be transported to the GnRH receptors of the anterior pituitary. Alternatively, the two salts may differ in half life in circulation which could lead to lesser efficacy of the GnRH hydrochloride salt compound. Furthermore, these differing GnRH salt compounds may have different tissue perfusion patterns that contribute to variation in capacity to induce LH release from the anterior pituitary. Basic research exploring pharmacological features of these differing GnRH salts is lacking and if conducted would lead to an enhanced understanding of the biological effects of these GnRH salts. The findings from conducting the logistic regression analysis indicate that the greater the circulating P4 concentrations the lesser the ovulation induction with use of all types of GnRH products and salts in the current study, and these findings are consistent with those from previous reports. For example, Giordano et al. (2012) also evaluated the ovulation-induction effect of circulating P4 concentrations at the time of the GnRH treatment. Cows with greater (3.5 ng/mL) P4 concentrations had a lesser amplitude of the GnRH-induced LH release than cows with lesser (0.2 ng/mL) P4 concentrations (3.3 compared with 15.7 ng/mL). When there is consideration of previous and present research findings, it is obvious when P4 concentrations exceed 1 ng/mL the amount of LH secreted as a result of GnRH administration is less (Giordano et al., 2012). In a subsequent study, Giordano et al. (2013) evaluated the effect of relatively larger GnRH doses on ovulatory response. Interestingly, doubling the GnRH dose resulted in an increase in ovulatory response only in cows with P4 concentrations exceeding 1 ng/mL at the time of GnRH administration, and there was no effect of GnRH dose in cows with P4 concentrations of < 1 ng/mL. Thus, depending on the reproductive management schedule used on a farm and the stage of the estrous cycle at beginning of the reproductive programs, increasing the GnRH dose might be something to be explored by the herd veterinarian to maximize synchronization in the breeding protocol.