br Conclusion br Author contributions br Competing interests

Conclusion
Author contributions
Competing interests
Acknowledgments
Introduction Reproductive performance is a major factor affecting profitability of dairy farms. The use of GnRH and prostaglandin F2α (PGF2α) in a coordinated fashion allows cows to be inseminated as a result of the use of a timed artificial insemination (TAI) protocol without the need for detection of estrus (Pursley et al., 1995, 1997). The use of Ovsynch protocol results in an increase in the percentage of cows inseminated earlier after the end of the voluntary waiting period, thus increasing the proportion of cows that become pregnant earlier in lactation (Pursley et al., 1997). The Double-Ovsynch (Souza et al., 2008) hormonal treatment regimen is a later development of the original Ovsynch program, and is widely used in high producing dairy herds in USA. In the United States, several GnRH products are commercially available and approved by the Food and Drug Administration to be used as a part of breeding protocols for dairy and beef herds. These products may have different gonadotropin salts and diluent composition that may affect whether there is an ovulation as a result of treatment with the Biotin-16-dCTP (Martínez et al., 2003; Souza et al., 2009). As a result, it is possible that there are lesser ovulatory responses as a result of GnRH administration for induction of ovulation with hormonal treatment regimens which ultimately results in a lesser cow fertility. In a previous study (Martínez et al., 2003), both the surge release of LH and ovulatory responses were less for cows treated with gonadorelin hydrochloride compared with cows treated with gonadorelin diacetate-based products; however, the effect of type of GnRH on P/AI was not assessed in the previous study. Results of another study indicated there were no differences in P/AI between cows where gonadorelin hydrochloride and gonadorelin diacetate were used in the hormonal treatment regimen for conducting the first postpartum AI (Poock et al., 2015). Poock et al. (2015) used a hormonal treatment regimen in cows for conducting the first postpartum AI that included two PGF2α treatments 14 days apart or enrolled cows to a resynch protocol. Both factors (poor wave pre-synchronization with the two PGF treatments during presynch as well as random stage of the estrous cycle at start of resynch) will set cows to start Ovsynch at variable stages of follicle growth, which will lower the ovulatory responses to the first GnRH of Ovsynch (Souza et al., 2008; Ayres et al., 2013). In addition, when there was use of a single PGF2α treatment, there was an increase in P/AI as a result of the relatively greater ovulatory response to the 1st GnRH injection during the Ovsynch protocol; however, the advantages from the greater ovulation rate may be negated when complete luteal regression is not accomplished (Poock et al., 2015).
Materials and methods
Results Ovulatory response on D17 after treatment with the Double-Ovsynch hormonal regimen tended to differ as a result of type of GnRH salt compound administered. With use of the regression model, there was detection of a trend (P =  0.07) for a lesser ovulation rate when there was administration of hydrochloride-based GnRH compared with the Diacetate salt. Ovulation response as a result of administration of GnRH salts on D17 did not differ with administration of the three commercial GnRH products (Fig.1). Circulating concentrations of P4 at D17 of Double-Ovsynch did not differ as a result of treatment with the different GnRH salts and averaged 3.7 ± 0.3, 3.5 ± 0.3, and 3.3 ± 0.3 in the CYS, FAC, and FER treatment groups, respectively. Similarly, proportion of cows with a CL on D17 of Double-Ovsynch was similar among the treatment groups (CYS = 98.4%; FAC = 95.3%; FER = 95.0%). As a result of a more precise analysis for predicted ovulatory responses as indicated by differing circulating P4 concentrations on D17, there were indications of lesser ovulation rates for cows treated with gonadorelin hydrochloride than cows treated with gonadorelin diacetate, as well as for cows treated with FAC compared with FER and CYS (Fig. 2). In addition, when there were greater P4 concentrations at the time of GnRH administration on D17, ovulatory response to the GnRH salt treatments was less (P <  0.01) for all treatment groups (Fig. 2). There was also no interaction in terms of ovulation response between P4 concentration on D17 and type of salt or GnRH product administered (Fig. 2).