Decreasing the dose of equine chorionic gonadotropin does not affect ovarian or pregnancy responses of purebred taurine and crossbred beef heifers

Highlights

• The fertility of taurine beef heifers is not decreased by the eCG doses reduction.

• The ovarian dynamic is influenced heifers cyclicity.

• There was no interaction between CL presence and eCG reduction doses.

Abstract

In this study there was evaluation of effects of different doses of equine chorionic gonadotropin (eCG: 200, 300, or 400 IU) administrated at progesterone (P4) plus estradiol-based timed AI (TAI). A total of 1080 heifers were included in the study. There was insertion of the intravaginal P4-device plus administration of 2 mg of estradiol benzoate IM. On D7, 12.5 mg of dinoprost tromethamine IM was administered and on D9, the P4 insert was removed and 0.5 mg of estradiol cypionate IM was administered. Heifers were categorized according to Reproductive Tract Status (RTS; 1−5) and were assigned to one of three treatments: 200 IU (n = 387), 300 IU (n = 357), or 400 IU (n = 336) of eCG. Estrous occurrence was evaluated at TAI 48 h later (D11). A subset of heifers (n = 213) had the largest follicle (LF) evaluated on D9 and on D11, and the formation of a new CL evaluated on D18.There was no effect of eCG treatment on LF on D11 (P = 0.79), occurrence of estrus (P = 0.92), and pregnancy at 30 days after AI (P/AI; 52.2%, 49.8%, and 51.5% for 200 IU, 300 IU, and 400 IU, respectively; P = 0.46). Regardless of the treatment, there was a greater P/AI when heifers had a functional CL, at initiation of the estrous synchronization treatment regimen. It, therefore, is efficacious to reduce the dose of eCG to 300 or 200 IU in purebred taurine and crossbred beef heifers without negative effects on ovarian, estrous or pregnancy responses.

Introduction

In beef cattle production systems, fertility is a key factor impacting the economics of beef production. Heifers that reach puberty earlier are more likely to become pregnant in the first breeding season and have greater lifetime productivity (Cushman et al., 2013; Wathes et al., 2014).

The increased fertility with use of progesterone (P4) plus estradiol (E2) treatment regimens is due to the additional stimulus of follicular development, enhancing the probability of ovulation and pregnancy (Sá Filho et al., 2009, 2010; Sales et al., 2011; Bó and Baruselli, 2014). Particularly in heifers and non-lactating cows, eCG treatment has also been used to minimize the negative effects of P4 on follicular development (Peres et al., 2009; Rigoglio et al., 2013). Furthermore, treatment with eCG leads to the formation of a larger corpus luteum (CL) and increased circulating P4 in subsequent estrous cycles (Baruselli et al., 2004; Bó and Baruselli, 2014; Sá Filho et al., 2010; Sales et al., 2011; de Sousa et al., 2016).

The conventional dose (400 IU) of eCG is a large proportion of the total treatment cost (48.7 %) (Sá Filho et al., 2010). Decreasing the dose of eCG while maintaining pregnancy results can be economically advantageous.

Results of studies in suckled Bos indicus cows indicate reducing the dose of eCG from 400 IU to 300 IU in P4 + E2-based estrous synchronization treatment regimens does not affect pregnancy rate (Sá Filho et al., 2009). In addition, results of other studies indicate satisfactory efficiency of smaller doses, such as 200 IU, of eCG in suckled B. indicus beef cows and heifers (Peres et al., 2009; Doroteu et al., 2015). In Bos taurus suckled beef cows, the decrease from the conventional dose used (400 IU) to 300 IU of eCG also resulted in satisfactory results (Pessoa et al., 2016). With a GnRH + PGF-based TAI treatment regimen (Marquezini et al., 2013), however, there was not any improvement in pregnancy rate per TAI (P/AI) after administering 200 IU of eCG. In heifers, results of studies indicate there can be use of 300–400 IU of eCG at the time of progestin removal with desireable outomes (Baruselli et al., 2004; Bó et al., 2012, 2016). There, however, are no studies in which there was comparison of different doses and its effects on rate of follicular development and pregnancy Evaluating the effect when there is administration of smaller than the conventional dose of eCG to purebred taurine and crossbred beef heifers could be important in reducing the costs of TAI programs and deserves further investigation.

It was hypothesized that in purebred taurine and crossbred beef heifers there could be use without affecting P/AI percentages of the P4 + E2-based treatment regimen with TAI combined with administration of a smaller than conventional dose of eCG. The aim of the present study, therefore, was to evaluate the effects of the eCG doses (400, 300, or 200 IU) administered at the time of P4 insert removal on ovarian follicular growth, diameter of the newly formed CL, and on P/AI percentages.