Administering an additional prostaglandin F2α injection to Bos indicus beef cows during a treatment regimen for fixed-time artificial insemination
Introduction
Estrous synchronization treatment regimens based on estradiol + progesterone (P4) facilitated the use of fixed-time artificial insemination (FTAI) in Bos indicus cattle in South America (Vasconcelos et al., 2014), with satisfactory pregnancy results per FTAI independently of cow estrous cyclicity status at the beginning of the treatment regimen (Meneghetti et al., 2009). A prostaglandin F2α (PG) injection is an important component of these treatment regimens by inducing the regression of the corpus luteum (CL), and consequently decreasing circulating P4 concentrations to facilitate the synchronous timing of ovulations among cows (Roche, 1974; Schams and Berisha, 2004). If luteolysis is incomplete, residual P4 concentrations near the time of FTAI negatively affect pregnancy per FTAI in dairy and beef cows (Pereira et al., 2013; Rodrigues et al., 2018). Research efforts, therefore, have focused on refining PG administration during FTAI treatment regimens to result in a complete CL regression.
In dairy cows assigned to a GnRH-based treatment regimen for FTAI, the inclusion of an additional standard PG injection increased the proportion of cows with complete luteolysis compared with the original single PG administration (Brusveen et al., 2009). Ribeiro et al. (2012) reported improved luteolysis and pregnancy per FTAI in dairy cows administered the PG dosage in two injections compared with a single standard injection. Pereira et al. (2015) noted similar benefits of an additional PG injection to dairy cows assigned to an estradiol-based treatment regimen for FTAI, and such outcomes were observed in cows with or without a CL when the estrous synchronization treatment regimen was being administered. With all these experiments, the additional PG injection was administered 24–48 h after the initial PG administration of the treatment regimen. Results from Pereira et al. (2015) indicate the additional PG injection may result in improvement in pregnancy rate per FTAI as a result of other factors rather than induction of complete luteolysis.
Prostaglandin F2α is known to stimulate LH release by the pituitary gland (Warberg et al., 1976). In beef cattle, PG administration enhanced GnRH-induced LH release in postpartum B. indicus cows (Randel et al., 1996) and in estrous cyclic or anestrous B. taurus females (Cruz et al., 1997). The prevalence of anestrous cows at the beginning of the breeding season is a major reproductive problem in B. indicus--based cattle enterprises, primarily because of inadequate pulsatile LH release from the anterior pituitary as a consequence of poor herd nutrition (Yavas and Walton, 2000; Vasconcelos et al., 2014). It was hypothesized that administering an additional PG injection will increase pregnancy rates per FTAI in B. indicus cows, and this outcome would be independent of the luteolytic functions of PG. Two experiments were conducted to test this hypothesis. In Experiment 1, there was comparison of pregnancy rates per FTAI in Nelore cows administered one or two PG injections during an estradiol-based treatment regimen for estrous synchronization. Experiment 2 was conducted to compare these same treatment regimens on physiological and reproductive responses of Nelore beef cows without a CL during an estradiol-based treatment regimen for FTAI.
Section snippets
Materials and methods
All animals utilized in the present study were cared for in accordance with acceptable practices and experimental protocols reviewed and approved by the Sao Paulo State University (UNESP) – Animal Use Ethics Committee (0180/2018). There is a depiction of both experimental treatment regimens in Fig. 1.
Experiment 1
There were no differences in DPP and cow BCS at FTAI between treatment groups (Table 1). Pregnancy rates per FTAI were greater (P = 0.01) in cows of the PG2- as compared with the PG1-treatment group (Table 1).
Experiment 2
There were no treatment × parity interactions (P ≥ 0.37) for any of the variables analyzed. Hence, all results are reported for multiparous and primiparous cows according to the main treatment effect.
There were no differences (P ≥ 0.44) in DPP and BCS at FTAI between cows of the two
Experiment 1
This experiment was conducted to determine if administering an additional PG injection would improve pregnancy per FTAI of B. indicus cattle when there was an estradiol-based treatment regimen used for estrous synchronization. Cows included in this experiment are typical of those in a commercial herd in tropical areas of South America, including the less-than-optimal BCS that commonly occurs in cattle within these regions (Cooke et al., 2020). Pregnancies per FTAI were greater in the PG2
Conclusions
Administering an additional PG injection during an estradiol-based FTAI protocol increased pregnancy per FTAI in B. indicus cows with a less than optimal BCS (e.g. < 5.0). This outcome was not associated with luteolytic effects of PG, but rather was the result of an enhanced follicle development and response to the estrous synchronization treatment regimen in cows that were likely to have an insufficient release of LH pulses for dominant follicle development. Inclusion of the additional PG
Declaration of Competing Interest
Nothing to declare.
Acknowledgements
The authors thank Zoetis Brasil (São Paulo, Brazil) and Estrotect (Spring Valley, WI, USA) for product donation, Agropecuária Fazenda Brasil for use of animals and facilities (Nova Xavantina, Brazil), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) for funding Isabella M. Noronha.
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