In beef cattle, greater productivity is directly associated with the reproductive efficiency of the herd. Therefore, protocols to induce synchronization of ovulation are commonly used to induce earlier postpartum cyclicity [1]. However, the efficiency of ovulation synchronization is variable and usually lower in cows with lower than ideal body condition scores [2].

After parturition, the prolonged anestrous period in beef cows reduces herd reproductive efficiency [3]. The hormonal mechanism is the low LH pulsatility during this period that is commonly the result of nutritional stress [4], parity [5] and the presence of a calf [6]. Such factors may affect the final follicular growth, resulting in patterns of follicular growth that do not lead to ovulation [7]. In cows with compromised energy status due to the low nutrient availability in extensive production systems, a reduction in the secretion of GnRH and, consequently, LH is observed [8]. This inhibition of the hypophyseal-hypothalamic axis occurs as a consequence of the negative feedback originating from the increased concentrations of non-esterified fatty acids and neuropeptide Y due to the mobilization of body fat [9,10]. Moreover, cows with a negative energy balance have lower glucose concentrations; glucose is the major energy metabolite used by the central nervous system [11]. Concomitant with this scenario, the presence of the calf ensues the release of endogenous opioids, suppressing the release of GnRH and LH [6,12,13]. Thus, novel hormonal strategies are being developed to increase LH pulsatility and, consequently, the efficiency of ovulation synchronization protocols [14]. However, metabolic and endocrine differences between Bos taurus and Bos indicus cows must be considered for the development of new hormonal strategies that increase the efficiency of ovulation synchronization protocols. Thus, Bos taurus cows ovulate a larger follicle than Bos indicus cows and consequently have a larger corpus luteum. Furthermore, the clearance rate of steroid hormones is greater in Bos taurus than Bos indicus cows [15].

Protocols that utilize a combination of progesterone and estradiol have demonstrated satisfactory results, with pregnancy rates of approximately 50 % [2,16]. In such protocols, the action mechanism of progesterone is mediated by estradiol, which alters the amount and localization of progesterone receptors in the hypothalamus [18] and increases LH receptors in the granulosa and theca cells of the pre-ovulatory follicle according to the intrafollicular/seric estradiol concentrations [17,19]. The main advantage, therefore, of ovulation synchronization protocols that use progesterone is to decrease the latency of the first ovulation in anestrous cows [20]. However, the efficiency of ovulation synchronization is approximately 85 % [21] and is lower in cows with low BCS at the initiation of the protocol [22]. Thus, the period of exposure to progesterone during TAI protocol, in cows that do not respond to the ovulation synchronization protocol may be insufficient to increase the required LH pulsatility for adequate follicular growth and ovulation [14]. In these cows, ovulation synchronization protocols induce the formation of smaller than ideal follicles [23,24]. In addition, short luteal phases were observed in cows that underwent their first postpartum ovulation synchronization [25], possibly because of an inadequate period of exposure to progesterone [26] and lower estradiol concentration on proestrous [27,28]. Recently, exposure to progesterone for 10 days before the initiation of the ovulation synchronization protocol was found to increase the follicular diameter at the onset of the protocol, at the time of removal of the intravaginal progesterone insert and to induce a 1.7-fold increase in the pregnancy rate in suckled Bos indicus cows [14]. Therefore, the objective of the present study was to evaluate the effect of injectable progesterone before the estradiol/progesterone-based ovulation synchronization protocol on the final follicular growth rate, ovulation rate, and pregnancy rate in suckled Bos taurus beef cows, similar to study in suckled Bos indicus cows [14]. We hypothesized that exposure to progesterone before the ovulation synchronization protocol would increase the fertility of anestrous Bos taurus cows submitted to the synchronization of ovulation.