In the beef cattle industry, reproductive efficiency directly impacts the profitability of cow-calf systems. To improve reproductive efficiency, timed artificial insemination (TAI) has been widely implemented in South America to increase the number of pregnant cows without the need to detect estrus. Moreover, the hormonal protocols used for TAI reduce the interval between parturition and conception [1], particularly in suckled Bos indicus cows in postpartum anestrus [2]. In the Brazilian cattle industry, the most used hormonal protocols in Bos indicus beef breeds include both 17β-estradiol (E−17β) and progesterone (P4).

A fundamental step in the hormonal protocol for TAI is the treatment to induce and synchronize ovulation. Ovulation is typically induced by the administration of E−17β at or after the removal of the P4 device, or by using GnRH analogs 12–18 h before TAI. Among the exogenous E−17β options, E−17β benzoate (EB) and E−17β cypionate (EC) are the most commonly used. The administration of 1 mg EC at 48 h before TAI in Bos indicus cows increases the proportion of cows in estrus at the time of TAI compared to the administration of GnRH analog at TAI [3]. However, around 20–40 % of the females receiving EB or EC to induce ovulation still have a small dominant follicle (DF) (<11.5 mm of diameter) and/or do not express estrus at TAI, which is negatively associated with rates of pregnancy per TAI (P/TAI) [[3], [4], [5]]. Moreover, in females bearing a small DF and with low circulating E−17β [6] or not showing estrous behavior [7], ovulation induced only by GnRH analog is associated with a low P/TAI and an increased rate of pregnancy loss. On the other hand, in females previously treated with EC at the time of P4 device removal, the addition of GnRH treatment at TAI improves ovulation rate and subsequent P/TAI in cows with small DF [8].

Therefore, the reduced pregnancy success in cows with small DF or not showing estrous at TAI is associated with reduced circulating E−17β and may be mitigated by an additional E−17β treatment around TAI. In this regard, in dairy cows receiving GnRH analog for ovulation induction, the treatment with 1 mg E−17β at 24 or 8 h before TAI improved the proportion of cows exhibiting estrus [9,10]. Moreover, untreated cows that had an endometrial thickness <8 mm and were treated with E−17β had greater P/TAI compared to untreated cows [10]. The injection of E−17β in Holstein's cows promoted an earlier E−17β peak in circulation compared to EB and EC [11]. The circulating E−17β peak occurs 2 h after treatment, followed by a gradual reduction, regardless of the dose [12]. In addition, the dose of E−17β administered increases the luteinizing hormone (LH) within 18–24 h [13].

Thus, the use of E−17β at TAI may improve ovulation synchrony and benefit the modulation of the uterine environment in beef females with small DF or not showing estrus. The central hypothesis of the current experiments is that administration of E−17β at TAI increases the proportion of females exhibiting estrus, the synchrony of ovulation, and modulates the uterine environment in beef females with small DF, Exp1 and 2, or that do not demonstrate estrus, Exp 3, ultimately increasing P/TAI. The general aim of these experiments was to evaluate the effects of E−17β administration at TAI on the time of ovulation, uterine environment, estrous expression, and P/TAI, in Nelore cows with small DF or that did not demonstrate estrus at TAI.