Reducing the age at first conception and calving of heifers is a pivotal goal to enhance productive and reproductive efficiency in beef cattle systems [1], [2]. In Nelore (Bos indicus) heifers, known for reaching puberty later than Bos taurus heifers [3], hormonal strategies to advance reproductive maturity before timed-artificial insemination (TAI) protocols have proven to be an effective tool for mimicking the onset of puberty [4], [5], [6], [7], [8]. These pharmacological interventions aim to stimulate the maturation of the hypothalamic-pituitary-gonadal axis and induce the first ovulation in prepubertal females [9], [10], [11], [12], while also optimizing the uterine environment, creating better conditions for conception and pregnancy maintenance [13], [14]. A literature review conducted by our research group [8] demonstrated that, in general, ovulation induction protocols increase first ovulation rate, improving uterine (UT) development, expression of estrus before TAI, and fertility in prepubertal heifers.

In this context, initial studies indicated that administration of exogenous progesterone (P4) in prepubertal beef heifers stimulates the pulsatile secretion of GnRH and LH, inducing estrus and ovulation [8], [10], [11], [12]. Circulating P4 appears to reduce the expression of hypothalamic estradiol (E2) receptors, thereby reducing the negative feedback of E2 on GnRH secretion [15]. Although early ovulation induction protocols have been based primarily on P4 treatments, the combination with an E2 ester as an ovulation inducer [16] appears to be beneficial in improving ovulatory response and fertility [4], [5], [17]. In addition, studies suggest that exposure of heifers to P4 and E2 has beneficial effects on uterine development [18], [19], [20], as evidenced by increased uterine diameter and tone, which results in conception and pregnancy maintenance [21], [22], [23]. This effect likely occurs because exposure to these protocols mimics a physiological proestrus [23] potentially leading to ovulation and improved uterine development. The presence of a corpus luteum (CL) at the onset of a TAI protocol has been consistently associated with greater expression of estrus near TAI and higher pregnancy per AI (P/IA) in these females [4], [5], [6], [7], [15].

Although intravaginal P4 devices are widely used, long-acting injectable P4 (iP4) has gained prominence as a practical alternative for P4 supplementation in commercial beef cattle operations, allowing for a single handling before the beginning of a TAI protocol [24]. While ovulatory response tends to be lower in protocols that rely on iP4 alone, some studies suggest that exposure to iP4 alone may stimulate uterine development, which should have a positive effect on fertility[14]. Silva et al. [13] observed that heifers receiving 150 mg of iP4 24 d before a TAI protocol had higher uterine scores compared to non-exposed heifers, resulting in greater P/IA in ∼13 mo old Nelore heifers.

Nevertheless, there is still an intriguing variability in fertility results observed among studies that evaluated strategies for induction of ovulation, particularly in prepubertal yearling Nelore [8]. Alves et al. [7] observed that the presence of CL at the onset of a TAI protocol did not influence P/AI, as Nelore heifers at ∼13 mo old, with or without CL, had similar fertility (48.0% vs. 48.5%, respectively), regardless of the induction protocol to which they were subjected. These results suggest that ovulation may not be essential for improving fertility, as suggested by previous studies. In other words, the benefit of exposing heifers to induction protocols may be more closely related to P4 exposure than to ovulation itself, which justifies evaluating the simplified induction protocol using iP4 as a more practical and viable alternative in terms of management. Thus, understanding whether ovulation is truly necessary to optimize fertility in this category or whether improved uterine development alone would be enough, is crucial for advancing strategies that enhance the reproductive efficiency of young heifers [13], [14].

Therefore, the objective of this study was to evaluate the reproductive outcomes of yearling Nelore heifers submitted to ovulation induction strategies based on association of an intravaginal P4 device and EC or, exclusively, administration of iP4. Two main hypotheses were tested: 1) exposure to the P4+E2-based hormonal protocol results in a greater percentage of heifers with CL and greater UT development at the beginning of the TAI protocol than the treatment with iP4; 2) heifers exposed to the P4+E2-based hormonal protocol have greater fertility and lower pregnancy loss (PL) after the first TAI service, compared with those treated with iP4.