Beef cattle herds are composed by Bos indicus and Bos taurus cattle, and both have physiological particularities that affect reproductive performance [1], [2]. Due to their high adaptability [3], [4], Bos indicus cattle, such as Nelore, have been widely raised in beef farms located in tropical climate regions. In this context, several reproductive strategies have been developed to optimize reproductive and productive performances specifically for these cow-calf operations. It is known that implementation of reproductive strategies based on hormonal treatments, such as fixed-time artificial insemination (TAI) programs, can enhance genetic improvement and economic gains of beef herds [5], [6]. Moreover, particularly for heifers, optimizing reproductive outcomes is remarkably important since this category usually has the most developed genetic in herds.

Adjustments in estradiol (E2) plus progesterone (P4)-based TAI protocols for Nelore cattle were previously tested, evaluating protocol lengths and the duration of permanence of P4 device, usually varying from 7 to 9 d [7], [8], [9], [10], [11], [12]. Previous studies reported the predominance of three follicular waves during the estrous cycle in Nelore heifers [2], [13]. Therefore, shorter protocols may be better suited for heifers, as they induce a follicular wave with a duration more similar to the physiological duration of 3-wave heifers [14]. Conversely, longer protocols with extended follicular waves may promote early oocyte maturation and/or heifers ovulating before the scheduled TAI, and both situations are reported to reduce fertility [15], [16], [17].

Due to the a more pronounced suppressive effect of P4 on LH [18] release and follicular growth in Bos indicus females [1], strategies were developed aiming to reduce circulating P4 concentrations during the TAI protocol, such as administration of prostaglandin F2α (PGF) on Day 0 of the TAI protocol [1], [7], [8], or anticipating PGF treatment by 2 d in relation to the removal of the P4 device, in 9-d protocols [11]. Regarding the additional PGF dose, Carvalho et al. [1] reported positive effects on the diameter of the dominant follicle (DF) when PGF was administered on Day 0 of an 8-d E2/P4-based protocol in cyclic beef heifers. However, other studies reported no effect on P/AI when the PGF was administered either on Day 0, in 7- and 8-d protocols [19], [20], or on Day 7, in a 9-d protocol [21].

None of these studies, however, directly compared an E2/P4-based 7-d protocol, initiated with an additional PGF treatment, to 9-d protocols with PGF administered at different times. Therefore, the objective of this study was to compare reproductive outcomes of Bos indicus (Nelore) heifers submitted to a 7-d P4 device TAI protocol with an additional PGF at the beginning, or 9-d P4 device protocols with PGF administered at different times. Three main hypotheses were tested: 1) Besides resulting in a smaller DF at P4 device removal and at TAI, 7-d protocol induces greater P/AI; 2) Within 9-d protocols, an additional PGF dose (Day 0) or anticipation of PGF administration (Day 7) increases DF diameter, expression of estrus and premature ovulation; 3) 9-d protocols result in more premature ovulation than 7-d protocol, which can impair pregnancy per AI (P/AI) in those groups.