Reproductive efficiency is a key driver of livestock production systems with calf crop being a key determinant of profitability in beef cow-calf operations. Calf crop in a beef cattle operation depends on the number of calves weaned times the weaning weight of the calves, both of which are dependent upon reproductive efficiency of the operation. Key drivers of calf crop are the pregnancy rate at the end of the breeding seasons (BS) and how early in the BS the females become pregnant [[1], [2], [3]]. In Bos indicus (Nelore) cattle, anovulatory condition is a critical issue associated with reproductive performance since a low proportion of cows become cyclic early postpartum [4]. Many studies have reported that synchronization of ovulation protocols combined with TAI can induce cyclicity and increase reproductive efficiency and genetic gain [5,6].

Currently, there are many options for TAI protocols and hormonal manipulations that allow producers to implement this biotechnology with high efficiency and fertility [4,6]. One important factor that can influence the efficiency and profitability with any reproductive management strategy, including TAI, is pregnancy loss (PL). The majority of PL is not detected with the traditional methods of pregnancy diagnosis using ultrasound or rectal palpation, since it occurs in the embryonic phase, before 30 days of gestation; however, later embryonic and fetal mortality also occurs and can reduce reproductive and economic performance [[7], [8], [9], [10]]. Therefore, understanding the mechanisms and factors that influence pregnancy maintenance are biologically and economically relevant.

Factors that affect pregnancy per AI (P/AI) usually based on a pregnancy diagnosis at 30–45 days in pregnancy have been discussed in the literature and include parity, cyclicity status, nutrition, body condition score (BCS), expression of estrus and hormonal treatments [4,6,[11], [12], [13], [14]]. Although some of these factors may also impact PL (summarized in Ref. [9]), these effects have not been extensively reported, particularly in Bos indicus Nelore cattle.

Thus, the aim of the present study was to evaluate PL in Bos indicus beef cows according to several factors, such as parity, BCS, presence of corpus luteum (CL) at the beginning of the TAI protocol, expression of estrus near TAI and effect of sire. This study also evaluated the effect of hormonal manipulations that have been commonly used to increase fertility, such as adding a prostaglandin F2α (PGF) treatment at the beginning of TAI protocols, using 0.5 or 1.0 mg of estradiol (E2) cypionate to induce final ovulation, and adding a gonadotropin-releasing hormone (GnRH) treatment at TAI in estradiol (E2)/progesterone (P4)-based protocols. We anticipate that these results may help in designing future studies and elevate the discussion of the critical topic of PL.