Reproduction is the main factor that determines productivity in cow-calf systems, and pregnancy loss is recognized as a major reproductive challenge in these cattle operations [1]. A recent meta-analysis by Reese et al. [2] indicated that most pregnancy losses in beef cattle are resultant from early embryonic mortality during the initial month of gestation, followed by late embryo + early fetal mortality until 100 days of gestation. The majority of studies compiled by Reese et al. [2] evaluated early embryonic mortality, whereas limited studies have evaluated pregnancy losses throughout the gestation period. Hence, additional research is warranted to characterize pregnancy losses in beef females [2], particularly in Bos indicus nulliparous heifers managed to be inseminated as yearlings. Recent studies reported elevated pregnancy losses in this female category [3,4], but without comparison with other parities in the same experimental design.

Pregnancy loss is mainly caused by suboptimal uterine conditions and embryo competence, and such conditions are regulated by genetic and non-genetic factors [5,6]. Heat stress and inflammatory conditions are examples of non-genetic factors that cause pregnancy loss in dairy cattle [5]. The relationship between non-genetic factors and pregnancy losses in beef cattle still requires investigation [5,7]. Haptoglobin is an acute-phase protein widely used as inflammation marker in cattle [8,9], and serum haptoglobin concentrations were negatively associated with pregnancy rates to fixed-time artificial insemination (FTAI) in suckling B. indicus-influenced beef cows [10,11]. No research, however, has evaluated if serum haptoglobin concentrations impacts pregnancy losses in beef females. Therefore, this experiment was designed to evaluate this rationale, and to characterize pregnancy losses in B. indicus nulliparous heifers, primiparous cows, and multiparous cows from day 30 of gestation to calving.