Dynamics of the Reproductive Changes and Acquisition of Oocyte Competence in Nelore (Bos taurus indicus) Calves during the Early and Intermediate Prepubertal Periods

SIMPLE SUMMARY: Brazil has the largest cattle commercial herd in the world, mostly of the Nelore breed (Bos taurus indicus), and it is one of the largest in vitro bovine embryo producers. However, Nelore cows experience puberty later than taurine breeds (Bos taurus taurus), which can lead to a delay in genetic improvement. Therefore, including calves as oocyte donors for in vitro embryo production programs could be an attractive step for reducing the interval between generations, accelerating the herd’s genetic gains, and contributing to the sustainability of milk and meat production systems. Nevertheless, calves’ oocytes are reported as less competent in generating embryos and establishing pregnancies than those collected from adult females. In addition, most studies have been carried out on taurines. Thus, this study’s goal was to search for a better understanding of prepubertal Nelore females’ reproductive development, related to the performance of their oocytes in terms of in vitro production of embryos. This can provide support for making decisions on the use of this animal category as an oocyte donor and prospects for hormonal protocols or changes in the culture media for in vitro embryo production. ABSTRACT: The purpose of this study was to characterize the reproductive physiology, oocyte competence, and chromatin compaction in Nelore calves in the early-prepubertal period (EPP) and the intermediate-prepubertal period (IPP). Calves aged 2–5 (EPP) and 8–11 months old (IPP) were assigned to Trial 1 (morpho-physiological–endocrine evaluations, n = 8) or Trial 2 (oocyte donors, n = 8) vs. the respective control groups of cows (n = 8, each). All morphological endpoints, except the antral follicle count, increased from the EPP to the IPP. The EPP LH-FSH plasma concentrations were similar to cows, whereas LH was lower and FSH was higher in the IPP than in cows. . Cows produced more Grade I (12.9% vs. 4.1% and 1.7%) and fewer Grade III COC (30.1% vs. 44.5% and 49.0%) than the EPP and IPP calves, respectively. The IPP calves’ oocyte diameter was similar to those from cows but greater than those from EPP females (124.8 ± 8.5 and 126.0 ± 7.5 μm vs. 121.3 ± 7.5 μm, respectively). The expression of the chromatin compaction-related gene HDAC3 was downregulated in calves. The proportion of the blastocyst rate to the controls was lower in EPP than in IPP calves (43.7% vs. 78.7%, respectively). Progressive oocyte competence was found during the prepubertal period, which can help to decide whether to recover oocytes from calves.