Contrasting parameters of higher vs. lower fertility Bos indicus sires: assessment of sperm binding to oviduct cells and adjustments in the timed artificial insemination protocol.

The aim of this study was to investigate fertility markers associated with sire fertility using an in vitro test, and to evaluate whether adjustments in timed artificial insemination (TAI) protocols could enhance pregnancy per AI (P/AI) in Bos indicus cows. In Exp. 1, semen from six sires with higher fertility (HF; n = 3) or lower fertility (LF; n = 3) was evaluated for sperm binding to oviductal epithelial cell (OEC) aggregates, sperm motility, and membrane integrity. In Exp. 2, a total of 1133 Nelore cows were submitted to a TAI protocol and randomly assigned to receive GnRH 16 h before TAI (G-16) or at TAI (G0), and were inseminated with one straw of semen from HF sires, one of LF sires (1LF), or two of LF sires (2LF), in a 2 × 3 factorial arrangement: G-16/HF (n = 203), G-16/1LF (n = 187), G-16/2LF (n = 174), G0/HF (n = 182), G0/1LF (n = 198), G0/2LF (n = 189). In Exp. 1, semen from HF sires had improved sperm kinematic parameters, while membrane integrity was similar between fertility groups. Additionally, sperm binding/mm of OEC was greater for HF than LF sires at 12, 24, and 36 h. Unexpectedly, in Exp. 2, neither fertility group, timing of GnRH treatment, nor their interaction affected P/AI. However, within G-16 group, cows with greater body condition score (BCS ≥ 3.0) had higher P/AI than those with BCS < 3.0 (62.2 [194/312] vs. 51.2% [129/252]). Moreover, within LF sires, cows with BCS ≥ 3.0 had higher P/AI than cows with BCS <3.0 (60.6 [221/419] vs. 50.5% [164/329]). In conclusion, semen from HF sires had higher sperm binding to OEC in vitro and improved kinematic parameters. However, these findings did not result in differences in P/AI. One possible explanation is that treatment with GnRH, whether administered 16 h before or at TAI, may have improved synchronization of ovulation relative to insemination, potentially reducing disadvantages associated with LF semen. Nevertheless, the sperm binding assay remains a promising tool for assessing sire fertility, although further optimization and validation are required.