Increasing the frequency of ejaculate collection in young dairy bulls increases semen production and field fertility

Younger bulls typically produce lower volumes of semen per ejaculate with a lower sperm concentration than older more mature, bulls and often fail to meet semen demand using standard collection frequency schedules. The objective of this study was to assess the effect of ejaculate collection frequency on semen output, sperm quality and field fertility in young bulls under commercial conditions. Holstein Friesian bulls aged 366 ± 8 days (mean ± SEM) were assigned one of two ejaculate collection frequencies: (i) HF (n ¼ 14 bulls), where ejaculates were collected twice a day, five days in each two-week period or (ii) LF (n ¼ 12 bulls), where ejaculates were collected once a day, two days per week. The trial period continued until each bull reached both 20 ejaculates and 1000 marketable frozen semen straws. Subjective motility was assessed on all ejaculates pre-freeze and post-thaw (at 0 and 2 h). A subset of ejaculates were assessed post-thaw by computer-assisted sperm analysis for motility, kinematics and morphological defects and by flow cytometry for viability, membrane fluidity, acrosome integrity, reactive oxygen species and DNA fragmentation. A total of 13,846 inseminations (9,541 for HF and 4,305 for LF) were carried out on dairy cows and heifers. HF reached the 1000 straw threshold 41 days earlier than LF (P < 0.01) with the same number of ejaculates. Ejaculate volume and sperm concentration were not affected by treatment but the first ejaculate of the day (HF only) had a greater volume (P < 0.001) and sperm concentration (P < 0.05) than the second ejaculate. HF had higher pre-freeze total (P < 0.01) and gross (P < 0.05) motility than LF. HF had higher post-thaw (2 h) total and gross motility than LF (P < 0.05). Ejaculate rejection rates did not differ between treatments. There was no effect of treatment, week or ejaculate number of the day (HF only) on post-thaw motility and kinematic parameters or sperm viability, membrane fluidity, acrosome integrity and DNA fragmentation. However, HF had lower superoxide production than LF (P < 0.05). Pregnancy per artificial insemination was 64.5 ± 1.0% and 59.9 ± 1.1% for the HF and LF bulls, respectively (mean ± SEM; P ¼ 0.05). In conclusion, collecting ejaculates more frequently from young bulls significantly reduced the number of days required to obtain 1000 straws, increased semen quality in terms of lower superoxide production and increased field fertility