Fertilization Characteristics of Spermatozoa Collected from Bulls Grazing Tall Fescue Pastures

Consumption of toxic endophyte-infected (E+) tall fescue pastures is known to have a negative impact on bull reproductive performance. Since decreased cleavage rates of embryos fertilized with spermatozoa from bulls grazing E+ tall fescue pastures have been observed in several studies using differing sets of bulls, technicians, pastures, and other methods of inducing tall fescue toxicosis (ergotamine tartrate), it is hypothesized that spermatozoa function from bulls grazing E+ is impaired in ways undetectable by gross semen examination.

During a three-month grazing study, 6 Angus bulls were utilized to determine the effects of grazing E+ tall fescue pastures on growth performance and spermatozoa function. Bulls were appointed to graze Kentucky 31 tall fescue (Festuca arundinacea Schreb.) infected with Neotyphodium coenophialum, an ergot alkaloid producing endophyte (n=3) or Jesup tall fescue infected with non-ergot alkaloid producing endophyte (NTE) MaxQ^TM (n=3). Bulls were grouped by body weight (BW) and scrotal circumference (SC) to graze pastures from April 18-June 26, 2007. Blood samples, BW, SC, semen, and rectal temperatures (RT) were collected every 7 d. Scrotal temperatures (ST) were obtained before semen collection each week in June. Semen was evaluated for gross motility, morphology, and Computer Assisted Semen Analysis (CASA) parameters. Semen from a subset of bulls (n=2 per treatment) was used to assess spermatozoa ability to function utilizing in vitro assays.

Growth performance was decreased in E+ bulls compared to bulls grazing NTE tall fescue pastures (P = 0.002). Concentrations of prolactin were reduced in bulls grazing E+ compared to bulls grazing NTE tall fescue pastures (P = 0.055). Motility post-thaw and during a 3-hour stress test were decreased (P = 0.024 and P < 0.0001, respectively), in addition to altered CASA parameters for spermatozoa. Penetration was reduced in oocytes fertilized with spermatozoa from bulls grazing E+ (64.54 ± 3.28%) compared to NTE tall fescue pastures (87.42 ± 1.63%, P < 0.0001) coupled with hastened meiotic completion, and reduced intracellular calcium parameters. These findings indicate impaired spermatozoa function in bulls grazing E+ tall fescue pastures that extends beyond gross semen characteristics, and may provide direction for future studies.