The latest entry in the field of semen evaluation is computer assisted semen analysis (CASA). Heterospermic insemination has been used to increase pregnancy rates from low fertile bulls. The objective of this study was to evaluate, with the aid of CASA, heterospermic semen characteristics and pregnancy rates using different concentrations of bull semen in a timed artificial insemination protocol. Semen was collected from two bulls of known fertility by artificial vagina and all CASA motility parameters were evaluated individually and combined. Straws were filled using a semi-defined semen extender (Andromed, Minitüb, Tiefenbach, Germany) as follows: single bull A and single bull B (12 × 10⁶ of progressive motile cells after thawing); Mixed bull semen: A + B (12 × 10⁶ of progressive motile cells after thawing) and Supermix bull semen: A + B (28 × 106 of progressive motile cells after thawing). All cows received a P4 intravaginal device (DIB, Syntex, Argentina) and 2 mg of EB i.m. (Syntex) on Day 0, 500 mg cloprostenol (Estroplan, Syntex) at the time of DIB removal (Day 8), and 1 mg EB i.m. on Day 9. Fixed-time insemination (FTAI) was performed at 52 to 56 h after DIB removal. A total of 249 cows were randomly allocated to be inseminated with bulls A and B (n = 76), with Mixed A + B (n = 87), and with Supermixed A+B at a high concentration (n = 86) by a single inseminator. Pregnancy rates were evaluated at 38 days after insemination by transrectal ultrasonography. Means and standard deviations or each characteristic were calculated, compared, and statistically analyzed. The following sperm motility parameters were determined with the Ceros 12.1 sperm analyzer (Hamilton Thorne Biosciences, Inc., Beverly, MA, USA) on at least 1000 spermatozoa: velocity average path (VAP), velocity straight line (VSL), curvilinear velocity (VCL), amplitude lateral head (ALH), beat cross frequency (BCF), straightness (STR), linearity (LIN), and percentage of rapid or static cells. There were no significant differences (P > 0.05) in VAP, VSL, VCL, ALH, STR, or LIN. There was a numerically higher percentage of rapid cells in the Supermix semen. Pregnancy rate from bulls A and B was 61% and from Mixed A + B 60%, while that from Supermixed A + B was 69%. Results from the analysis indicate that semen concentration is an important element to be considered in a timed artificial insemination program. Numerically higher pregnancy rates were obtained with double semen concentration in the straw. More research is required to evaluate the interaction between different breeds within a timed artificial insemination program.

This research was supported by Centro Genético Bovino de EOLIA sa and Syntex sa Argentina.