324 ADJUSTING SPERM CONCENTRATION USED TO INSEMINATE SUPERSTIMULATED BEEF COWS, IN ORDER TO AVOID DECLINE IN EMBRYO PRODUCTION

Abstract

The quantity and quality of semen may affect conception rate after artificial insemination (AI). The concentration of motile spermatozoa, after thawing, varies according to the sire and lot of frozen semen used. The minimum amount of spermatozoa required in a semen straw was determined to obtain pregnancy after a single AI of a non-superovulated animal. However, superstimulation of embryo donors results in the availability of many more oocytes (an average 10–20) for fertilization than in a non-superovulated female (1 oocyte). The purpose of the present work was to verify if by adjusting the concentration of motile spermatozoa, in straws with low sperm concentration, the percentage of viable embryos is comparable to those obtained using straws with high sperm concentration after thawing. Nelore cows (Bos taurus indicus) were superstimulated with a protocol termed P36 (Barros CM et al. 2003 Theriogenology 59, 524 abst), in which the ovulation is induced by exogenous LH (12.5 mg, Lutropin^®, Vetrepharm, London, Ontario, Canada), administered 36 h after PGF_2α. One sample of each lot of semen was analyzed by CASA (computer-assisted semen analysis), and motile sperm concentration, after thawing, was adjusted to a minimum of 25–30 × 10⁶ spermatozoa, which is approximately 3 to 4 times higher than the sperm concentration used for a regular AI. Fixed-time AI (FTAI) was performed 12, 24, and sometimes 36 h after exogenous LH. The number of semen straws necessary to obtain at least 25 × 10⁶ spermatozoa varied from 2 to 6 (Groups 2 to 6, respectively). Since at least two semen straws were used per animal, there is no Group 1. The number of FTAI was adjusted according to the number of straws used, i.e., 2 straws (FTAI 12 and 24 h after LH), and 3 or more straws (12, 24, and 36 h after LH). Mean total structures (oocytes, viable embryos and degenerate embryos), mean viable embryos per flushing, and viability rate (percentage of viable embryos/total structures) were, respectively: 12.2, 8.9, and 73.6% (Group 2, n = 19 flushings); 13.5, 9.6, and 70.9% (Group 3, n = 101); 13.3, 9.4, and 70.9% (Group 4, n = 22); 5.5, 4.0, and 72.7% (Group 5, n = 4); and 24.0, 13.0, and 54.2% (Group 6, n = 1). When the results from Groups 4, 5, and 6 were pooled, total structures, viable embryos, and viability rate were: 12.5, 8.7, and 69.8% (n = 27). The statistical analysis was performed using the ratio of viable embryos/total structures for each flushing, transformed in square root followed by arc sin. Data from groups 4, 5 and 6 were pooled before comparing to the other groups by ANOVA. In order to facilitate the comprehension of the results, data were presented as viability rate instead of means of arc sin. There was no difference when comparing pooled data from Groups 4, 5, and 6 with the other groups (2 or 3; P = 0.87; ANOVA). It is concluded that by adjusting the concentration of motile spermatozoa in straws with low sperm concentration (Groups 4, 5, and 6), the viability rates are comparable to those obtained using semen with high sperm concentration (Group 2 or 3).

Nogueira has a fellowship from FAPESP (Sã Paulo).