Relationships among Estrous Behavior, Superovulatory Response and Grade 1 Embryo Sex Ratio in Superovulated Holstein Heifers

Abstract

Abstract. Abstract. In this study, we first attempted to determine whether the timing of artificial insemination affects the sex ratio of seven-day-old embryos in superovulated Holstein heifers. The superovulatory treatment consisted of eight decreasing doses of FSH for 4 days and 2 doses of PGF2α given with the last two doses of FSH. The superovulated heifers were given a GnRH analogue 48 h after the first PGF2α treatment and were artificially inseminated 48 h (n=10) or 56 h (n=8) after the first PGF2α treatment. There were no significant differences in the percentages of unfertilized ova and transferable embryos (grades 1 to 3) between the two groups. The proportions of female grade 1 embryos did not significantly differ from the expected ratio of 50:50 (49.3% at 48 h and 52.5% at 56 h). We then compared the estrous behavior and superovulatory responses of the heifers with a proportion of female embryos of 50% or less (n=7, Low group) to those of the heifers with a proportion of female embryos of more than 50% (n=9, High group). The Low group had a longer duration of estrus and a higher superovulatory response than the High group. These findings offer little encouragement for prediction of the population of female embryos collected from superovulated heifers. Further studies are necessary to evaluate to what degree maternal hormone levels are related to estrus duration and sex ratio. Key words: Estrous behavior, Sex ratio, Superovulation, Superovulatory response (J. Reprod. Dev. 53: [1015][1016][1017][1018][1019][1020][1021] 2007) he dairy and beef industries mainly use female or male animals, respectively. Numerous efforts have been directed toward developing procedures to skew the sex of offspring [1][2][3]; however, it is difficult to draw clear conclusions from the literature, as different laboratories have reported conflicting results under similar conditions. Significant enrichment of X chromosome-bearing spermatozoa has only been demonstrated by sex selection of spermatozoa based on chromatin differences (cell sorting by flow cytometry) [4]; however, this procedure is not widely used due to the cost of the equipment and the limitations in relation to sorting rate.On the other hand, the sex ratio of offspring has been altered for several species by varying the timing of insemination, such as in humans [5,6], hamsters [7,8], mice [9] and sheep [10]. In cattle, some researchers have shown that varying the timing of artificial insemination (AI) does not influence embryo sex ratio [11,12]. The overall sex ratio of bovine embryos produced in vivo and in vitro does not differ from the expected ratio of 50:50 [13][14][15].

Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels around the time of conception

“…Ideta et al (2007) reported that heifers bearing males had a significantly longer duration of oestrus, on the average, than those bearing females. These authors write: 'further, studies are needed to evaluate to what degree maternal hormone levels are related to oestrus duration and sex ratio'.…”

Subjecting Holstein Heifers to Stress During the Follicular Phase Following Superovulatory Treatment May Increase the Female Sex Ratio of Embryos

“…Martinez et al [13] showed that the sex ratio of calves could be altered by the timing of AI, with early insemination during estrus resulting in more females and late insemination resulting in more males. However, we reported that changing the timing of AI did not influence the sex ratio of embryos collected from superovulated heifers [14]. We also reported that frequent transrectal examinations and/or blood samplings during the follicular phase increases the female sex ratio of bovine embryos, although the mechanism of this is unknown.…”

“…In cattle, females are preferred in the dairy industry. In our previous study [14], the proportion of female embryos collected from superovulated Holstein heifers that were frequently placed in stanchions and subjected to transrectal examination of the ovaries during the follicular phase tended to be higher than the expected 50% (66.7%, 26/39). The goal of the present study was to test the validity of this observation by using a greater number of superovulated and artificially inseminated heifers.…”