Total oocytes recovered
Fig 1 shows the results of the total average of oocytes collected by OPU in Holstein cows during the four seasons of the year. The total production of oocytes among seasons differed (P<0.05). The highest number of oocytes was obtained in winter, followed by autumn (P<0.05). The lowest oocyte production was observed in spring and summer, without a not significant difference between them (P>0.05).
In this study, it was observed that the number of oocytes obtained per cow was lower in the seasons (spring and summer) with more intense heat, this probably was due to the effect of higher environmental temperature, which affect the number of oocytes
(De Rensis et al., 2015), while in the wintertime a greater quantity of oocytes produced per cow was observed. This adverse effect of high ambient temperature on reproduction has been reported in various studies
(Hansen, 2019; Wolfenson and Roth, 2019). Results in terms of oocytes obtained in this work and from previous studies may be due to the modification of the follicular dynamics of cows during high environmental heat, which alters follicular dominance and decreases the size of medium-sized follicles
(Roth, 2021). Also, HS alters follicular development and compromises the development potential of oocytes
(Paula-Lopes et al., 2012) and this, in turn, could affect the number of suitable follicles at the time of OPU. High environmental heat alters the hypothalamic-pituitary-gonadal axis, ovarian follicles in the different stages of development, these could modify the general pattern of follicular wave dynamics
(Wolfenson and Roth, 2019) thus, affect the oocytes obtained from cows subjected to OPU.
In vitro embryo production
Fig 2 shows the average number of total embryos derived from oocytes collected by OPU in Holstein cows during the four seasons of the year. The highest number of total embryos were produced in winter and autumn, compared to the spring and summer seasons (P<0.05).
Regarding embryo production, the results were consistent with the competition of oocytes, since fewer embryos were produced in the summer, followed by spring and there was a greater embryo produced in winter and autumn. These results may be explained by the findings of
(Walsh et al., 2011) who documented that the inability of embryonic development is due to the poor quality of the oocyte, which, as explained above, was found in the most intense hot seasons.
In various studies it has been proven that HS has unfavorable effects on the oocyte, these same effects being the ones that could lead to a reduction in embryo production. In this study, cleavage was not evaluated, however and according to
Gendelman and Roth (2012), the rate of cleavage regardless of season is high when compared to fertilized oocytes that complete their development into blastocyst. It could be that this is independent of the fact that the oocytes in the season of intense heat are fertilized and may even cleave, as the competition of the oocytes is affected as previously verified. Fertilized ovum may not have progressed in their development and probably for this reason embryonic production was lower. In another experiment, it was reported that heat stress during
in vitro fertilization decreases fertilization success
(Sakatani et al., 2015), which supports the explanation of the lower embryonic production in the seasons with intense heat compared with seasons with less severe heat
(Payton et al., 2004).
Competition of oocytes
Fig 3 shows the percentage of competition of oocytes collected by OPU throughout the year that developed to embryos in Holstein cows. The percentage of oocyte competition through the times of the year significantly differed. That is, a higher (P<0.05) percentage of competition was observed during winter, followed by autumn and spring seasons and the poorest competition occurred during summer (P<0.05).
In this sense, the competence of oocytes is a complex process that involves the integral maturation of the oocyte and the acquisition of the capacity to achieve its development
(Khanday et al., 2019; Hansen, 2019). In the same way that was observed in the above-mentioned variables, the competition of the oocytes was greater in the temperate seasons compared to seasons with intense warm weather. Some reports indicate the effect of season on the development of oocytes
(Silva et al., 2016; Gendelman and Roth, 2012) and as in the present study, a decrease in their growth and maturation was reported.
Again, the results of the present study could also be supported by what was proposed by
Wolfenson and Roth (2019), who mention that HS for prolonged periods could alter the follicular microenvironment, damaging the competence of the cell and for this reason, the oocytes would have less capacity to cleavage and become an embryo. These findings could explain the decrease in fertility of Holstein cows in hot months in the region where the present study was carried out
(Mellado et al., 2013).
Oocytes collected from Holstein cows during the summer have reduced oocyte competition and this is expressed by delayed division and a low rate of embryo development to the blastocyst stage. This through the mechanism by which HS affects the competition of oocytes implies cellular and molecular damage, resulting in a failure in the maturation and fertilization of the oocytes
(Roth, 2021).