Based on in vitro assessment, 6% DMF was a less effective cryoprotectant when used with a slow freezing rate, whereas 10% EG resulted in acceptable survival. It was noteworthy that the pellet method was the most promising for both sperm survival and fertility rate. This method has already been successfully adapted to turkey sperm freezing [22] with 42% sperm survival. Unfortunately, fertility was not reported. The pellet method produced higher fertility rate in the present study than that reported earlier in guinea fowl [12] using DMF with moderate freezing rate (64 vs 20%, respectively). The slow protocol with 10% EG had a good survival rate of live, intact spermatozoa, but fertility was very low. Consequently, using only in vitro tests to assess the effectiveness of sperm cryopreservation methods can be very misleading. Notable, there were higher rates of morphologically abnormal sperm in slower protocols than in the faster ones (nitrogen vapour or pellet method). The significantly highest rate of abnormal spermatozoa was detected in slow protocol using EG. Regarding specific defects, damage to the midpiece was the most common abnormality in all protocols (data not shown). Based on the present experiment, cryoprotectant had a lesser impact on sperm morphology than the rate of freezing. To our knowledge, there are no data available regarding sperm abnormalities of frozen thawed guinea fowl semen. However analysed the abnormal sperm ratio in frozen-thawed gander spermatozoa, the rate of midpiece injuries, namely “bent neck”, were the most prevalent [23], similar to findings with emu spermatozoa [24].
When semen was preserved by the slow protocol, only 18–30% of eggs were fertilized; moreover 80–90% of them contained early dead embryos. According to earlier reports frozen–thawed avian spermatozoa are associated with an increased incidence of early embryonic mortality. Although only a single spermatozoon is required to activate an ovum, in the case of avian species, polyspermic fertilization occurs and an optimal number of sperm number are necessary for good fertility. The role of the other spermatozoa penetrated into the ovum is not fully understood yet. Previous studies reported that both too low [25], [26] and high [27] sperm concentration increase the chance of embryo mortality. Presumably, in the case of the present slow protocol there were not enough live, morphologically normal spermatozoa for the complete embryo development. Since the effect of freezing protocols was compared in the study, approximately similar amounts of sperm were used for inseminations. Certainly, the higher the sperm concentration, the higher the rate of intact spermatozoa and therefore the fertility. However, it is known that there is a limit in insemination doses since significant part of sperm is rejected from the cloaca if the doses are higher than 200 µL.
Furthermore, perhaps morphologically normal spermatozoa contained cryopreservation-induced damage to nuclear structures, including DNA [28]. In humans, DNA-damaged spermatozoa fertilized ova, but the rate of early embryonic death increased [29]. During storage of equine sperm, there is also a measurable increase in DNA damage as detected by the comet assay with both cooled and frozen storage [30], [31].
In conclusion, the pellet method was the most effective way for conservation of guinea fowl spermatozoa with a high rate of fertility. There are no data available about the long term duration of sperm in pellet form. Tselutin et al., [17] prefer slow freezing protocols to pellet method for special long term gene conservation purposes in the case of domestic fowl. Further investigations are in progress to check the effects of storage conditions in the case of pellet form.