Trials to skew the primary sex ratio or preselecting offspring with a predefined sex have been considered to be of interest for a number of years. Several methods and techniques have been used to this purpose. The present study evaluated the effects of red Light Emission Diode (LED) radiation on primary sex ratio and qualitative parameters of bull sperm during the swimup method. Fresh semen samples of four Holstein bulls were used in these experiments. Two experimental groups were designed (with or without LED radiation). The LED radiation group received red LED irradiation during the swim-up procedure. The photo-stimulation pattern consisted of sequential light-exposure periods (the light phase for 10 min followed by the dark phase for 10 min and the final light phase for 10 min). In each experiment, 5 tubes (#1 to 5) were considered for the swim-up procedure. The sperms were collected at 5, 10, 20, 30 and 60 min of swimming up time. Samples were analysed by Computer Assisted Sperm Analysis (CASA) and quantitative real-time PCR. Results of CASA parameters show significant differences between the two experimental groups for most parameters estimated at 10 and 30 min. Results of the comparative threshold cycle (ΔCt] reveal that the Y-bearing sperm swim faster than the X ones and the percentage of Y-bearing sperm in the control group was significantly greater. In conclusion, using red LED during the swim-up method could enhance sperm CASA parameters and increase the Y/X- chromosome bearing sperm ratio. Our results showed that red LED irradiation improves motility of sperm, which in turn can improve fertility. Additionally, the Y/X bearing sperm chromosome ratios are different in different times of the swim-up procedure.

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