Bull’s age effect on the ratio of x and y chromosome-bearing spermatozoa

The reproductive efficiency improvements that can be achieved in bovine herds highly depend on the male, as the bulls account for approximately 80% of the gene pool. Likewise, age can impact the sires’ reproductive efficiency through its seminal quality, which is strongly influenced by the bull’s sexual maturity. The study aimed to assess the bull’s age effect on the X and Y chromosomes-bearing spermatozoa ratio in semen. Sixteen Brahman bulls (Bos indicus) were grouped into two age range and their reproductive capacity was assessed. Genomic DNA was extracted from a single collection of semen to quantify by qPCR (quantitative polymerase chain reaction) the PLP and SRY genes, used as sexual chromosome markers, and autosomal GAPDH as the reference gene. The values obtained were compared using the delta Ct (2-∆Ct) method and validated with the use of a commercial sample of semen sexed for the X chromosome. The SRY gene had a proportion of 43% in bulls of 4-5 years of age and 32% in the group of 9-10 years, while the proportions of the PLP gene were 57% in bulls of 4-5 years and 68% in the 9-10 years group. This deviation from the expected ratio (1:1) in bulls of nine and ten years old with a preponderance of the X chromosome content might suggest a possible influence of age in the sex chromosome proportions in the spermatozoa; however, these results were not statistically significant.