Sexual rest is a transient condition, which compromises conception rates, characterized by large volumes of ejaculate with high percentages of dead sperm observed in bulls. The biochemical mechanisms leading to this ejaculate pattern are not fully understood. Six adult resting Nellore bulls were submitted to Breeding Soundness Evaluation by four consecutive semen collections through the electroejaculation method during a 30 min period. Each ejaculate had its semen phenotypic parameters; morphology and physical aspects were evaluated. To assess enzymatic activity (superoxide dismutase, catalase, and glutathione S-transferase), lipid peroxidation (concentrations of malondialdehyde and nitric oxide), fatty acid, and proteomic profile aliquots of spermatozoa from the first and fourth ejaculates were used. All sperm parameters differed between the first and fourth ejaculates. Spermatozoa from the first ejaculate showed lower enzymatic activity and a higher concentration of lipid peroxidation markers. Among the 19 identified fatty acids, 52.7% are polyunsaturated. Relative abundance analysis showed that C12:0 and C18:0 fatty acids differed between the first and fourth ejaculates, being the fourth ejaculate richer in spermatozoa. The proteomics analysis identified a total of 974 proteins in both sample groups (first and fourth ejaculates). The majority of identified proteins are related to cellular processes and signaling. Quantitative proteomics showed 36 differentially abundant proteins, 6 up-regulated proteins in the first ejaculate, and 30 up-regulated proteins in the fourth ejaculate. Spermatozoa from bulls at sexual rest have less antioxidant capacity, causing changes in their fatty acid composition and protein profile, which generates the observed sperm pattern and lower fertilization capacity.

Graphical Abstract