118. Role of Oral Melatonin in Improving Semen Quality and Thermal Regulation of the Testes in Bulls

Abstract This study examined the impact of oral melatonin supplementation on fresh and post-thawed sperm traits as well as scrotal temperatures. Yearling Angus bulls (n = 21) were randomly allocated into two groups, melatonin (MEL; n = 11) or control (CON; n = 10) diets from October 2024 to January 2025. MEL bulls were supplemented with 200 μg/kg of body weight of melatonin dissolved in ethanol (5.31 ± 0.05 mL), while CON bulls were supplemented with an equivalent ethanol vehicle control (5.21 ± 0.05 mL) daily via the CALAN gate feeding system. Semen, obtained via electroejaculation were collected on D 0, 28, and 56. Subsequently, a 1 mL aliquot of raw semen was removed for extension and cryopreservation. Kinematic parameters were analyzed in fresh and post-thawed semen via computer-assisted sperm analysis (CASA). Further, cryopreserved semen, from each collection, was also used for flow cytometry. Additionally, scrotal temperatures were collected via FLIR E75 thermal camera on D 0, 14, 28, 42, 56, 70, and 87. Sperm and scrotal temperature data were analyzed using the MIXED procedure of SAS. MEL had greater (P = 0.01) sperm motility on D28 compared with CON (83.7 ± 3.3 vs 72.0 ± 3.1). Additionally, on D56 MEL had greater progressive motility (P = 0.02; 38.2 ± 4.6 vs 24.7 ± 2.5), LIN (P = 0.02; 42.4 ± 1.9 vs 35.7 ± 1.4), and STR (P = 0.01; 76.7 ± 1.8 vs 68.9 ± 1.6) compared with CON. Interestingly, MEL had lesser ALH (P = 0.01; 8.4 ± 0.2 vs 9.3 ± 2.3), VAP (P = 0.005; 103.0 ± 2.6 vs 115.6 ± 2.5), VCL (P = 0.008; 193.6 ± 6.1 vs 220.5 ± 5.7), and VSL (P = 0.03; 77.8 ± 2.2 vs 85.5 ± 2.1) on D0 compared with CON, however by D28 there were no differences between the groups (P > 0.1). When the change in sperm parameters, expressed as a relative difference, were compared between fresh and post-thaw semen MEL had a lesser relative increase in LIN (P = 0.007; 2.0 ± 2.0 vs 11.2 ± 1.7) compared with CON on D56. For flow cytometric analysis, MEL had greater intracellular calcium staining intensity (P = 0.02; 20595 ± 1174.9 vs 16486 ± 1036.2) on D28, indicating an increased concentration of intracellular calcium. Lastly, there were no differences in scrotal temperatures between groups during the treatment period (P > 0.1). Melatonin supplementation from fall to winter had both acute and long-term impacts on sperm kinematic and flow cytometric parameters. Factors related to motility were improved without negatively affecting scrotal temperatures.