41 Impact of oral melatonin supplementation on sperm quality and testicular artery hemodynamics in the bull

The objective of this study was to examine the impact of oral melatonin supplementation on sperm quality and testicular artery hemodynamics in the bull. Yearling Angus bulls (n = 21) were randomly allocated into two groups, either melatonin fed (MEL; n = 11) or control fed (CON; n = 10) for 90 days of treatment. MEL bulls were supplemented with 200 mg/kg of body weight of melatonin dissolved in ethanol, while CON bulls were supplemented with an equivalent ethanol vehicle control. Supplementation was top-dressed in a grain mix, fed daily via the CALAN gate feeding system from October 2024 to January 2025. Body weight was collected, and melatonin supplementation was adjusted weekly. Baseline measurements were collected 2 ± 1 d prior to treatment administration. Scrotal circumference (SC) and semen samples were collected on d 28, 56, and 84. Semen samples were obtained via electroejaculation and evaluated by computer-assisted sperm analysis for morphology and motility parameters. Testicular artery hemodynamic measurements (TBF; total blood flow, RI; resistance index, PI; pulsatility index, S/D; systolic to diastolic ratio, MnV; mean velocity of the left and right testis), were collected on d 14, 28, 42, 56, and 70 via Doppler ultrasonography. In addition, total blood flow relative to body weight (BF/BW) and average blood vessel diameter (DIA) were calculated. Data were analyzed using the MIXED procedure of SAS specific for repeated measures with treatment, time, and the interaction as fixed effects. Initial covariates included the baseline variable of interest, age, weight, SC, and temperature humidity index. Covariates were removed using backwards manual selection with variables having a P < 0.1 being left in the model. Melatonin supplementation increased sperm motility (P = 0.0496) in MEL (82.52 ± 2.41) compared to CON bulls (75.52 ± 2.22) and wobble tended to decrease (P = 0.051) in MEL (51.93 ± 0.36) compared to CON bulls (53.03 ± 0.32). Progressive motility, beat cross frequency, average path velocity, curvilinear velocity, and straight-line velocity improved (P < 0.05) over time. In contrast, distal droplet count, linearity, and straightness decreased (P < 0.05) over time. There was no effect (P > 0.05) of treatment on testicular artery hemodynamics. However, PI, RI, S/D TBF, and BF/BW changed (P < 0.05) over time. PI, RI, and S/D decreased (P < 0.05) from d 14 to 56. In contrast, TBF and BF/BW increased (P < 0.05) from d 14 to d 56. MnV tended (P = 0.0687) to increase and DIA tended (P = 0.0576) to decrease from d 14 to d 70. Melatonin supplementation from fall to winter resulted in a notable increase in sperm motility despite the lack of alterations to testicular artery hemodynamics due to treatment.