Theses and Dissertations

ORCID

0009-0004-9735-3255

Advisor

Lemley, Caleb

Committee Member

Fan, Peixin

Committee Member

Feugang, Jean

Committee Member

McGee, Marcus

Date of Degree

8-12-2026

Original embargo terms

Immediate Worldwide Access

Document Type

Dissertation - Open Access

Major

Agricultural Science (Animal and Dairy Science)

Degree Name

Doctor of Philosophy (Ph.D.)

College

College of Agriculture and Life Sciences

Department

Department of Animal and Dairy Sciences

Abstract

Bull subfertility is a prevalent issue with significant consequences for cattle production. Unfortunately, few management interventions exist to alleviate bull subfertility and improve bull reproductive efficiency. Melatonin, a key regulator of circadian rhythms and seasonal reproduction, improves reproduction in non-seasonally breeding mammals, yet the benefits of oral supplementation in the bull remain underexplored. Therefore, this study aims to elucidate the extent to which melatonin supplementation could serve as a management intervention to improve bull reproductive efficiency. Yearling Angus bulls (n = 21) were utilized to examine the effects of 90 days of oral melatonin supplementation on sperm quality, blood plasma and seminal plasma hormone concentrations, testicular artery hemodynamics, scrotal temperatures, testicular transcriptomic profiles, semen and preputial microbial niches, and bull social and reproductive behaviors. Melatonin fed bulls (MEL; n = 11) had greater (P < 0.05) blood plasma, seminal plasma, and testicular melatonin concentrations compared to control bulls (CON; n = 10). In fresh sperm, MEL bulls had greater motility and motion related kinematic parameters (P < 0.05). Additionally, melatonin increased sperm intracellular calcium concentrations after cryopreservation (P = 0.02). During melatonin administration there were no differences in any hemodynamic measurements, scrotal temperatures, blood plasma, seminal plasma, or testicular testosterone concentrations, or antioxidant markers (P > 0.05). Melatonin supplementation resulted in changes in both semen and preputial microbial community composition with more uniform community clustering being evident at D28 in MEL niches (P < 0.05). Lastly, social and reproductive behaviors were altered due to melatonin administration. MEL bulls performed fewer mounts but received a greater number of mounts (P < 0.05). In addition, MEL bulls performed more self-grooming and allogrooming compared to CON bulls (P < 0.05). Taken together, in vivo melatonin supplementation altered various aspects of bull reproductive physiology. Melatonin’s effects on fresh sperm parameters appear to be beneficial for natural service fertility, however further research investigating the manner that in vivo melatonin supplementation influences sperm cryotolerance is needed. Furthermore, species level characterization of the semen microbiome is essential to elucidate how melatonin targets specific microbes and the effects these microbes have on sperm and overall bull fertility.

Sponsorship (Optional)

This publication is a contribution of the Mississippi Agricultural and Forestry Experiment Station. This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch project under accession number 1011100. This research is supported in part by the U. S. Department of Agriculture, Agricultural Research Service, project 6066-31000-015-00D.

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