Theses and Dissertations

ORCID

0000-0001-5281-5381

Advisor

Fan, Peixin

Committee Member

Rude, Brian

Committee Member

Lemley, Caleb

Committee Member

Tao, Sha

Date of Degree

8-12-2026

Original embargo terms

Embargo 2 years

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

Rumen microbes play an indispensable role in fermentative digestion and energy harvest in ruminants, but their contributions to neurophysiology and stress responses remain largely unexplored. This dissertation investigated the role of the rumen microbiome in neurotransmitter metabolism and its relationships with heat stress responses in lactating dairy cows. First, we developed a machine learning model using rumen microbiota 16S rRNA gene sequencing data from previous studies. The model accurately distinguished heat-stressed from non-heat-stressed cows (AUC = 0.851) based on microbial signatures, including enriched Ruminococcaceae UCG-001 in heat-stressed cows, confirming heat stress-induced alterations in rumen microbiome. We then quantified neurotransmitters in rumen fluid and blood plasma and found substantially higher concentrations of key neurotransmitters in rumen, including-excitatory neurotransmitter glutamate and the inhibitory neurotransmitter γ-aminobutyric acid (GABA). In vitro culture experiments demonstrated the important role of rumen microbes in neurotransmitter metabolism, while long-read metagenomic sequencing identified potential microbial genes and hosts involved in these pathways. Microbial capacity for neurotransmitter metabolism varied among taxa: genes involved in glutamate metabolism were abundant and widely distributed, whereas dopamine metabolism genes were less abundant and detected only in few less-abundant taxa, such as Xanthomonas spp. We further examined relationships between rumen neurotransmitters and heat stress responses in lactating Holstein and Jersey cows. Compared with heat-stress-relieved Holsteins, heat-stressed Holsteins spent more time drinking (P < 0.0001), exhibited more agonistic behaviors (P < 0.0001), and had lower milk protein percentage (P = 0.001). Heat-stressed Jerseys spent more time standing (P = 0.007), less time lying (P = 0.003), and engaged in more social interactions (P = 0.02) than Holsteins. Rumen glutamate tended to increase under heat stress in Holsteins but not Jerseys (P = 0.09). Moreover, rumen glutamate was positively correlated with agonistic behaviors in heat-stressed Holsteins, whereas GABA tended to be negatively correlated with agonistic behaviors in heat-stressed Jerseys. Collectively, these findings demonstrate the neuroactive potential of rumen microbiome and suggest that rumen-derived neurotransmitters may contribute to behavioral and physiological responses to heat stress, providing a foundation for mechanistic studies, supporting the development of microbiome-based strategies to improve heat stress resilience, welfare, and productivity in dairy cattle.

Sponsorship (Optional)

This research was funded by National Institute of Food and Agriculture, U.S. Department of Agriculture, under the award number 2024-67015-42622 to PF, Mississippi Agricultural and Forestry Experiment Station Strategic Research Initiative program, and Office of Research and Economic Development Undergraduate Research Program.

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