# Longitudinal characterization of mature beef bull seminal fluid and blood serum metabolome during a 65-day breeding season

## TL;DR

This study tracks metabolic changes in beef bulls during a breeding season, focusing on how workload affects purine metabolism in seminal fluid and serum.

## Contribution

The study provides new metabolomic insights into how bull workload during the breeding season influences metabolic pathways related to fertility.

## Key findings

- Purine and pyrimidine metabolism pathways were most affected in seminal fluid and serum during the breeding season.
- Moderate workload bulls showed increased xanthine, hypoxanthine, and inosine in seminal fluid over time.
- High workload bulls showed decreased purine metabolite abundance, possibly due to increased sperm turnover.

## Abstract

The metabolic impact of the breeding season on beef bulls has not been extensively studied. The change in plane of nutrition and intensive activity demands induce body weight loss and metabolic changes that may influence fertility and long-term management. The use of metabolomics to characterize the metabolic changes or adaptations during the breeding season will aid in developing hypotheses for future research in beef bulls. The objective of this study was to identify metabolites in both serum and seminal fluid (circulating vs. excretory) that were affected temporally throughout a 65-d breeding season and by bull workload (bull: cow). Over a two-year period, purebred Angus bulls (year 1: n = 8, year 2: n = 9) owned and housed by the Cal Poly beef unit were surveyed using serum (collected via the coccygeal vein) and seminal fluid (collected via electroejaculation) at the beginning (d0), middle (d30), and end (d65) of a 65-d, post-AI breeding season. Bulls were subjected to one of three workload groups: high (HIGH), moderate (MOD), or low (LOW). Samples underwent HPLC-MS to identify metabolites within the fluids. Data were stratified for sample day and workload and compared using ANOVA for sample day and workload in a 3 × 2 factorial design. Within seminal fluid and blood serum, purine and pyrimidine metabolism pathways were the most affected (P ≤ 0.02). Three purine metabolism metabolites showed significant (P ≤ 0.03) workload × sample day interactions within seminal fluid. Xanthine, hypoxanthine, and inosine showed an increase (P ≤ 0.03) in abundance in moderate workload bulls throughout the 65-d breeding season and a decrease in abundance in high workload bulls. However, we speculate that a higher workload, likely with more total ejaculates and higher sperm cell turnover, may lead to an increased purine concentration for DNA synthesis. These foundational metabolomic data indicated that an important supplementation strategy to consider may be folate because of the relationship with purine metabolism. Overall, bull workload, particularly a high workload inducing catabolism, may provide a benefit for metabolic health and reproductive performance during the breeding season.

Bull workload during the breeding season primarily impacts purine metabolism in seminal fluid.