# The fertility of a sub-population of stallions is negatively affected by ambient climatic conditions, mediated through DNA damage

## TL;DR

This study shows that heat stress from climate conditions can reduce fertility in some stallions by damaging sperm DNA.

## Contribution

The study identifies a sub-population of stallions whose fertility is negatively impacted by heat stress through DNA damage, in a commercial breeding context.

## Key findings

- Stallions stabled at night and in paddocks during the day are exposed to high ambient climatic conditions.
- 18 stallions showed reduced fertility linked to heat stress, with six showing consistent DNA damage and fertility decline.
- Heat-induced subfertility is likely mediated by oxidative DNA damage in sperm.

## Abstract

Exposure to systemic heat stress in male mammals adversely affects sperm production, fertility, and DNA integrity. To date, few studies have investigated this phenomenon in horses, particularly in industry-relevant environments. Therefore, this study examined the relationship between ambient climatic conditions and fertility within a population of commercially fertile stallions. Post-coital semen samples (n = 804) were collected weekly from 46 Thoroughbred stallions during two successive breeding seasons (NSW, Australia; 22 weeks total). Semen samples were processed via single-layer colloidal centrifugation on-site to remove contaminants, seminal plasma and poor-quality sperm cells. The remaining sperm fraction was resuspended in Biggers, Whitten and Whittingham (BWW) medium for analyses, including sperm concentration, motility, and DNA damage. Fertility data (first cycle and per-cycle pregnancy rates) were collected from farms. Loggers were placed in the stables and paddocks of stallions to record ambient temperature and humidity, on 3-min cycles. Our results indicate that current management regimens, involving the stabling of stallions at night, with paddock access during the day, expose stallions to highest ambient climatic conditions. Polynomial distributed lag modelling identified 18 stallions whose fertility was adversely affected by heat stress, based on correlations between fertility and ambient climatic conditions (p ≤ 0.05). Of these 18 candidate stallions, six (13% of the overall population) exhibited repeated positive relationships between ambient climatic conditions and sperm DNA damage, and repeated negative relationships between fertility and sperm DNA damage (r ≥ 0.50; p ≤ 0.05). These results suggest that heat-induced subfertility was most likely mediated via oxidative damage to the sperm DNA, and that current management strategies should be assessed to prevent exposure of stallions to maximal climatic conditions.

The online version contains supplementary material available at 10.1007/s00484-025-03123-y.