Heat Stress and Bull Semen: From Endocrine and Molecular Dysregulation to Targeted Intervention

Heat stress, a critical environmental stressor intensified by global warming, increasingly threatens bull semen quality. This review synthesizes the multi-level mechanisms underlying this impairment. Heat-stressed bulls exhibit behavioral changes that serve as early indicators of reproductive dysfunction, alongside pronounced declines in routine semen parameters and alterations in the seminal plasma biochemical microenvironment. At the endocrine level, heat stress disrupts the hypothalamic–pituitary axes, suppressing testosterone synthesis and perturbing the testicular microenvironment to compromise spermatogenesis. At the molecular level, altered expression of heat-stress-related genes and epigenetic reprogramming—involving non-coding RNAs, DNA methylation, and m6A methylation—collectively disturb this process. Notably, the “gut–testis” axis emerges as a key pathway: heat-induced gut microbiota dysbiosis compromises intestinal barrier integrity, and immune and endocrine signals from translocated microbial metabolites ultimately impair testicular function and semen quality, as evidenced by cross-species studies. At the application level, the above mechanistic evidence, which is largely derived from model animals or non-bovine livestock, still requires direct and systematic validation in bulls. Collectively, this review provides an integrative mechanistic framework for heat stress-induced deterioration of bull semen quality and highlights critical directions for developing targeted mitigation strategies under global climate change.