Heat Stress Effects on Semen Quality in Dairy Bulls Under Commercial AI Conditions: A 5-Year Comparative Study Between the Netherlands and New Zealand

Abstract: Heat stress impairs spermatogenesis and reduces sperm quality in dairy bulls, yet most studies rely on small datasets from single locations. This study examined the effect of the temperature-humidity index (THI) on fresh and post-thaw semen quality in dairy bulls from different breeds between two countries with contrasting thermal environments, using a large retrospective dataset from a commercial AI centre. A total of 38,888 ejaculate records collected between 2020 and 2025 in the Netherlands (NL; n=25,107; 566 bulls; 8 breeds) and New Zealand (NZ; n=13,781; 596 bulls; 3 breeds) were analysed. Daily THI was calculated from meteorological station data and merged with each ejaculate by collection date and facility. Response variables included total sperm cells per ejaculate and post-thaw progressive motility. Responses were log-transformed to stabilize variance and modelled using linear mixed models with THI as a natural cubic spline, adjusting for year, breed, jump number, and bull age, with bull identity as a random effect. THI ranged from 23 to 88 in NL and from 46 to 83 in NZ. NZ bulls were exposed to THI above 60 on 80.4% of collection days versus 46.8% in NL. In NL, progressive motile sperm concentration declined significantly with increasing THI (p<0.01), from 8.44 x10^6 at THI<50 to 7.94 x10^6 at THI>=80, a reduction of 5.9%. Post-thaw progressive motility in NL showed a modest but statistically significant decline (p<0.001), from 51.8% at THI<50 to 51.6% at THI 70–80 (-0.22 percentage points). In NZ, where THI rarely falls below 50, post-thaw progressive motility was 43.9% at THI 50–60 and 45.7% at THI 70–80, a non-significant increase of 1.8 percentage points (p>0.05). Comparing the same THI classes across countries, NL showed a decline of 0.31% points while NZ showed an increase of 1.80% points, with no significant THI effect in NZ. Bull age was a significant predictor of post-thaw semen quality in both countries (p<0.001), though the within-bull spline model indicated this was driven primarily by population-level trends rather than acute individual responses. These findings demonstrate that heat stress effects on bull semen quality are trait- and environment-specific. The absence of THI-driven motility declines in chronically heat-exposed NZ bulls, contrasting with the gradient observed in NL, challenges universal THI thresholds across production systems and highlights the role of breed composition and long-term thermal adaptation in population-level heat sensitivity.