Seasonal and phenotypic effects on sperm quality in a taurine breed acclimated to the tropics versus a specialized commercial Zebu breed

Heat stress is a growing challenge for cattle production in tropical environments, particularly for Bos taurus breeds, which are generally less heat‑tolerant. Several locally adapted breeds have evolved morphological and physiological traits that combine characteristics of both Bos taurus and Bos indicus subspecies. We evaluated seasonal changes in semen quality and their association with environmental conditions and external body morphology in Pantaneiro (locally adapted Bos taurus) and Nelore (Bos indicus) bulls in the Brazilian Pantanal. Environmental conditions differed markedly between seasons, with the rainy season presenting a higher discomfort index. During the dry season, the index ranged from comfort to danger (45–83), whereas in the rainy season it ranged from alert to emergency (74–84). Fresh semen quality showed no major seasonal or breed differences, with motility averaging around 83% in both groups. In contrast, post-thaw sperm kinematic parameters, evaluated using the computer-assisted sperm analysis (CASA) system, showed clear breed differences. Pantaneiro bulls had higher motile sperm (43% vs. 31% in the dry season; 45% vs. 34% in the rainy season; P = 0.04), progressive sperm (26% vs. 17% in the dry season; 23% vs. 16% in the rainy season; P = 0.02), and the proportion of spermatozoa classified as rapid by CASA (32% vs. 20% in the dry season; 30% vs. 21% in the rainy season; P = 0.01) compared to Nelore bulls. Average path velocity (VAP) and straight-line velocity (VSL) were also higher in Pantaneiro bulls, particularly during the dry season. The viability of post-thaw sperm membranes and chromatin integrity followed the same pattern: the proportion of cells with intact acrosome, intact plasma membrane and high mitochondrial membrane potential was higher in Pantaneiro (41% in the dry season, 52% in the rainy season) than in Nelore bulls (34% in both seasons). External morphological traits, such as body size, skinfold thickness, hair length and skin color, were negatively correlated with spermatic kinetics and membrane integrity (r ≤ − 0.30). Overall, Pantaneiro bulls outperformed Nelore bulls under the same conditions. Both breeds maintained acceptable semen quality under heat stress, but Pantaneiro bulls showed more stable and superior post-thaw performance. These results reinforce the conservation value of this genetic resource. As a Bos taurus well adapted to tropical environments, it is an important resource for breeding programs targeting resilience and reproductive efficiency.