Adaptive approaches to reproductive management of high-yielding dairy cows under thermal stress

Heat stress represents a major constraint for reproductive efficiency in modern dairy herds, particularly in high-producing cows. The present study evaluated the influence of seasonal variation in the temperature–humidity index (THI) on the reproductive performance of Holstein cows. It assessed the effectiveness of hormonal synchronization protocols under thermal load. The research was conducted at the private agricultural enterprise “Fortuna” in the Chernihiv region, Ukraine. A total of 165 Holstein cows were divided into three groups according to milk productivity: 4500 to 6000 kg, 6000 to 7500 kg, and 7500 to 9000 kg per lactation. Seasonal dynamics of conception rate and service period duration were analyzed in relation to monthly THI values. The results revealed a pronounced seasonal pattern of reproductive performance. During thermoneutral periods in spring and autumn, the conception rate ranged from 44.1 to 56.2 percent. In contrast, during the summer months, when THI ranged from 74 to 79 units, fertility declined sharply to 20.1-31.9 percent. The negative impact of heat stress was most evident in high-yielding cows, where conception rate during the summer period was 2.8 times lower than the autumn peak, and the service period increased by 18 to 49 days. More than 61 percent of cows in this Group exceeded a 100-day service period, indicating substantial reproductive maladaptation under thermal stress. Application of hormonal synchronization protocols improved reproductive outcomes during periods of elevated environmental temperature. The Double Ovsynch protocol demonstrated greater efficiency than the standard Ovsynch scheme, increasing conception rate in high-yielding cows by 8.4 % during the summer period. The results indicate that thermal stress induces metabolic and endocrine disturbances that impair follicular development and reduce reproductive efficiency. Overall, the findings demonstrate that effective reproductive management under conditions of increasing climatic pressure requires the combined use of reproductive biotechnologies and improved environmental management. Integration of hormonal synchronization protocols with strategies to reduce metabolic and thermal stress represents an important approach for maintaining fertility in high-producing dairy herds.