Impact of temperature humidity index on estrus behaviour and conception rates of Jersey crossbred cows in the mid country intermediate zone in Sri Lanka

Abstract

Heat stress is a significant challenge in dairy farming in Sri Lanka, particularly for highproducing Bos taurus breeds like Jersey cows. Unlike the more resilient Bos indicus cows, Jersey cows generate greater metabolic heat and are therefore more vulnerable. The Temperature- Humidity Index (THI) is widely recognised as an indicator to quantify heat stress, and values above 72 generally cause thermal discomfort. This adversely influences both reproductive and productive performances. The impact of THI on reproduction has not been previously studied in this region. Hence, this study evaluated the impact of THI on reproductive behaviour and conception rates of Jersey crossbred cows maintained under intensive management. The study was conducted on a mid-country intermediate zone research farm where Jersey crossbred cows—F2/F3 generations derived from imported Australian cattle—are maintained by the National Livestock Development Board of Sri Lanka. THI values were recorded over a six-month period from January to June using both digital temperature-humidity gauges and weather station data. The six months were divided into two phases: Phase 1 (January-March) and Phase 2 (April-June). A generalised linear model was used to analyse differences between the two phases (using SAS 9.4). THI values differed significantly between Phase 1 and Phase 2. Phase 2 consistently exhibited higher THI values across the 06:00 - 10:00, 10:00 - 14:00, 18:00 - 22:00, 22:00 - 02:00, and 02:00 - 06:00 time frames (p < 0.0001). The visibility of pronounced estrous signs was lower in Phase 2, with marginal significance (p = 0.05), while the number of successful AIs was significantly higher in Phase 1 (n=16) than in Phase 2 (n=5). The reduced display of estrous signs interferes with visual detection and timed AI, ultimately lowering conception rates and impacting milk production efficiency and farm profitability. In conclusion, these findings emphasise the necessity of managing heat stress in Jersey crossbred cows to maintain optimal reproductive performance. Further studies are needed to evaluate the effects of THI mitigation and timed AI on improving reproductive outcomes in crossbred Jersey cows in the region.