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.
