Abstract
The magnitude of heat stress in dairy cattle is assessed using temperature humidity index (THI) widely in the world. The present study was conducted to assess the appropriateness of incorporating THI in recommending dairy cattle breeds to different areas of Sri Lanka. The study considered 10 different cattle genotypes commonly reared for milk production in the country. Daily THI values were computed by employing a thermal model, THI = Tdb - [0.55 - (0.55 × RH/100)] × (Tdb - 58) using data collected from 26 meteorological stations from 2005-2014. The THI values for different regions and the threshold THI values for milk production of 10 genotypes were used to produce THI maps. Among the crosses, Jersey crossbreds, except Jersey × Friesian cross, showed a high coping ability with comparatively high milk production capacity. Local cattle were not affected within the THI range observed in the country. THI based recommendation for distribution of dairy cattle could be made by identifying the variation of coping ability of cattle genotype and minimizing the vulnerability to climate change. Therefore, costly management interventions needed to mitigate the heat stress and related low productivity of dairy cattle under smallholder production systems could be abated. Key words: Climate change, dairy cattle breeds, heat stress.
Highlights
Responses of animals in challenging thermal environments are gradual, extending from coping to reproduction and production functions which is the last in the sequence of responses (DeShazer et al, 2009)
The temperature humidity index (THI) values were calculated using the equation which combined the effects of ambient temperature and Relative Humidity (RH) to assess the heat load intensity
The THI values ranged between 62 and 83, suggesting a fluctuation of THI across the country according to the general climatology of the country which is mainly governed by the Island position in the warm Indian Ocean, presence of a central massif being perpendicular to two approaching monsoons from opposite directions in each year and the close proximity to vast land mass of Indian sub-continent (Punyawardena, 2007)
Summary
Responses of animals in challenging thermal environments are gradual, extending from coping to reproduction and production functions which is the last in the sequence of responses (DeShazer et al, 2009). Several indices have been developed to measure level of comfort of the animal by combining different environmental or meteorological parameters depicting a unique thermo-physiological effect (Hahn et al, 2009). The threshold varies according to various factors such as genotype of the animal, age or life stage, level of nutrition and prior conditioning describing the combinations of environmental requirements for livestock. In this context, Hahn et al (2009) argued that air temperature alone does not make an adequate representation of the hot thermal environments on animal. THI could be used as a basis of categorization of geographical regions of a country according to suitability for different livestock breeds
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