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-The objective of the present study was to assess the effective thermal conductivity of the hair coat (k ef , mW.m -1 .K -1 ) of Holstein cows in a tropical environment, as related to conduction and radiation in the absence of free convection. The average k ef was 49.72 mW.m -1 .K -1 , about twice the conductivity of the air (26 mW.m -1 .K -1 ) and much less than that of the hair fibres (260 mW.m -1 .K -1 ). The low k ef values were attributed mainly to the small cross area of individual hairs, ρ ef /ρ f (17.2% and 21.3% for black and white hairs, respectively). White coats were denser, with longer hairs and significantly higher k ef (53.15 mW.m -1 .K -1 ) than that of the black hairs (49.25 mW.m -1 .K -1 ). The heritability coefficient of the effective thermal conductivity was calculated as h 2 =0.18 the possibility was discussed of selecting cattle for increased heat transfer through the hair coat.

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