Effects of air temperature on body temperatures and sensible heat loss of Friesian and Jersey calves at 12 and 76 days of age

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Abstract
1. Measurements have been made of sensible heat flow, body temperature and some hair coat characteristics, at 10 different air temperatures, on each of 24 Friesian and Jersey calves aged 12 and 76 days.2. Minimum values for thermal conductance of the trunk tissues were 29·5 and 37·3 kcal/m2.hr per °C for the 12-day-old Friesian and Jersey calves respectively; corresponding values for the 76-day-old calves were 22·5 and 19·9 kcal/m2.hr per °C.3. Weight of hair per m2 surface area was greater for Friesian calves and for younger calves; Friesian calves had longer hair and deeper coats. Coat depth increased at low temperatures in all calves.4. The air temperatures at which the ears became vasoconstricted were 18·8 and 21·8°C for the 12-day-old Friesian and Jersey calves respectively; the corresponding values for the 76-day-old calves were 10·1 and 17·8°C. Vasoconstriction occurred in the back legs at a lower air temperature than in the ears or on the trunk surface.5. The intake of dry matter as fresh grass was 1·15 kg/day for Friesians and 0·72 kg/day for Jerseys, aged 76 days; the Friesians weighed 74 kg and the Jerseys 59 kg at this age.