Effects of ambient temperature, age and wind speed on the thermal balance of layer-strain fowls.

1. In White Leghorn laying hybrids aged 14 to 399 d the effects were measured of ambient temperature (Ta) and wind speed (WS) on heat production (HP), evaporative heat loss (EHL), nonevaporative heat loss with or without considering heat storage (NEHL1 or NEHL2), conductance (c), total insulation (I) and colonic temperature (Tc) as well as on biological optimum temperature (BOT), thermoneutral temperature (TNT) and threshold temperature for evaporative heat loss (Te). 2. All thermal balance variables were affected by age, Ta and WS. 3. The relationships between Ta and HP or NEHL2 were best expressed in terms of a polynomial function at low WS, and in terms of a linear function at medium and high WS; the relationship between Ta and NEHL1 at all WS studied was best described in terms of linear functions, the slope of which increased with decreasing Ta and rising WS; those between Ta, EHL and c were best described in terms of exponential functions and between Ta and I in terms of quadratic functions, the rate of increase of which was higher at low Ta and medium and high WS. 4. The description of relationships between the age of birds and their HP, EHL, NEHL1 and NEHL2 was performed after logarithmic transformation and yielded a linearly decreasing function with advancing age. 5. Threshold temperatures increased as WS increased from 0.2 to 1.2 m/s; for EHL (Te) by up to 7.5 K, for Tc (BOT) and for HP (TNT) by 10 K. 6. The effects of age, Ta and WS on HP and both forms of NEHL were summarised by multiple regression equations.