Thermoneutral zone and resting metabolic rate of growing White Leghorn-type chickens.

1. Metabolic rate was determined once a week in pullets from commercial flocks of the local White Leghorn strain from the age of 1 d to 13 weeks. 2. The equations relating minimal resting metabolic rate and body weight were: for body weights of 40 to 163 g Y = 1.83 W0.986, and for 163 to 1 200 g Y = 11.64 W0.623, where Y = O2 consumption (ml/bird h) and W = body weight (g). 3. The relationship between lower critical temperature (Tcl) and age may be described by two equations: for pullets of 1 to 21 d of age Y = 35 - 0.286 X, and for 21 to 70 d of age Y = 63.3 X-0.254, where Y = Tcl (degrees C) and X = age (d). 4. Tcl related to body weight is also described by two equations: for body weights of 40 to 163 g Y = 42 - 1.18 W0.480, and for 163 to 800 g Y = 42 - 3.60 W0.260. 5. These equations could be used to calculate the Tcl on a day-to-day basis related to age or body weight.