Fasting metabolic rate in hens. 2. Strain differences and heritability estimates.

Short-term measurements of fasting metabolic rate (FMR) of two medium heavy breeds, Rhode Island Red (RIR), Light Sussex (SS), and their reciprocal crosses, were recorded between 40 and 60 weeks of age. Pure breeds differed significantly in fasting metabolism corrected for body weight, feather loss, and activity. During lighted hours, FMR of RIR exceeded FMR of SS hens by .8 kJ/hr. For the dark period the difference was 1.5 kJ/hr. Fasting metabolic rates of reciprocal crosses fell between parental means. Heterosis in egg mass output of 5 to 6% and 3 to 4% in feed conversion ratio, respectively, did not increase FMR of crosses. Heritability estimates for FMR within stocks were moderately high (FMRD: h2S = .23 to .43). An adjustment for body weight, feather loss, and activity decreased heritabilities, but a genetic component in FMR still persisted (h2S = .05 to .22).