Effects of the naked-neck gene on traits associated with egg laying in a dwarf stock at two temperatures.

Naked-neck (NaNa and Nana+) and normal hens, all dwarf (dw), were compared for laying performance in two groups at 15 to 20 or 30 degrees C. Naked-neck hens, especially homozygotes, weighed less only at the cooler temperature. Body fatness was not different. Normal, but not naked-neck birds, showed a depression in food intake and egg output at 30 degrees C. In the hot environment mean egg weight and egg mass output were highest for naked-neck homozygotes. At the cooler temperature mean egg weight was also higher for this genotype, at fixed body weight. The decline in egg mass with time seemed greater for fully-feathered layers at high temperature. At moderate temperature residual food intake was slightly increased by the naked-neck gene, but owing to the reduced body weight, food utilisation was not poorer for the NaNa or Nana+ genotypes. At higher temperature Nana+ and especially NaNa hens utilised food more efficiently. An increase in wattle size and a slight decrease in body temperature were associated with the Na allele.