Breed Differences in Heat Tolerance of Day-Old Baby Chicks

The purpose of the tests reported here was to determine if breed and sex differences occurred in day-old baby chicks when exposed to high environmental temperature. A difference in susceptibility to high temperatures has been observed in White Leghorn hens and other dual-purpose breeds (Hutt, 1938). MATERIALS AND METHODS Day-old baby chicks were placed in standard fibre-board summer-size chick boxes, the dimensions of which were 24″×18″×6″. The boxes were sealed except for a single ⅝-inch hole in the lid of each compartment and kept in a constant temperature room for varying times at approximately 100°F. The temperatures inside the boxes at chick level were measured by means of thermocouples protected by a wire screen and a potentiometer (Leeds and Northrup 8662). At the termination of each test all chicks were autopsied and the sex ascertained. The time of exposure was determined by the percentage kill as estimated by the sound . . .