Differential effects of heat stress in three strains of laying hens

Hy-Line Brown, W36, and W98 hens were housed for 2 wk at 22 degrees C, exposed to 35 C heat stress (HS) for 2 wk, and had 2 wk of recovery at 22 C. Production parameters ( egg production, feed intake, egg quality measures, and mortality), acid-base status ( pH, pCO(2), HCO3-, pO(2)), intestinal calcium uptake, and expression of hepatic heat shock protein-70 were measured. All production parameters except mortality were reduced by HS in all 3 strains. There were strain x temperature interactions for egg production, feed intake, shell thickness, specific gravity, and yolk weight, with the least severe reduction in W98 hens. The W36 hens were intermediate for egg production and feed intake, and the Brown and W36 hens were equivalent for shell thickness, specific gravity, and yolk weight. There were no interactions for egg, albumen, and shell weights, Haugh units, and intestinal calcium uptake, and all were reduced by HS. Blood pH and pCO2 were reduced by HS, and hepatic heat shock protein-70 was increased, with no differences among strains. Mortality rates during HS were 16, 8, and 4% for the Brown, W98, and W36 hens, respectively, but were not subjected to statistical analysis. The results suggest a mechanism in the W98 bird that appears to confer an advantage during HS with regard to egg production.