Impact of heat stress on the post-hatch growth, morphometrics, and allometry of the chicken ( <i>Gallus gallus</i> )

1. Heat stress negatively affects animal growth. This study examined the impact of heat stress on a modern broiler (Ross 708) chicken compared to a heritage meat-type chicken line (UIUC).2. To mimic the effects of a heat wave, birds were exposed to daily, cyclic high ambient temperatures between post-hatch d 21-42, while control birds were maintained at thermoneutral temperature throughout this period.3. On d 28 and 42 post-hatch, eight heat-stressed and eight control birds from both lines were weighed, necropsied and total organ masses were determined for the brain, breast muscle, liver, small intestine, and spleen.4. Normalised tissue mass at d 42 under thermoneutral conditions for the Ross 708 breast muscle was 2.4-fold larger than that of the UIUC chickens. The cross-sectional area of the Ross 708 breast muscle cells was also twofold larger than that of the UIUC birds. In contrast, UIUC birds had larger heart, spleen, duodenum, jejunum, ileum and brain masses than the Ross 708 chickens when raised at the thermoneutral temperature.5. These results indicated that heat-stress shifted growth resources away from immune, skeletal muscle, neuronal and hepatic systems to maintain the heart and small intestine.