| Abstrakt: |
Romney-cross ewe lambs (n = 27) were subjected to controlled environmental conditions to induce acute heat stress. The goals of the study were to: (1) determine appropriate physiological indicators of acute heat stress, (2) determine threshold rectal temperatures (Tr) for catecholamine and cortisol release, (3) determine effects on meat quality, and (4) assess the effect of dehydration on the above measures. There were 3 treatments: control (ambient temperature and humidity), heat stress (33C, and 85-100% humidity), and heat stress combined with water deprivation. The duration of the treatment period was 12 h. Respiration rate (Rf) and rectal temperature (Tr) were highly correlated with increasing temperature humidity index (THI) (r > 0.75, P < 0.001), whereas heart rate was less responsive to THI (r = 0.30, P < 0.05). The welfare of these lambs was at risk at Tr greater than 40.5C, a point at which respiration rate was maximal and unable to prevent further increases in Tr. Plasma cortisol concentrations were increased in heat-stressed lambs after Tr reached approximately 40.7C. Plasma catecholamines were only elevated in lambs when Tr was greater than 42C. The majority of lambs subjected to heat stress had a Tr less than 42C, and there were no significant effects on meat quality. Despite exhibiting increases in plasma protein concentrations, there were no indications that dehydrated lambs were under additional stress during heat challenge in comparison with hydrated lambs. |
