الفهرس 
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Abstract
This study was carried out at Wageningen Agricultural University, The Netherlands. Heat tolerance of neonatal chicks was investigated by exposing chicks either for 48 h to a constant high environmental temperature (35, 37 and 39 C) or by exposing them once or twice for 8 h to a very high environmental temperature (41 C). Heat tolerance of chicks was measured in the short (during heat exposure) and long term (after heat exposure) . Parameters determined for short term effects were: body weight loss and water loss for specific body parts, (rectal) body temperature, heat production and mortality. Parameters determined for long term (4 weeks) effects were: daily gain, feed intake, feed conversion (feed: gain), body temperature and mortality.
Factors, of which effects on heat tolerance were measured, were: the water vapor pressure during incubation (45% vs 55% relative humidity), holding time in the hatcher after hatch (long vs short, i. e. early vs late hatching chicks), sex (male vs female), artificial administration (injection, s.c.) of saline with and without vitamin C (treated vs non-treated chicks) and breed (Fayoumi vs Hisex Brown).
During heat exposure, the chicks were accommodated in one of two identical climate respiration chambers. During this
period, chicks had no food or water available. After heat exposure, the chicks were accommodated for 4 weeks in normal grower cages at normal thermal conditions. During this growing period, chicks had free access to a commercial diet and water.
1) When exposed to heat as applied, the chicks could not maintain their initial heat production during second day. This indicates that the temperatures used were above the upper critical temperature.
2) Chicks hatching late and coming from eggs incubated at low (45%) relative humidity had a higher heat tolerance than chicks from other groups. These chicks lost less weight and water during heat exposure. After exposure, mortality in those groups was lowest. This means that an incubation relative humidity 10% lower than normal did not negatively affect heat tolerance of Hisex Brown. Low incubation relative humidity affected Fayoumi chicks more negatively than Hisex Brown chicks.
3) Sex did not affect the heat tolerance of neonatal chicks.
4) Eggs placed in an incubator to produce chicks to be exposed to high temperature should be uniform with respect to weight. Chicks from such a group will hatch in a short time span. As soon as possible after hatching, transportation should start.
5) Injection of saline improved heat tolerance of neonatal chicks, because it reduced body weight and water loss during exposure, and reduced mortality during the growing period. Vitamin C seemed to have a negative affect on heat tolerance when given in combination with saline. No effect of vitamin C or saline on production characteristics (gain, feed : gain) in the growing period.
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5) Fayoumi chicks lost less body weight and body water than Hisex Brown chicks. Heat production of Fayoumi chicks was higher at both days of exposure than that of Hisex Brown chicks. The similar body temperature and mortality in Fayoumi and Hisex Brown chicks,indicate that Hisex Brown chicks tolerate heat as well as Fayoumi chicks.
7) Chicks exposed twice to heat load lost more body weight and body water than those exposed once. Exposure to heat load twice increased mortality after heat exposure in comparison to heat exposure only once.