الفهرس 
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Abstract
The present study was carried out at the Poultry Physiology Lab, Department of Poultry Production, Faculty of Agriculture, Ain Shams University, Egypt, during the period June to August, 2018.
The main objective of this study was to evaluate the effect of early and late post – weaning heat conditioning of rabbits on productive performance, blood biochemical constituents, antioxidant defense system and heat – shock protein (HSP) gene expression in two strains of growing rabbits.
A total number of 64 weaned (six weeks old) kids from both of New Zealand white (NZW) and Baladi Black (BB) strains were used in the study (32 kids per each strain). The initial average of body weight was 500 and 450 g for NZW and BB rabbits, respectively. After an adaptation period of one week, rabbits of each strain were divided randomly into two groups: control and heat stressed (HS) groups. The control groups were housed under the prevailing in door ambient air temperature and relative humidity of the breeding room (26 – 28 ±1°C and 31- 34 % RH) while the HS groups were exposed to daily four hours of high temperature (36 ± 1 ° C) and 55-60% RH. This exposure procedure was repeated at the end of the experiment period (13 weeks of age). Data of response were productive performance, blood parameters as well as a portion of the spleen tissue, to estimate heat shock proteins 70 and 90 kDa.
The main results could be summarized as follows:
After the first heat exposure:
• Live body weight was significantly heavier in NZW than Baladi Black (BB) rabbits.
• Heat stress exposure increased LBW of stressed rabbits than the controls.
• A significant increase in LBW in response to strain × HS interaction was recorded, with the NZW rabbits being the best.
• Daily gain was significantly heavier in NZW than Baldi Black (BB) rabbits.
• Heat stress exposure increased daily gain of rabbits than the control group.
• A significant increase in daily gain in response to strain × HS interaction was noticed with the NZW rabbits being the best.
• Feed consumption was significantly increased for BB rabbits than NZW ones.
• Heat exposure increased feed consumption in general especially for NZW rabbits.
• Regardless heat exposure, feed conversion ratio was significantly better for NZW than BB rabbits.
• Heat exposure had bad effect on feed conversion ratio compared to the control.
• A significant increase in feed consumption and feed conversion ratio in response to strain × HS interaction was recorded with NZW rabbits being the best.
• Water consumption was significantly increased for NZW than BB rabbits.
• Heat stress exposure increased water consumption than the control group.
• A significant increase in water consumption in response to strain × HS interaction with the NZW rabbits being consumed more water than BB ones.
• Rectal temperature was not significant affected by rabbits strain, while
• heat exposure increased rectal temperature than control group.
• A significant increase in rectal temperature in response to strain × HS interaction with the BB rabbits showed better thermo-tolerance than NZW.
• Blood plasma proteins (TP, Alb, Globulin, A/G ratio) did not significantly affected by rabbit’s strain, but they were increased significantly after heat exposure.
• A significant increase in TP, Alb and Globulin in response to strain × HS interaction with the NZW rabbits having the higher levels.
• Plasma Cholesterol and triglycerides were significantly decreased but HDL was significantly increased in BB than NZW rabbits.
• Liver function-related enzymes activity (ALT and AST) was not significantly affected by the strain and heat exposure of rabbits. This was also observed for plasma creatinine and Urea levels as kidney function indicators.
• A significant increase in AST and ALT enzymes, and both creatinine and urea levels in response to strain × HS interaction in NZW rabbits.
• Hemoglobin (Hb) concentration was significantly higher in NZW than BB rabbits, however early age heat exposure and strain by HS interaction had no effect.
• Plasma tri-iodothyronine (T3) concentration was not significantly affected by rabbits strains, but early age heat exposure and both strain X HS interaction were significantly increased its level especially in HS NZW rabbits.
• Insulin- like growth (IGF) concentration of growing rabbits was not significantly affect by strain, while HS and the interaction effect caused a significant increase in its concentration.
• Plasma Corticosterone concentration was significantly higher in NZW than BB rabbits and in early heat stressed than the control rabbits.
• A significant increase in Corticosterone concentration in response to strain × HS interaction with the NZW rabbits showed the higher level.
After the second period heat exposure:
• Live body weight was significantly higher in NZW rabbits than BB ones.
• Early- age heat exposure of rabbits had resulted in a significant effect on live body weight of both strains after the second exposure period with NZW rabbits showing heavier weights. This was also observed for daily gain results which are better for NZW than BB rabbits.
• Feed consumption was significantly increased for NZW rabbits than BB ones.
• A significant increase in feed consumption in response to strain × HS interaction effect with the BB rabbits consumed more feed than NZW strain
• however, feed conversion ratio was improved by the interaction response.
• Water consumption was significantly increased as age advanced in NZW than BB rabbits respectively. Heat exposure exaggerate this criteria compared by the control non- heat exposed rabbits.
• A significant increase in water consumption in response to strain × HS interaction with the NZW rabbits recorded the highest value.
• Rectal temperature (RT) was significantly reduced in heat acclimated than control rabbits, regardless the strain effect.
• Blood plasma proteins (TP, Alb, Globulin, A/G ratio) was not significantly affected by rabbits strains, while late heat exposure at marketing age had significantly increased their levels compared to the control group.
• A significant increase in TP and albumin, but not globulin, in response to strain × HS interaction was recorded with the NZW rabbits being the best.
• Plasma lipids fractions (Cholest., Tri-g. ) were significantly decreased where BB had the lower values than NZW rabbits, while HDL level was significantly increased. This trend of reduction was also recorded as a physiological response to heat exposure and /or HS x strain interaction.
• ALT and AST enzymes activity did not significantly affected by strains and heat treatments; however the HS by strain interaction increased their activity especially in BB rabbits.
• Creatinine and Urea levels were significantly increased in NZW than in BB rabbits at the marketing age.
• Hemoglobin concentration was significantly higher in NZW than BB rabbits, but HS has no effect. However, a significant increase in Hb concentration as a response to strain × HS interaction was observed in NZW rabbits than BB ones.
• Tri-iodothyronine (T3) concentration was not significantly influenced by strain, but HS- acclimated rabbits showed significantly higher values.
• A significant increase in T3 in response to strains × HS interaction with the NZW rabbits being has the high level.
• Insulin- like growth (IGF) concentration of rabbits was not significantly affected by strain, while heat stress exposure increased its level than control group. Moreover, a significant increase in IGF concentration in response to strains × HS interaction was noticed for the NZW rabbits.
• Corticosterone concentration was significantly increased in NZW than BB rabbits, and in HS- exposed rabbits than controls. Also, its level increased in response to strains × HS interaction.
Gene expression results:
• There is a significant effect of early age heat exposure on heat shock protein ( HSP 70 and HSP 90), expression in spleen tissues, regardless rabbits breed. Both HSPs were significantly higher in heat stressed rabbits than control ones.
• At 13 weeks of age, early- age heat stressed rabbits showed significantly high level of gene expression for heat shock protein -related genes (HSP70 and HSP90) indicating the adaptation of rabbit to high temperature.
• Similar results are observed for both hepatic Hsp70 and hsp90 gene expression which increased in thermal conditioned rabbits compared to their relative control.
CONCLUSION
It is concluded that early age heat exposure of weaning rabbits could be used as an alternative tool to alleviate the negative impact of heat stress at older age.