الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Summer depression of reproductive performance of dairy cattle is a worldwide problem causing sever economic losses in dairy cattle. As the high temperature and humidity have a negative effect of on fertility. Moreover Heat stress laeds to decreased feed intake which in turn causes reduction in milk production in dairy cattle as well as depressing the secretory function of the udder.The current study aimed to investigate if the dairy farms in north Egypt is subjected to heat stress (HS) during Summer season and to investigate the effect of summer season on reproductive performance of these cows. the higher conception rate was recorded during months of cold weather , while the lower conception rate was recorded hot weather months because of HS. And increased number of services per conception as well as increased days open recorded in summer calving cows.The THI values are the most practical means of assessing the exposure of cattle to heat stress, The effect of monthes of the years on CR in relation to THI Representing highest THI than the critical level (74) during June, to September, with anegative effect on CR.Cortisol rate as well as HSP 70 rate were significantly lower in Winter in comparison to Summer. While Nitric oxide rate was significantly higher in Winter in comparison to Summer.from this study we could conclude that: During summer season in north Egypt there is a heat stress as expressed by THI (i.e THI ≥ 74; indicate heat stress).