الفهرس 
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Abstract
Two experiments were conducted to study the effect of using some feed additives to alleviate heat stress in Holstein dairy cows during transition to early lactation under hot summer Egyptian climatic condition. In the first experiment 36 cows were randomly assigned into 4 dietary treatments ( each of 9cows), the control group was fed on the basal diet( BD), the 2nd group was fed on BD + Saccharomyces cervicae (SC 10g/h/d), the 3rd group was fed BD + Mannanoligosaccharids (MOS 10g/h/d), and the 4th group was fed on BD + a combination of SC+MOS. In the 2nd experiment 36 cows were assigned into 4 dietary treatments ( each of 9cows), the control group fed on BD only , the 2nd group was fed on BD+ Cr Propionate (Cr Pr), the 3rd group was fed BD + Cr Methionin (Cr Met) and the 4th group were fed on BD+ Cr Yeast (Cr Y). Cr supplements were added to achieve 500ppb Cr. All animals was fed on TMR formulated diet to meet their requirements ( NRC, 2001 ). Atmospheric temperature, (C0) relative humidity (RH), THI, body surface temperature indices, rectal temperature and respiration rate were recorded throughout the experimental period. BW, BCS, milk yield, milk composition (fat, protein, lactose, milk urea nitrogen, milk somatic cell count and milk Cr residues). Blood biochemical parameters (glucose, NEFA, cortisol, SOD, glucagon and progesterone), moreover some reproductive parameters were determined at different relevant weeks throughout the experimental period. Generally, the environmental data results confirmed that heat stress conditions in both experiments showed a trend to increase body surface temperature indices at all points, rectal temperature, respiration rate, milk somatic cell count and to decrease milk yield, FCM, milk fat, milk protein, blood glucose and reproductive parameters traits. The SC supplementation had significantly (P<0.05) decreased rectal temperature values postpartum, sustained better milk yield, FCM, and milk somatic cell count values and had significantly (P<0.05) increased rumen PH, blood glucose and decreased rumen ammonia nitrogen, blood NEFA and blood SOD. On the other hand Cr fortification significantly (P<0.05) improved milk yield and FCM. The second group that supplemented with Cr pr surpassed other groups and achieved highest milk yield. A numerical increase in milk fat content, meanwhile a significant (P<0.05) positive effect on milk protein, milk lactose were noticed beside a decrease in MUN. Neither the BW nor BCS was affected. Blood glucose was reduced in most wks. Moreover, Cr fed groups exhibited significant (P<0.05) lower values of NEFA. Also there were improvements on the values of service per conception but days elapsed to first heat were not significantly (P>0.05) affected. Levels of serum cortisol tended to have insignificantly (P>0.05) lowered values throughout the second experiment. It is to be concluded that, such commercially available additives could be fortified to the ration of high producing dairy cows to ameliorate the adverse effects of heat stress on the performance and productivity under the Egyptian hot summer conditions.