الفهرس 
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Abstract
ABSTRACT
The aim of this study focused on evaluating the performance of pure HO and MO X HO crossbred cows by investigating the expression variance of the PRLR gene and its association with productive traits and evaluation of reproductive performance and disorders in the two breeds under Egyptian conditions. Moreover, studying the impact of environmental variables in three farms of Holstein-Friesian dairy cows in different geographical zones on milk yield and some reproductive traits. Also studying the performance of two farms of pure Holstein-Friesian dairy cows that had evidently varied environmental conditions, including productive, reproductive traits and the incidence of some managerial disorders.
The results in study (1) clarified that cross breed between MO X HO is not the best solution for increasing the milk yield as pure HO cows had higher milk yields during all lactations than MO X HO cross bred cows by approximately (3.1, 7.3, and 8.6%) during the (first, second, and third parties, respectively). Pure HO had more stability and ability to maintain a higher level of milk yield because the reduction in PRLR gene expression level did not differ between the second and third parties, while in MO X HO cross bred cows there was a significant decrease in the third parity compared to the second parity. MO X HO cross bred cows had significant superior advantages for reproductive performance traits than pure HO specially during summer season and showed lower AFC, NSPC, DOPN, and CI, however it recorded higher incidence of the reproductive problems (retained placenta, endometritis, and repeat breeder). While study (2) revealed that WS, RH, and temperature were the main indicators for the level of heat stress. Farms with higher temperature degree displayed higher THI values and lower RH. The highest positive correlation was found between temperature and THI (r = 0.984). Area of high THI values exhibits lower WS. DMY decreased when THI increased. Farms situated in the lower latitude had longer length of exposure to heat stress and losing more milk than farms in high latitude. Heat stress had cumulative effect, impairing the reproductive performance and a negative economic impact by declining the DMY, increasing the NSPC and reducing both CR and PR. Results from study (3) declared that multiparous cows had less M305 and DIM than primiparous cows regardless of the fam environmental conditions. Heat stress had a minor effect on primiparous and a major effect on multiparous cows. Lameness increased in the farm with high RH than the farm with higher THI, while mastitis increased in the farm with higher temperatures and higher THI. The highest percentage of metritis and RP recorded in July at the farm with lower THI values (32.60% and 19.34%) for metritis and RP respectively. It could be cocluded that pure HO cows produce more milk together with higher relative expression of PRLR gene versus MO X HO crossbred cows at the three parities. THI can be used as prospective indicator for heat stress and milk yield losses particularly in hot humid environments, where milk yield decreased when THI increased.