الفهرس 
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Abstract
The present investigation was carried out at the El-Alamiah dairy farm, which belongs to Universal Company for Agricultural Development and Soil Reclamation located 90 Km south Alex - Egypt. 5844 lactation records of 1483 Holstein-Friesian and Friesian dairy cows covering the period from 1996 to 2018 were used in the present study. Cows were grouped according to their productive and reproductive performance. Milk yield was measured weekly on fixed day. Cows were milked in milking parlors two times daily. Cows were artificially inseminated using frozen-thawed semen. A cow was stopped from milking when its daily milk yield decreases to about 10 kg of milk despite of calving date. Depending on body weight, level of milk yield and milk fat percentage, cows were fed according to NRC (1989). The objectives of the present investigation were to indicate the effect of the length of DP on the subsequent lactation performance through evaluating the performance of milk production traits, reproductive traits, health traits and lifetime traits of Holstein Friesian cows, raised in Egypt, and determine the best length of dry period based on milk yield, reproduction, lifetime performance and cow health. The main results obtained could be summarized as following:i. performance of the studied herd 1. Lactation length (LL) and days open (DO) were high being 128.5 and 139.8 days, respectively. Also, length of dry period (DP) ranged from 21-480 d with an average of 75.6 d. 2. The average of non-productive period of a cow was 35 months out of longevity of 81 month and productive life length (PL) of 53 months. 3. A cow produced 28423 kg lifetime milk with an average 11.45 kg per day in 3.91 complete lactation number (CLN). 4. Mastitis and Lameness Scores were near zero (0.0082 and 0.0128, respectively). ii. Factors affecting dry period length 1. All studied factors had significant impact on length of dry period (DP).
2. First parity had the highest DP (101.75d) which tended to decrease in the following parities to reach the lowest value in parity 5 and above (61.29 d).
3. Length of DP was affected significantly by calving season. The winter calvers had the shortest DP (67.41 d). iii. Factors affecting lactation performance
1. Sire, dry period length, parity, calving year and calving season had highly significant effects on most of studied traits
2. first calving affected Mastitis numbers, productive life length and longevity length. The year of first calving had an effect on cow health and lifetime traits.
3. Age at first calving (AFC) had no significant on productive, reproductive and health traits. Also, lifetime traits were not affected by AFC except Lon and LDY.iv. Effect of DP on the subsequent lactation performance
1. Increasing DP length to 90 d increased 305-d MY and LL, but all other productive traits decreased. 44
2. DP category of 76-90 d showed the highest TMY and 305 d-MY (7806.29 and 6197.82 kg, respectively) and the longest LL (333.83 d). So, it can be concluded that this particular DP resulted the best production performance of cows. Also, it showed the longest DO and CI of 203.22 d and 15.27 month, respectively. On the contrary, the shortest DP (< 31 d) showed the best reproductive performance.
3. The 76-90d DP and that > 91d were associated with the highest lameness numbers (0.7976 and0.0104, respectively) but with no significant effects.
4. Increasing DP up to 61-75d caused increase in all lifetime traits which were decreased thereafter.v. Relationships between Dry Period and subsequent lactation traits
1. The correlation coefficients between DP and all productive traits were significantly negative and ranged between -0.064 to -0.038.
2. Correlations of DP with all reproductive traits were significantly positive with a range of 0.034 to 0.148. But, DP had no significant correlations with the health traits.
3. The regression coefficients of TMY, 305dMY, LTY and LDY on DP were significantly negative (-4.4, -5.35, -35.26 and -0.018 kg, respectively). Therefore, increasing dry period will cause decrease in milk yield of the lactating cows. Also, the regression coefficients of DO and CI on dry period were positive (0.22 and 0.23 d, respectively). from the obtained results, it was concluded that, the dry period of 61-75 days is the most optimum for Holstein Cows raised under subtropical warm climate. However, more studies to confirm these results under the Egyptian conditions are required.