| Summary: | This thesis includes two studies, in the first study 693 complete lactation records of 693 Holstein cows were collected from a commercial farm (Dina Farm) during years 2011 and 2012. The aim of the first study was to evaluate some productive and reproductive traits of Holstein cows raised under two different housing systems. The first housing system (FHS) was characterized by the presence of six units of cooling operated manually, with shed height of 4.5 m and 18% shaded area. The second housing system (SHS) was characterized by the presence of 16 automatic cooling units, with a shed height of 8 m and 38% shaded area. Data on 305 days milk yield (305-dMY), daily milk yield (DMY), days open (DO) and number of services per conception (NSPC) were collected and analysed using XLSTAT. The overall means of 305-dMY, DMY, DO and NSPC were 8550±330 kg, 26.9±1.04 kg, 147±8.8 days and 3.5±0.25 services, respectively. Cows housed under the SHS produced 11% higher 305-dMY (8980 kg vs. 8120 kg) (P < 0.05), 11% higher DMY, (28.3 kg vs. 25.5 kg) (P < 0.05) and non-significant shorter DO than those of FHS (138 days vs. 156 days). Cows maintained under the SHS had lower (P < 0.05) NSPC than those maintained under the FHS by 26% (3.1 vs. 3.9 services). These results could be attributed mainly to the more comfortable microclimate conditions in the SHS, mediated by greater shaded area, roof height and effective cooling system. The second study used 1152 complete lactation records for 576 Holstein cows, collected from two commercial farms (Alex. Copenhagen and Sami Asaad farm) during the period from 2007 to 2010 to determine the effect of average daily weight gain (ADG) of Holstein heifers from birth to conception on their age at first calving (AFC) and subsequent milk yield. Heifers were divided into four groups based on their ADG (650 g/day, 775 g/day, 875 g/day and 950 g/day for G1, G2, G3 and G4, respectively). Heifers of G4 reached the AFC 5.3 months earlier (P<0.001) than those of G1. Cows of G4 had a similar 305 days milk yield in the first lactation compared to G1 but scored non-significant higher milk yield in the second one by 535 kg. G4 had non-significant increase in daily milk yield and milk yield/day of cow age until the end of the first lactation (MY1/DCA) compared with G1. Milk yield/day of cow age of G4 until the end of the second lactation (MY2/DCA) increased (P<0.01) by 12.7 % compared to G1. Days open of G4 in the first lactation (DO1) was shorter (P<0.05) than that of G1 by 23 %. This trend was extended to the second lactation, however, it was insignificant. The present results may indicate that productive and reproductive performance of the faster growing Holstein heifers is better than that of slower growing ones.
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