الفهرس 
Chapter I : IntroductionOnly 14 pages are availabe for public view
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Abstract
Introduction Days open considered a significant source of variation on
milk yield in Holstein cows. Cows those conceived shortly after calving had lower 305-day and total milk yield. In addition, that days open had a highly significant effect on peak milk yield (Tekerli, 2000 Holstein raised in Egypt as a high productive cows scored longer days open and this lead to prolonged calving interval (Ahmed et al 2002) ,so days open has an important role in determining calving interval and milk production,( Ali et al., 2003)
As days open increase ,fewer calves are produced per year leading to lower calf sales and fewer replacement heifers, also the breeding costs increase due to higher services per pregnancy .Moreover, the milk sold per
day decreases due to longer average days in milk 150 ). Furthermore,
increasing of days open due to more repeat breeders increase the veterinary
costs (Angela, 2004) Cows with a short dry period during their second gestation produced 89.1% as much milk as cows with a 60-days dry and that cows in their third or greater gestation produced 95.1% as much milk as cows with a 60-days dry (Rastani and Grummer, 2006 Reducing the duration of the dry period from 55 to 34 days improves
reproductive efficiency based on fewer days open and more cows confirmed
pregnant at 150days in milk (DIM). There is no negative effect of shortening
the dry period on cow health (Watters 2006 Moreover, there are some advantages of reducing dry period length including simplified management of dry cow through movement to one
group nutritional strategy, increase income from milk in the current lactation, decrease metabolic disorder and reduce strain on dry cow facilities
(Grummer and Rastani, 2004).