الفهرس 
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Abstract
The present study was carried out at the experimental farm of Animal Production Department, Faculty of Agriculture, Minia University.To study effect of two level of dietary rumen degradable protein and effect of some protected amino acids supplementation (Methionine, lysine and threonine), on some productive performance and some physiological response of dairy cattle. Sixteen Holstein cows were selected, twelfth in their third and four in its fifth lactation season, after two week of parturition, were served in this study during April to August 2015. The average body weight of these cows was 500 ± 18kg. Rations were formulated to meet 115% of the cow’s metabolizable energy (ME) and 110 % metabolizable protein (MP) requirements for maintenance and production, according to AFRC (1993) recommendations for dairy cattle. In this experiment, two levels of dietary rumen degradable (65 and 75% of the total dietary protein, 11.66 or 13.71% of dry matter) were studied. These two rations were supplemented or unsupplemented with protected amino acids (methionine, lysine and threonine). Economical feature, supplementation of PAA as (6.25 g protected methionine /h/ day, 18.24 g protected lysine /h/ day and 13.14 g protected threonine /h/ day, T2) or (19.22 g protected methionine /h/ day, 56.13 g protected lysine /h/ day and 34.80 g protected threonine /h/ day, T4) to lactating cows ration increased the feeding cost by 9.37 and 26.17 % when compared with feeding T1 or T3(un supplemented rations), respectively. However, the daily return (DR) was increased by 9.63 and 6.93 % upon feeding rations T2 or T4 compared with T1 and T3.