الفهرس 
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Abstract
The present study was performed to investigate the effect of feeding prebiotic (mannan-oligosaccharides and β-glucans combination) and organic acids mixture (calcium formate, calcium propionate &citric acid) either single or in a combination on broilers fed low protein diets on growth performance, carcass traits and abdominal fat, meat cholesterol, triglycerides and chemical composition, blood parameters and immune response of broilers.
A total number of 200 birds one- day old unsexed broiler chicks (Ross 308 breed) were obtained from local commercial source, weighed and randomly distributed to 10 equal groups each of 20 chicks. The initial average weight of the experimental chicks was 40g. Birds were fed according to two phases feeding program: starter (0 – 21 days of age) and grower-finisher (22–42 days of age). Experiment diets containing four levels of dietary crude protein (100, 95, 90 & 85% of NRC protein requirements) and two feed-additive program. The experimental diets were as follows: (T1) The basal control diet (100% of NRC protein requirements) without any feed additives , (T2) 95% of NRC protein requirements supplemented by prebiotic; (T3) 95% of NRC protein requirements supplemented by organic acids; (T4) 95% of NRC protein requirements supplemented by combination of prebiotic and organic acids; (T5) 90 of NRC protein requirements supplemented by prebiotic; (T6) 90% of NRC protein requirements supplemented by organic acids; (T7) 90% of NRC protein requirements supplemented by combination of prebiotic and organic acids; (T8) 85% of NRC protein requirements supplemented by prebiotic; (T9) 85% of NRC protein requirements supplemented by organic acids & (T10) 85% of NRC protein requirements supplemented by combination of prebiotic and organic acids. The feed Additives were commercial products in powder form. Additives and their dosages were: (prebiotic at level starters & growers 1 g/Kg), (organic acid mixture at level of 4.5 g/Kg) and (combination at level of 0.5g/kg prebiotic plus 2.25g/kg organic acids). Mortality rate, growth performance, carcass traits and abdominal fat, meat cholesterol, triglycerides and chemical composition, blood parameters and immune response of broilers and economical feed efficiency were assessed.
The results of experiment showed that:
1. Throughout the experiment, there was only two cases of mortality. One of them in T2 which fed on 95% NRC protein diet supplemented with prebiotic, while the other case in T5 which fed on 90% NRC protein diet supplemented with prebiotic, and these cases could have accounted to natural cases
2. The addition of 1g/kg prebiotic to 95% of NRC protein diet (T2) of broiler has the best body weight, cumulative weight gain and better feed conversion compared to control group and other treated groups.
3. The addition of 1g/kg prebiotic to 90 and 85% of NRC protein diets recorded non-significant effect on body weight and cumulative weight gain compared with control group which fed diet has 100% of NRC protein.
4. The inclusion of 0.5g/kg prebiotic plus 2.25g/kg organic acids to broiler fed diets containing 95, 90 and 85% of NRC protein (T4, T7 and T10, respectively) recorded non-significant effect on body weight and cumulative weight gain during the starter period (0-3 w), however, the body weight and gain of broilers during grower finisher period (4-6 w) were significantly (P<0.05) decreased compared with control.
5. The addition of organic acids to diets containing 95, 90 and 85% NRC protein (T3, T6 and T9, respectively) recorded the lowest body weight, gain and worst FCR.
6. The best FCR was recorded in prebiotic supplemented groups fed on diets containing 95 and 90% of NRC protein requirements (T2 and T5, respectively). While the worst FCR was recorded in organic acids supplemented groups.
7. The consumption of prebiotic and organic acids either single or in combination had no significant effect on hot carcass percentage, eviscerated carcass percentage, dressing percentage, relative weight of liver, heart and gizzard compared with control group. However, a significant (P<0.05) reduction of abdominal fat content was observed in the prebiotic, organic acid and combination groups.
8. There was a significant decreased (P<0.05) in cholesterol and triglycerides content of broiler meat (breast and thigh) among all prebiotic, organic acids and their combination treated groups compared with the control one.
9. No significant differences in dry matter and ash content of broiler breast and thigh meat among all treated groups including the control one. However, the fat content was significantly (P<0.05) decreased and protein content of broiler breast and thigh meat significant (P<0.05) increased.
10. No significant differences in the concentration of total serum protein, albumin, globulin and cholesterol content among all prebiotic and prebiotic plus organic acids treated groups and control one. While the serum triglycerides decreased significantly (P<0.05) by 16.41, 13.03 and 21.11% in prebiotic fed groups (T2, T5 and T8, respectively) and by 18.80, 24.96 and 18.99% in prebiotic plus organic acids fed groups (T4, T7 and T10, respectively) than control.
11. No significant (P<0.05) differences in serum albumin and cholesterol content in broilers fed diets have 95, 90 & 85% of NRC protein supplemented by organic acids ( T3, T6 and T9) , respectively was observed. However, the serum total protein and globulin significantly (P<0.05) increased in broiler fed organic acids supplemented by 95% of NRC protein requirements diet (T3) and numerically in broiler fed organic acids supplemented by 90% of NRC protein requirements diet (T6), A/G ratio decreased significantly (P<0.05) in broiler fed organic acids supplemented by 95% of NRC protein requirements diet (T3) and numerically in broiler fed organic acids supplemented by 90% of NRC protein requirements diet (T6), while there were no significant differences in globulin and A/G ratio for broilers fed 85%NRC protein diet (T9). Serum triglcerides decreased significantly (P<0.05) by 15.72%, 24.6 and 24.91% in broiler fed 95, 90 and 85% of NRC protein diet, respectively than control one.
12. The inclusion of prebiotic to diets have 95, 90 and 85% of NRC protein had no significant effect on spleen, thymus and bursa relative weight.
13. There was a significant (P<0.05) increase in bursa relative weight in broilers fed organic acids supplemented diets (T3, T6 and T9), while spleen and thymus relative weights insignificantly affected by addition of organic acid.
14. The inclusion of 0.5g/kg prebiotic plus 2.25g/kg organic acids to broiler fed diets have 95, 90 and 85% of NRC protein requirements increased numerically the relative weight of bursa,