الفهرس 
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Abstract
The study targeted to evaluate the effect of supplementing two levels of the spore-forming bacteria Bacillus subtillis (Bs) + Bacillus licheniformis (Bl) as a mixture 1:1 (2×109 cfu/g of each) on performance, health, and blood parameters in Holstein calves. This work included two experimental stages. The first stage was a growth trial that lasted for two months. In the 1st month, 36 neonatal Holstein calves (5 days old, with an average weight of 44.27±0.37 (males) and 42.87±0.55 (females) Kg) were randomly assigned into three homogeneous groups (12 calves/group, 6 / each sex). Animals were fed a diet consisting of whole milk plus a starter mixture, which was offered ad libitum on the 8th day of age. Once a day, 0, 10, and 20 g of the tested probiotics were dissolved in whole milk of the afternoon meal for 30 successive days to represent G1 (control), G2, and G3, respectively. The trial was continued for another month with no probiotics supplements. The attained results revealed that significant (P<0.001) increase in the overall total DMI in G2 during the month post-treatment compared to those in G1 and G3. The LBW and ADG in G2 calves were better (P<0.001) than those in the other tested groups, which led to a fast reaching to weaning weight in G2. The FCR had remarkable (P<0.001) improvements during the post-treatment interval with G2 and G3. Also, at all intervals, ECPU and ECR were considerably (P<0.001) enhanced in G2 compared to G1 and G3. The probiotics inclusion resulted in advanced (P=0.02) overall mean values of ADG, FCR, ECPU, and ECR in females against that in the male calves. Cases of diarrhea and pneumonia declined with probiotics addition, particularly in G2 compared with G1. Calves of G2 had a concentration of serum globulin higher (P=0.01) than other groups. Both levels of probiotics resulted in sizable (P=0.001) increases in concentrations of serum IgG and substantial (P=0.001) reductions in albumin, triglycerides, and cholesterol compared to those of the control. The best relative profit of the tested diets was observed in G2. In the second investigational stage, a digestion trial that lasted for a month was carried out to evaluate the impacts of the same probiotics on nutrient digestibility, rumen fermentation, and some blood constituents. The used animals were 18 growing Holstein heifers (8 months old), the same females of the same three tested groups in the first stage. The animals were fed TMR and received daily drenches of the same tested probiotics levels (0, 10, and 20g as G1 (control), G2, and G3, respectively) before the morning meal. The findings indicated that the digestibility of CP, CF, and NFE and the percentage of TDN and DCP were notably (P<0.001) improved in G3 compared to the control group. Also, G2 has valuable advances (P<0.001) in the digestibility of CP and CF and ruminal MCP contrasted with G1. Serum total protein and globulin concentrations of were elevated (P=0.024 and P<0.001, respectively) with G3 compared to the other groups. In contrast, the concentrations of triglycerides in serum were significantly (P=0.006) decreased with both levels of probiotics relative to the control. In summary, supplementing the newborn or growing Holstein calves with a daily dose of 10 of Bs + Bl had promising effects on digestibility, rumen fermentation, performance, and health status.