Effects of selected zootechnical feed additives as alternatives to zinc-bacitracin antibiotic growth promoter in broiler diets
This study was designed to investigate the effects of selected zootechnical feed additives [probiotic (live Bacillus subtilis), organic acids, protease enzyme and chelated minerals] combinations as alternatives to zinc-bacitracin (ZnB) antibiotic growth promoter in broiler diets on growth performance, blood parameters, meat quality and tibia bone parameters. Eight hundred Cobb broiler chicks were evenly distributed to 40 pens to which five dietary treatments: negative control (T1) (commercial broiler diets with no antibiotics); T2 (positive control (commercial broiler diets with zinc-bacitracin); T3 (T1 + chelated minerals + protease enzyme); T4 (T1 + chelated minerals + protease + organic acids) and T5 (T1 + chelated minerals + protease + probiotic) were randomly allocated. The inclusion levels of chelated minerals, protease enzyme, organic acids and probiotic were 0.03, 0.05, 0.5 and 0.02%, respectively. The chelated minerals additive was a composite of 19.3% copper, 36% zinc, and 44.7% manganese by weight. A maize grain-soybean meal-based starter diet was fed to chicks from 0-13 days of age. On days 14, 15 and 16, after a 16 hour fast, the birds were challenged with a high protein diet (40% soybean meal and 10% poultry by-products) and a finisher diet was fed thereafter till day 35. Intake and weight gain data were used to calculate average daily feed intake (ADFI), feed conversion ratio (FCR) and average daily gain. There was no week × diet interaction effect on feed intake (FI), body weight gain (BWG) and FCR. There were no dietary effects (P>0.05) on FI, average daily gain and FCR. Haematological parameters influenced (P<0.05) by the diet were haemoglobin and haematocrit (HTC) only. Chickens fed T5 (9.57 ± 0.32 g/dl) and T1 (9.52 ± 0.32 g/dl) had the highest haemoglobin levels while T2 resulted in the lowest levels. T1 (0.37 ± 0.01%) also caused the highest number of haematocrit followed by the T5 (0.36 ± 0.01%) and T2 (0.31 ± 0.1%) resulted the lowest value of haematocrit. Dietary treatments did not have any effect (P>0.05) on erythrocytes, leucocytes, neutrophils, lymphocytes, monocytes, eosinophils, basophils and normoplasts. The serum biochemistry indices, alanine transaminase (ALT), sodium and total serum protein were significantly (P<0.05) influenced by the dietary treatments but not aspartate aminotransferase (AST), potassium, albumin, urea, calcium and cholesterol. T2 (8.50 ± 1.57 IU/L) resulted in the highest level of ALT compared to the lowest of treatment T3 (2.25 ± 1.57 IU/L). Feeding T1 (33.37 ± 1.14 g/l) caused the highest total protein meanwhile T2 (29.75 ± 1.14 g/l) diet resulted in the least total protein. The T1 continues with same trend of causing the highest level even in sodium while T3 (144.00 ± 1.25 mmol/l) resulted in the lowest value. The T1 (0.39 ± 0.05 mmol/l) diet also had the highest urea levels and T3 (0.20 ± 0.05 mmol/l) the lowest. There were no dietary effects (P>0.05) on bone development parameters. Similarly, dietary treatments did not influence (P>0.05) all carcass traits. Diet had no effect (P>0.05) on all internal organs apart from the spleen and proventriculus. The highest weight of spleen was observed in chickens fed T3 (3.81 ± 0.32 g) while T1 (2.12 ± 0.32 g) had the lowest weight. T3 (12.63 ± 0.61 g) had the heaviest proventriculus weight while T2 (9.63 ± 0.61) had the lowest weight. The results of external organs of broilers also showed a lack of significant effect (P>0.05) of diet on breast, drumstick, wing, head and shank weights. However, diet had an effect (P<0.05) on neck weight; T1 (55.50 ± 2.08 g) promoted the heaviest necks while T3 (44.88 ± 2.08 g) promoted the lightest neck weight. With regards to meat quality measurements, there was no dietary effect (P >0.05) on pH, dripping loss and shear force. All meat colour parameters were not influenced by diet apart from redness. Overall, the proposed zootechnical feed additives were shown to have potential as alternatives to zinc-bacitracin antibiotic growth promoter in broiler diets. Collectively, the results of this study can be used in informing formulating antibiotic-free diets that will not have any negative effects on growth performance and the meat quality of broilers.