The study was conducted on 672 one-day-old female Hybrid Converter turkeys divided into 6 dietary treatments (7 replications per group) which differed in DL-methionine (DL-Met) supplementation levels. The Met content in diets between 1 – 4/5 – 8 weeks of feeding was as follows: 4.1/3.7, 4.7/4.1, 5.6/4.6, 6.1/5.0, 6.8/6.5 and 7.5/7.4 g · kg-1. In groups 3 and 5, dietary Met levels corresponded to those recommended by NRC (1994) and B.U.T. (2012), respectively. The lowest Met content of the diet, without supplemental pure Met, caused a significant decrease in the bird body weight gains. In comparison with group 1, a considerable increase in dietary Met concentrations led to a decrease in the plasma levels of urea ( = 0.042), total protein and albumin ( < 0.001) in groups 4 – 6, and in the activity of γ-glutamyl transferase in all other groups ( < 0.001). The highest glutathione peroxidase (activity and the lowest lipid peroxide concentration in the turkeys’ blood were noted in treatments 4 and 5. The lowest and the highest Met content significantly decreased ferric reducing ability of plasma values ( < 0.001 vs the other groups). Plasma immunoglobulin A concentrations were significantly ( < 0.001) higher in groups 1 and 3 than in the remaining groups. In conclusion, the diet without Met supplementation compromised the growth performance and antioxidant status of turkeys. Diets supplemented with Met according to B.U.T. (2012) recommendations or at levels slightly lower exerted the strongest antioxidant effect.
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