Growth, body composition, and marginal efficiency of methionine utilization are affected by nonessential amino acid nitrogen supplementation in male broiler chicken.

The main objective of this experiment was to study the effect of CP level in the diet on the efficiency of Met utilization. Male Ross broilers were grown in floor pens in groups of 10. Three pens were allocated 1 of 16 experimental diets from d 8 to 21 posthatch. Dietary Met concentration ranged from 1.8 to 7.0 g/kg with 183 (low protein; LP) or 229 (normal protein; NP) g of CP/kg of diet. Inclusion rates of DL-Met and nonessential amino acids were varied to achieve these differences. Prececal net disappearance was additionally studied for the NP diet without Met supplementation in 4 pens of 10 birds each. Net disappearances were 83 and 78% for CP and Met, respectively, and ranged from 66% (cystine) to 96% (Ala) for other amino acids. Birds significantly and nonlinearly responded to increased Met intake in BW gain, which was significantly lower for NP than for LP. Although the concentration of protein in gained BW was unaffected by Met and CP levels, the contents of fat and energy in gained BW were lower with NP than with LP and rose with increasing Met intake until a plateau was attained. The content of Met in accreted whole-body protein rose with increasing Met intake and plateaued at about 2.0 g/16 g of N. With the exception of Lys and Gly, Met intake did not significantly affect the concentration of amino acids in accreted whole-body protein. The marginal efficiency of Met utilization was, at its maximum, 8% lower with NP than with LP. Concentrations of 3.4 and 3.6 g of Met/kg of diet were needed to achieve 95% of ymax in protein accretion with LP and NP, respectively. It was concluded that an increase in the dietary Met requirement often found with elevated CP concentrations was the consequence of a reduced capacity to use Met for protein gain.