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.

Response to lysine intake in composition of body weight gain and efficiency of lysine utilization of growing male chickens from two genotypes.

Male chickens of a broiler (B) and a layer (L) genotype were grown in floor pens from d 8 to 21 posthatch in groups of 10. Three pens per genotype were allocated to each of 10 experimental diets. The diets were offered ad libitum and they differed in lysine concentration from 3.8 to 16.8 g/kg. The source of supplemental lysine was L-lysine x HCl. All birds were killed at the end of the experiment, and representative birds (3 groups of 10 per genotype) were killed at the start for baseline measurements. Accretions of protein, fat, energy, and amino acids were determined by comparative body analysis. Responses were described with sigmoidal and exponential functions. Additionally, the net disappearance rate (NDR) of amino acids from the small intestine was studied with the basal diet (3.8 g of lysine/kg) using 6 replicated pens of 15 birds per genotype. Titanium dioxide was the indigestible marker. Net disappearance rates were not significantly different between genotypes for CP or any amino acid. Responses to incremental lysine concentration were nonlinear for both genotypes but distinctly different in magnitude between genotypes. Estimated y(max) values for 14-d BW, protein gain, and gain/ feed ratio were 534 (B) and 153 (L) g, 87.1 (B) and 28.7 (L) g, and 0.82 (B) and 0.71 (L) g/g. Protein accretion approached 95% of the estimated y(max) with dietary lysine concentrations of 12.5 (B) and 10.4 (L) g/kg. The amino acid profile of accreted whole body protein was different between genotypes, and was affected by supplementary lysine. Lysine content in accreted whole body protein approached upper values of 7.4 (B) and 5.6 (L) g/16 g of N with increasing dietary lysine concentration. Marginal efficiency of lysine utilization, determined as delta lysine accretion/delta lysine intake, showed maxima of 99% (B) and 74% (L). These maxima were achieved at intakes which were much lower than those needed for high protein accretion. It was concluded that the efficiency of amino acid utilization may depend on genotype, perhaps due to differences in the relative proportion of different protein fractions to whole body protein and due to differences in the ratio of synthesis and degradation of body proteins. Nonlinear relationships and different amino acid pattern of accreted body protein should be implemented in future models of requirements.