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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.

The response of White Pekin ducks to supplements of L-valine was studied during 3 wk posthatching. The basal valine concentration was 6.8 g/kg in a diet containing 18% CP and 2,990 kcal ME/kg (12.5 MJ ME/ kg). L-valine was supplemented in eight graded levels up to 12.7 g/kg at the expense of L-glutamic acid. Three pens of 14 ducks were allocated to each valine level. Diets were offered ad libitum. Body weight gain and feed/gain ratio were studied. At the end of wk 3, ducks were killed, processed to a homogenous mass, and analyzed for total body CP and amino acid content. Accretion of protein and amino acids was calculated. Additionally, a 5-d N balance study was conducted with separate ducks of the same hatch beginning on d 11 (6 ducks per treatment). The response of ducks to increasing valine concentration was described by exponential functions. Ducks significantly responded to the increasing valine concentration in growth, feed/gain ratio, and protein accretion. Ninety-five percent of y(max) in BW gain and protein accretion were achieved with 8.0 and 7.9 g valine/kg, respectively. The content of protein in gained BW was, on average, 149 g/kg without a significant valine effect. The valine content in accreted body protein was also unaffected by dietary valine (4.1 g/16 g N on average), which suggested that a major shift in body protein fractions did not occur. The overall efficiency of valine utilization was affected by dietary valine concentration and showed a maximum of 49%. Data from the balance study showed basically the same response of ducks, but the estimated optimum in dietary valine concentration was lower (7.0 g/kg). A comparison with published broiler data indicated that ducks and broilers in this age period were similar with regard to the valine content in gained protein and the efficiency of utilization of supplemented valine.

The effects of a supplementation of P from monobasic calcium phosphate (MCP; Ca(H2PO4)2) to low-P basal diets were studied in growing Pekin ducks. Body weight gain and feed conversion were studied in two separate periods between Days 1 to 21 (Experiment 1) and between Days 21 to 49 (Experiments 2 and 3). Retention of P was measured by comparative slaughter technique in Experiment 1. Additionally, two balance trials with quantitative determination of intake and excretion of P were conducted between Days 12 to 17 and between Days 30 to 35. MCP was supplemented in 7 or 6 graded levels at the expense of sand. In cases when ANOVA showed a significant effect of MCP supplementation, the response of ducks was described by nonlinear functions. No significant effect of supplemental MCP on growth, feed intake or feed/gain ratio was detected in the period between Days 21 and 49 with a basal P level of 3.0 g/kg. Between Days 1 and 21, ducks needed 5.1 g P/kg diet to achieve 95% of ymax in BW gain. The ymax for P concentration in gained BW, determined from balance trials, was 5.6 and 5.1 g/kg between Days 12 to 17 and Days 30 to 35, respectively. Ninety-five percent of ymax in P retention was achieved with a dietary P concentration of 6.2 and 4.3 g/kg between Days 12 to 17 and Days 30 to 35. The cumulative efficiency of utilization (retention/intake x 100) of dietary P from the basal diet was 49% (Days 12 to 17) and 43% (Days 30 to 35), and approached maximum with increasing supplementation of MCP of 55 and 53%, respectively, before it decreased again with further increase in MCP supplementation. The marginal efficiency of supplemental P (deltay/deltax) showed a maximum of 86% (Days 12 to 17) and 92% (Days 30 to 35), and this maximum was achieved where only 75 and 72% of ymax in P retentions were achieved. It is concluded that ducks require a lower P concentration in the diet with increase in age, but that the efficiency of utilization of P from inorganic salts is not clearly affected by age. Conclusions regarding the P requirement largely depend on the response criterion chosen. Based on P retention data, a dietary level of available P is recommended to be 3.4 (Days 1 to 21) and 2.3 g/kg (Days 21 to 49), although growth was unaffected by P even at lower concentrations of available P. Future comparative studies on the availability of P from ingredients should be conducted at a dietary P concentration that allows for identifying the maximum in utilization.