Based on the broken-line inflection point (Fig. 13.3), the digestible tryptophan requirement for maximal weight gain was 1.59 g kg-1, which is in very close agreement with the broken-line requirement estimate (1.57 g kg :) using feed efficiency as the response criterion (Baker et al., 2002). The digestible tryptophan requirement (1.59 g kg-1) ratioed to the average digestible lysine requirement of 9.60 g kg-1 (i.e. from Assays 1 and 2) was 16.6%, in close agreement with our earlier estimate of 16% (Baker, 1997).

The digestible tryptophan requirement estimated by taking the first intercept x value of the broken line (on the plateau) and the quadratic fitted line was 1.83 g kg-1 (Fig. 13.3). This requirement estimate parallels the 1.91 g kg-1 estimate obtained by taking 90% of the upper asymptote x value from the quadratic response curve.

Using the 16.6% Trp:Lys ratio determined herein together with our best estimate of the digestible lysine requirement (10.7 g kg :) for broiler chicks during the first 3 weeks of life (mixed sex feeding) as determined previously in chicks fed diets with an metabolizable energy (ME) level of 13.4 MJ kg-1 (Han and Baker, 1991, 1993), one arrives at a digestible tryptophan requirement of 1.78 g kg This agrees well with the requirement estimates of 1.83 g kg-1 obtained by determining the first intercept x value of the broken line (on the plateau) and the quadratic fitted line (Fig. 13.3). With a tryptophan digestibility of 86%

- 8 1.1 1.3 1.6 1.8 2.1 2.3 2.6 Dietary digestible tryptophan (g kg-1)
- 13.3. Fitted broken-line and quadratic plots of 13-day weight gain as a function of true digestible Trp in the diet (Assay 3). Data points are means of four pens of four male chicks during the period 8-21 days posthatching. The minimal true digestible Trp requirement determined by broken-line analysis using least squares methodology was 1.59 g kg 1 (/= 302.8 + 339.58 (x< 1.59); r2 = 0.934). The pens means data also were fitted to a quadratic regression equation y= -423.7 + 697.14x- 164.G3x2 (r2 = 0.922). The level of digestible Trp that maximized weight gain (i.e. upper asymptote) was calculated to be 2.12 g kg 1, with 90% of this value being 1.91 g kg 1. The first intercept x value of the broken-line (on the plateau) and the quadratic fitted line occured at 1.83 kg kg-1.

in maize-soybean meal broiler diets (Rhone-Poulenc, 1993), the total tryptophan requirement calculates to be 2.07 g kg 1 for chicks consuming conventional maize-soybean meal diets. This value is in good agreement with the NRC (1994) tryptophan requirement estimate of 2.00 g kg"1.

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