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%
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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