Graded doses of digestible isoleucine (4.5-9.5 g kg-1) resulted in quadratic (P < 0.01) responses in both weight gain and feed efficiency (Fig. 13.5). The broken-line requirement estimates were 5.89 g kg 1 for weight gain and 5.81 g kg-1 for gain/feed ratio (Baker et al., 2002). Thus, the ideal ratio of isoleucine:lysine was determined to be 61.4% (5.89 - 9.60 x 100). This value is lower than our original estimate of 67% (Baker, 1997).
The predicted digestible isoleucine requirement for chicks for 0-3 weeks posthatching is 6.57 g kg-1 (0.614 x 10.7) which is similar to the 6.79 g kg 1 requirement predicted from the first intercept value of the broken line plateau with the quadratic fitted line (Fig. 13.5). Extrapolation of this requirement estimate to chicks fed a maize-soybean meal diet where isoleucine digestibility is 91% (NRC, 1994) results in a total predicted isoleucine requirement of 7.22 g kg which is lower than the 8.0 g kg-1 estimate of NRC (1994).
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