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Because lysine is used as a reference AA for calculating ideal ratios, it is critical to obtain clear and accurate information on the lysine requirement. Therefore, we completed two identical bioassays (at different times) in which graded levels of digestible lysine (6.77-12.78 g kg-1) were fed (Figs 13.1 and 13.2). The broken-line requirement estimates for gain/ feed ratio were 9.64 (Assay 1) and 9.56 g kg-1 (Assay 2), both values being substantially higher than the broken-line requirement estimates for wight gain of 8.46 and 8.49 g kg-1 in Assays 1 and 2, respectively (Baker et al., 2002). The first intercept x value of the broken line (on the plateau) and the quadratic fitted line for feed efficiency occurred at 10.1 g kg-1 (Assay 1) and 10.4 g kg-1 (Assay 2) digestible lysine (Figs 13.1 and 13.2), and these requirement estimates closely approximated those obtained by taking 90% of the upper asymptote x value from the quadratic response curve. Regardless of the curve-fitting procedure used to estimate lysine requirements, the requirements obtained herein using a CGM semipurified diet containing about one-half of the dietary nitrogen from CGM and the other half from free AA were lower than those estimated previously in our laboratory for male chicks of the same age (Han and Baker, 1991, 1993).

The lysine requirement estimates of Mack et al. (1999) were done using a low protein maize-soybean meal diet. In two

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12.5

Dietary digestible lysine (g kg-1)

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Fig. 13.1. Fitted broken-line and quadratic plots of gain/feed as a function of true digestible Lys in the diet (Assay 1). Data points are means of four pens of four male chicks during the period 8-21 days posthatching. The minimal digestible Lys requirement determined by broken-line analysis using least squares methodology was 9.64 g kg 1 (/ = 694.4 + 54.90 (x< 9.64); r2 = 0.885). The pen means data also were fitted to a quadratic regression equation /= -300.2 + 174.68x-7.60X2 (r2 = 0.899). The level of digestible Lys that maximized feed efficiency (i.e. upper asymptote) was calculated to be 11.49 g kg-1, with 90% of this value being 10.34 g kg 1.The first intercept x value of the broken-line (on the plateau) and the quadratic fitted line occurred at 10.40 g kg-1.

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- 13.2. Fitted broken-line and quadratic plots of gain/feed ratio as a function of true digestible Lys in the diet (Assay 2). Data points are means of four pens of four male chicks during the period 8-21 days posthatching. The minimal digestible Lys requirement determined by broken-line analysis using least squares methodology was 9.56 g kg 1 (/ = 665.83 + 59.90 (x< 9.56); r2 = 0.870). The pen means data also were fitted to a quadratic regression equation (/= -502.70 + 210.88x- 9.42a2; r2 = 0.897). The level of digestible Lys that maximized feed efficiency (upper asymptote) was calculated to be 11.20 g kg 1, with 90% of this value being 10.08 g kg 1. The first intercept xvalue of the broken-line (on the plateau) and the quadratic fitted line occurred at 10.10 g kg 1.
- 13.1. Fitted broken-line and quadratic plots of gain/feed as a function of true digestible Lys in the diet (Assay 1). Data points are means of four pens of four male chicks during the period 8-21 days posthatching. The minimal digestible Lys requirement determined by broken-line analysis using least squares methodology was 9.64 g kg 1 (/ = 694.4 + 54.90 (x< 9.64); r2 = 0.885). The pen means data also were fitted to a quadratic regression equation /= -300.2 + 174.68x-7.60X2 (r2 = 0.899). The level of digestible Lys that maximized feed efficiency (i.e. upper asymptote) was calculated to be 11.49 g kg-1, with 90% of this value being 10.34 g kg 1.The first intercept x value of the broken-line (on the plateau) and the quadratic fitted line occurred at 10.40 g kg-1.

separate bioassays they found that the lysine requirement for maximal feed efficiency was substantially higher than that required for not only maximal weight gain, but also for maximal breast-meat yield. In our previous lysine work with 3-6-week-old broiler chicks (Han and Baker, 1994), the lysine requirement for maximal feed efficiency was similar to that required for maximal breast-meat yield.

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