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Quadratic (P < 0.01) responses in weight gain (Fig. 13.6) and feed efficiency occurred when graded doses (5.1-10.6 g kg-1) of digestible valine were fed (Baker et al., 2002). The broken-line digestible requirement estimates were 7.44 and 7.43 g kg-1 for weight gain and feed efficiency, respectively. The valine requirement (7.44 g kg-1) ratioed to the lysine requirement

Dietary digestible isoleucine (g kg-1)

Fig. 13.5. Fitted broken-line and quadratic plots of 13-day weight gain as a function of true digestible lie in the diet (Assay 5). Data points are means of four pens of four male chicks during the period 8-21 days posthatching. The minimal digestible lie requirement determined by broken-line analysis using least squares methodology was 5.89 g kg 1 (/= 292.0 + 72.25 (x< 5.89); r2 = 0.876). The pen means data also were fitted to a quadratic regression equation: y= -239.08 + 136.54x- 8.59x2; r2 = 0.810. The level of digestible lie that maximized weight gain (i.e. upper asymptote) was calculated to be 7.95 g kg 1, with 90% of this value being 7.15 g kg 1. The first intercept xvalue of the broken-line (on the plateau) and the quadratic fitted line occurred at 6.79 g kg-"1.

Dietary digestible isoleucine (g kg-1)

350 n

Dietary digestible valine (g kg-1)

Fig. 13.6. Fitted broken-line and quadratic plots of 13-day weight gain as a function of true digestible Val in the diet (Assay 6). Data points are means of four pens of four male chicks during the period 8-21 days posthatching. The minimal digestible Val requirement determined by broken-line analysis using least squares methodology was 7.44 g kg 1 (/ = 290.83 + 73.18 (x< 7.44); r2 = 0.963). The pen means data also were fitted to a quadratic regression equation: y= -618.42 + 200.39x- 1G.88x2; r2 = 0.950. The level of digestible Val that maximized weight gain (i.e. upper asymptote) was calculated to be 9.21 g kg 1 of the diet, with 90% of this value being 8.29 g kg-1. The first intercept x value of the broken-line (on the plateau) and the quadratic fitted line occurred at 8.11 g kg-1.

Dietary digestible valine (g kg-1)

Fig. 13.6. Fitted broken-line and quadratic plots of 13-day weight gain as a function of true digestible Val in the diet (Assay 6). Data points are means of four pens of four male chicks during the period 8-21 days posthatching. The minimal digestible Val requirement determined by broken-line analysis using least squares methodology was 7.44 g kg 1 (/ = 290.83 + 73.18 (x< 7.44); r2 = 0.963). The pen means data also were fitted to a quadratic regression equation: y= -618.42 + 200.39x- 1G.88x2; r2 = 0.950. The level of digestible Val that maximized weight gain (i.e. upper asymptote) was calculated to be 9.21 g kg 1 of the diet, with 90% of this value being 8.29 g kg-1. The first intercept x value of the broken-line (on the plateau) and the quadratic fitted line occurred at 8.11 g kg-1.

(9.60 g kg-1) was 77.5%, closely agreeing, therefore, with our original estimate of 77% (Baker, 1997) and with the recalculated ratio estimate (76%) of Mack et al. (1999), (see Table 13.1). Why the ideal valine:lysine ratio of broiler chicks is higher than that for pigs (68%) is not known (Baker, 1997).

Using 77.5% as an ideal ratio for valine, the predicted digestible valine requirement would be 8.29 g kg-1 (0.775 x 10.7), and this estimate agrees well with those calculated from the broken line/quadratic curve intercept method (8.11 g kg-1) as well as the value obtained by taking 90% of the upper asymptote from the quadratic fitted line (8.2 g kg-1). Dividing the 8.29 g kg-1 digestible valine requirement estimate by a maize-soybean meal valine digestibility value of 90% (NRC, 1994) yields a total valine requirement estimate of 9.21 g kg 1 for 0-21-day-old chicks fed a maize-soybean meal diet. The NRC (1994) total valine requirement estimate (maize-soybean meal basis) is 9.0 g kg-1. Thus, our 9.21 g kg 1 estimate herein is slightly higher. Previous work from our laboratory established that valine is among the four most limiting AA in soybean meal and in reduced protein maize-soybean meal diets (Han et al., 1992; Fernandez et al., 1994).

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