The digestible threonine requirement for maximal weight gain (Fig. 13.4) and feed efficiency was 5.35 and 5.33 g kg-1, respectively, based on the inflection points of single-slope broken-line fits (Baker et al., 2002). Using the higher of broken-line requirement estimates for weight gain and feed efficiency, the ideal threonine lysine ratio was 55.7%, considerably lower than our original estimate of 67% (Baker, 1997). The recalculated threonine lysine ratio from the Mack et al. (1999) study was 59% (Table 13.1) for chicks between 20 and 40 days posthatching. Thus, both our work herein and the data of Mack et al. (1999) suggest that the ideal ratio of threonine lysine is less than 60% for broiler chicks for growth periods ranging from hatching to 40 days posthatching. That the ideal ratio calculated from the Mack et al. (1999) data was somewhat higher than our estimate of 55.7% was likely due to the fact that older broilers exhibit slightly higher threonine :lysine ratios than younger broilers (Emmert and Baker, 1997).

Our threonine ratio of 55.7% multiplied by the digestible lysine requirement (10.7 g kg-1) of 0-21-day-old chicks (mixed sex feeding) predicts a digestible threonine requirement of 5.96 g kg-1, which closely approximates the 5.90 g kg-1 threonine requirement estimate obtained using the plateau intercept value of the broken line and the quadratic fitted line (Fig. 13.4). If one assumes that the digestibility of threonine in a maize-soybean meal diet is 87% (NRC, 1994), the predicted total threonine requirement would be 6.85 g kg-1 for chicks during the first 3 weeks posthatching. This estimate is well below the NRC (1994) requirement estimate of 8.0 g kg-1. Work from Kidd et al. (1997) and Smith and Waldroup (1988) suggests, also, that the

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- 8 4.3 4.8 5.3 5.8 6.3 6.8 7.3 7.8 Dietary digestible threonine (g kg-1)
- 13.4. Fitted broken-line and quadratic plots of 14-day weight gain as a function of true digestible Thr in the diet (Assay 4). Data points are means of four pens of four male chicks during period 8-22 days posthatching. The minimal digestible Thr requirement determined by broken-line analysis using least squares methodology was 5.35 g kg 1 (/ = 328.8 + 108.44 (x< 5.35), r2 = 0.867). The pens means gain data also were fitted to a quadratic regression equation: /= -559.3 + 265.86x - 19.54X2 (r2 = 0.855). The level of true digestible Thr that maximized weight gain (i.e. upper asymptote) was calculated to be 6.80 g kg-"1, with 90% of this value being 6.12 g kg 1. The first intercept x value of the broken-line (on the plateau) and the quadratic fitted line occurred at 5.90 g kg-"1.

- 8 4.3 4.8 5.3 5.8 6.3 6.8 7.3 7.8 Dietary digestible threonine (g kg-1)
- 13.4. Fitted broken-line and quadratic plots of 14-day weight gain as a function of true digestible Thr in the diet (Assay 4). Data points are means of four pens of four male chicks during period 8-22 days posthatching. The minimal digestible Thr requirement determined by broken-line analysis using least squares methodology was 5.35 g kg 1 (/ = 328.8 + 108.44 (x< 5.35), r2 = 0.867). The pens means gain data also were fitted to a quadratic regression equation: /= -559.3 + 265.86x - 19.54X2 (r2 = 0.855). The level of true digestible Thr that maximized weight gain (i.e. upper asymptote) was calculated to be 6.80 g kg-"1, with 90% of this value being 6.12 g kg 1. The first intercept x value of the broken-line (on the plateau) and the quadratic fitted line occurred at 5.90 g kg-"1.

NRC (1994) threonine requirement estimate for 0- to 3-week-old chicks is too high.

The ideal threonine:lysine ratio for young chicks of 55.7% is lower then the ideal threonine:lysine ratio of 65% for young pigs (Chung and Baker, 1992; Baker, 1997). Relative to pigs, avians have a very short gastrointestinal tract, and a large portion of the total threonine requirement is needed for gut protein synthesis (Bertollo et al., 1998). This could explain why avians require lower levels of threonine (relative to lysine) than pigs.

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