Threonine in broilers

For the essential amino acid threonine, there are not as much data available for response modelling. On the other hand, the commercial interest has increased considerably due to the availability of larger quantities of industrial L-threonine. Therefore, Fig. 25.7 displays another set of three experiments combined on a relative scale. Again, responses are non-linear and exponential functions fit the data well. Economic evaluations of these responses show a minimum feed cost per kg live weight gain at about 6.6 g kg-1 Thr with small variation due to absolute feed cost level (Fig. 25.8). With breast meat being set as the production goal, there is a shift to a higher optimum of about 7.0 g kg-1 dietary Thr.

For comparison, Fig. 25.9 reports a combination of two late finisher experiments from 42 to 56 days of age. With good agreements across trials, the fairly large biological responses are evaluated financially (Fig. 25.10), which is displayed for feed cost kg-1 breast meat only. The effect of feed cost on the most profitable dietary Thr content is again rather small, with optima around 6.9 g kg-1. Surprisingly, the dietary optimum is virtually identical with the optimum derived from the grower data (see Fig. 25.8). This, although concluded from different trials, would indicate the increasing importance of Thr for heavier birds, which is also underlined by the strong response in breast meat yield. Figure 25.10b looks into the effect of changing prices of L-Thr, covering a large range from

0.22

Feed cost US$1601-1 Minimum @ 10.4 g kg~

Feed cost US$1601-1 Minimum @ 10.4 g kg~

0.22

\Feed cost US$140 H

Minimum @ 10.3 g kg-1

: ♦ nn

v^Feed cost US$120 H

Minimum @ 10.1 g kg-1

t

i i i i

i i i i i i

Fig. 25.6. Effects of varying feed costs on economic optimum dietary lysine content, (a) Cost kg-1 live weight gain; (b) cost kg-1 breast meat yield.

US$2.80 to US$3.80 kg-1. The most profitable dietary Thr level, however, is almost unchanged (7.0 g kg-1 down to 6.8 g kg-1 total Thr) over such a broad range of input prices, indicating the favourable economics of even small Incremental changes in bird performance.

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