1. Introduction
In a series of experiments the application of ideal protein concepts provided a considerable impact on efficacy of
protein utilization, growth performance and carcass yields in growing chicken [1]-[8] and recommendations are
available about the expected ideal amino acid ratio (IAAR) in diets of meat type chicken as summarized by
Wecke and Liebert [8]. Currently, there are indications that optimal ratios of individual AAs related to lysine
might change depending on age period, feed protein source and dietary AA efficiency [7]-[11].
In addition to be part of body protein synthesis, sulfur containing amino acids (SAA) play a very important
role in different metabolic processes in growing animals [12] [13]. Consequently, recommended optimal SAA to
Lys ratios are currently not conclusive. Opposite to starter chicken with a SAA to Lys ratio between 71 and 72
to 100 [14]-[16], during grower period ratios between 69 and 75 to 100 are reported [14]-[19]. However, higher
proportions of minimum 77 [20] or 82 [21] of total SAA as related to Lys were proposed, especially when feed
efficiency and carcass yield were the reference criterion. Even the recommendations of both NRC [22] and
GRRS [23] varied from 71 to 100 vs. 82 to 100 for starter and 87 to 100 vs. 72 to 100 for grower chicken, respectively.
Based on 24 reports, Wecke and Liebert [8] summarized 74 ± 2 to 100 as optimal SAA to Lys ratio,
whereby 40 ± 4 to 100 counted as optimal for Met to Lys. Actual studies assessing these IAARs and taking into
account varying dietary Cys supply are scarce and inconclusive. It is indicated by Dilger and Baker [24] and
observations of Liebert et al. [25] that dietary Cys concentration has influence on efficiency of Met utilization in
chicks. In addition, the question arises if Met efficiency could be further improved by minimizing its metabolic
utilization as donor of methyl groups. Several betaine (BET) supplementation studies provide support for this
assumption [26]-[30].
Accordingly, the current experiments aimed to measure the dietary Met efficiency both at graded dietary
Cys:Met ratios and supplemented BET to re-evaluate the optimal Met:Lys ratio for growing chicken based on
further applications of the “Goettingen approach” [31].
1. IntroductionIn a series of experiments the application of ideal protein concepts provided a considerable impact on efficacy ofprotein utilization, growth performance and carcass yields in growing chicken [1]-[8] and recommendations areavailable about the expected ideal amino acid ratio (IAAR) in diets of meat type chicken as summarized byWecke and Liebert [8]. Currently, there are indications that optimal ratios of individual AAs related to lysinemight change depending on age period, feed protein source and dietary AA efficiency [7]-[11].In addition to be part of body protein synthesis, sulfur containing amino acids (SAA) play a very importantrole in different metabolic processes in growing animals [12] [13]. Consequently, recommended optimal SAA toLys ratios are currently not conclusive. Opposite to starter chicken with a SAA to Lys ratio between 71 and 72to 100 [14]-[16], during grower period ratios between 69 and 75 to 100 are reported [14]-[19]. However, higherproportions of minimum 77 [20] or 82 [21] of total SAA as related to Lys were proposed, especially when feedefficiency and carcass yield were the reference criterion. Even the recommendations of both NRC [22] andGRRS [23] varied from 71 to 100 vs. 82 to 100 for starter and 87 to 100 vs. 72 to 100 for grower chicken, respectively.Based on 24 reports, Wecke and Liebert [8] summarized 74 ± 2 to 100 as optimal SAA to Lys ratio,whereby 40 ± 4 to 100 counted as optimal for Met to Lys. Actual studies assessing these IAARs and taking intoaccount varying dietary Cys supply are scarce and inconclusive. It is indicated by Dilger and Baker [24] andobservations of Liebert et al. [25] that dietary Cys concentration has influence on efficiency of Met utilization inchicks. In addition, the question arises if Met efficiency could be further improved by minimizing its metabolicutilization as donor of methyl groups. Several betaine (BET) supplementation studies provide support for thisassumption [26]-[30].Accordingly, the current experiments aimed to measure the dietary Met efficiency both at graded dietaryCys:Met ratios and supplemented BET to re-evaluate the optimal Met:Lys ratio for growing chicken based onfurther applications of the “Goettingen approach” [31].
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