the same feedstuff. As discussed previously,
there is considerable variation in apparent
ileal digestibility values of amino acids
within each cereal grain. Of the essential
amino acids, the variation is especially
large for lysine and threonine. The deter-
mination of apparent ileal digestibility
values of amino acids in low-protein feed-
stuffs is routinely carried out with the
direct method in which the test feedstuff
provides the sole amino acids in the assay
diet. However, the total contents of crude
protein and the majority of the amino acids
in cereal grains are usually below the
threshold levels (Table 13.3). As sum-
marized in Table 13.3 for barley, wheat and
maize, there are large differences in the
contents of crude protein and amino acids
in the assay diets within the same cereal
grain (compiled from various research
groups) which arise mainly from differences
in the inclusion level and the crude protein
and amino acid contents. As a result, small
differences in dietary contents of crude
protein and amino acids below the
threshold levels will result in relatively
large variations in the digestibility values
of amino acids, especially those amino
acids present at low levels in cereal grains
(lysine, threonine and tryptophan) and/or
amino acids of which the ileal endogenous
recovery is relatively high (e.g. threonine),
as illustrated in Fig. 13.3.
The ranges of crude protein and amino
acid contents in the assay diets for deter-
mining apparent ileal digestibility values of
crude protein and amino acids in protein
supplements are summarized for soybean
meal, canola meal and sunflower meal,
which were compiled from different
research groups (Table 13.4). Similarly for
other protein supplements, there are differ-
ences in the dietary contents of crude pro-
tein and amino acids for the determination288 W.C. Sauer et al.
88
86
84
82
80
78
76
74
72
Apparent ileal LEU digestibility (%)Initial plateau value (85.6)Threshold level (13.3)
2 4 6 8 10 12 14 16 18 20
Dietary LEU levels (g kg^1 , on DM basis)LEU908682
787470
Apparent ileal LYS digestibility (%)Initial plateau value (86.0)Threshold level (9.4)
2 4 6 8 10 12 14 16 18
Dietary LYS levels (g kg^1 , on DM basis)LYS9287827772
Apparent ileal MET digestibility (%)Initial plateau value (89.0)Threshold level (2.4)
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Dietary MET levels (g kg^1 , on DM basis)MET85
80
75
70
65
60
55
50
45
Apparent ileal THR digestibility (%)Initial plateau value (77.0)Threshold level (6.3)
1.0 3.0 6.0 7.0 9.0 1.1
Dietary THR levels (g kg^1 , on DM basis)THR(A) (B)(C) (D)Fig. 13.3.The quadratic with plateau relationships between apparent ileal amino acid digestibility values
(Y: %, means ± SE) and dietary amino acid contents (X: g kg^1 , on dry matter basis). (A) Leucine (LEU); (B)
lysine (LYS); (C) methionine (MET); (D) threonine (THR) (Fan et al., 1994).