Farm Animal Metabolism and Nutrition

palatability, such as rye, wheat bran and
forage plants, the determination is often
carried out by the difference rather than by
the direct method. Kreienbring et al. (1988)
reported that the apparent ileal digesti-
bility values of amino acids in forage
plants determined by the difference
method varied considerably, indicated by
large standard deviations. As the amino
acid contents in these forage plants were
low and the inclusion levels in the assay
diets were also low, the large variation
probably resulted from the magnifying
effect induced by the calculation process of
the difference method. This indicates that
the difference method may not be suitable
for these low-protein feedstuffs. The regres-
sion method is most suited.
In summary, for the determination of
apparent ileal digestibility values of amino
acids in feedstuffs with a low protein
content, the regression method is the most
accurate approach. The difference method,
with a high inclusion level of a test feed-
stuff, is also a valid approach. The direct

method, which underestimates the ileal

digestibility values of amino acids, is not

valid for these feedstuffs.

Protein supplements

Information with respect to the effect of

methods of determination on the apparent

ileal amino acid digestibility values in

protein supplements was also obtained

with canola meal (Fan and Sauer, 1995a).

With respect to the difference method,

the apparent ileal digestibility values of

crude protein and amino acids in canola

meal were determined according to

Equation 13.7 at three levels of inclusion

(diets 2, 3 and 4). It seemed somehow con-

tradictory, at first, to note that the ileal

digestibility values of crude protein and

amino acids in canola meal calculated by

the difference method at the low level of

inclusion (244 g kg
^1 ) were higher (P

< 0.05 or 0.10) than those at the higher lev-

els of inclusion (305 and 366 g kg
^1 ). The

ileal digestibility values of amino acids in

canola meal were calculated by the

Methods for Measuring Ileal Amino Acid Digestibility 295

Fig. 13.5.Estimation of the apparent ileal digestibility of lysine (LYS) in the assay feed ingredient (barley) by
linearly regressing the ileal digestibility values (Y: %, means ± SE) of lysine in the assay diets against the
contribution levels (X) of lysine from the basal feed ingredient (canola meal) to its content in the assay diets
(Fan and Sauer, 1995a).