WATT PoultryUSA - December 2017 - 39
is defined by comparing the steepness of the test
and standard ingredient response curves. Figure 3
shows how varying the value for b (steepness of the
curve) affects the shape of the function. Setting the
exponential equations for the standard and test ingredients equal, we can show that
bt/bs = xs/xt = RBV.
While there seems to be no dispute over the
use of the slope-ratio assay in either the linear or
exponential form for minerals, vitamins or other
supplemental amino acids, there has been quite
some controversy over applying the same approach
to methionine sources. There are currently three
feed grade methionine sources available for livestock and poultry feed: D,L-methionine (DL-Met),
L-methionine (L-Met) and the hydroxy analog of
acid; DL-MHA). The first two sources are amino
acids while the third has a hydroxyl group in place
of the amino group. All three can be converted in
the animal's body into L-Met, the biologically active form. D-Met and DL-MHA must be converted
into L-Met, immediately raising a question as to the
RBV of these different options.
It has been suggested that comparing eﬃcacy
of methionine sources should only be done at
the standard industry supplementation rate. This
supplementation rate includes some provision for a
safety margin and falls in the plateau region of the
response, where RBV is meaningless. Such an approach prevents any comparison of eﬃciency. The
exponential slope-ratio assay only differentiates nutrient sources based on the supplementation rate at
which a measured response reaches the maximum.
The higher the RBV, the lower the supplementation
rate needed to meet the animal's requirement.
Hoehler et al (2005) provides an example of determining RBV with a non-linear data set. A total
of 2880 day-old chicks (Ross 208) were placed into
ﬂoor pens of 30 birds/pen for a 42 day feeding trial.
Pens were randomly assigned to one of 16 dietary
treaments resulting in 6 replicates per treatment. A
methionine deficient basal diet with no supplemental
methionine source was formulated to contain 0.59%
and 0.52% total sulfur amino acids in the starter
(d1-21) and grower (d22-42) feeds, respectively. The
treatments in each period comprised the basal diet
and 3 series of diets containing graded levels (0.04,
0.08, 0.12 and 0.20) of DL-Met, D,L-Met diluted with
glucose to 65% of the original concentration (DLMet 65%) and 88% purity DL-MHA. DL-Met 65%
was included as an internal standard for the method.
Results are shown in figures 4 and 5.
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