RA 00048.pdf - OAR@ICRISAT

Table 1. Percent crude protein, 96 hr in vitro dry matter disappearance (IVDMD) and indigestible protein in
eight sorghum genotypes.
Genotypes
Catechin
equivalent
Crude
protein
96 hr
IVDMD
Indigestible
protein
residue
IS0062
IS0418
IS8165
IS0616
0.29
0.27
4.28
5.93
13.78 94.8a*
12.11 93.1a
11.26 88.8b
14.20 83.0c
1.13
1.62
2.80
4.73
* Means in a column followed by the same letter do not differ significantly at the 0 05 level of probability using Newman-Keul's test.
nols and of the effects of polyphenols on sorghum
grain quality. We will discuss some of his
basic observations on the interactions between
tannins and the nutritional value of sorghum grain
in relation to our own research and to the
scientific literature.
Oswalt first recognized that in vitro dry matter
digestibility (IVDMD) of sorghum grains varied
significantly between genotypes, and that the
catechin equivalent values for tannin content
were negatively correlated with IVDMD as shown
in Table 1 (Schaffert et al. 1974). This was an
extremely important observation because it explained
the discrepancies which had previously
been observed between protein quality and biological
value in rat feeding experiments. Based on
the amino acid composition of sorghum grain, one
would predict that the biological value of any
particular variety of sorghum grain would be
directly proportional to its lysine content. This, in
fact, is true for low-tannin sorghum genotypes.
The data in Figure 1, however, illustrate that lysine
is not the first limiting component of biological
value for a group of high-tannin sorghum lines
from the world collection. There is an important
interaction between tannin content and protein
quality in sorghum which is not found in any of the
other major cereals. Cummings and Axtell (1973)
demonstrated this experimentally by feeding
whole grain and dehulled high-tannin sorghum
grain of IS 8260, as shown in Figure 2. Rat growth
is poor for the whole grain IS 8260, with and
without supplemental lysine. In contrast, denuding
IS 8260 grain improves biological value substantially,
and also allows a significant rat growth
response to the addition of supplemental lysine. A
similar technique was used in this study to
16-
14-
1 2 -
10
8
6
4
2
Low t a n n i n s o r g h u m
H i g h t a n n i n s o r g h u m
0 .20 .22 .24 .26 .28 .30 .32 .34 .36
L y s i n e
Figure 1. Relationship of biological value to
lysine concentration (g/100 g sample)
in high- and low-tannin sorghum lines
from the world collection in a 14-day
weanling rat feeding experiment.
determine the quantitative effects of tannins on
biological value. Using varying proportions of
high-tannin sorghum grain mixed with its dehulled
low-tannin counterpart, we were able to provide
diets containing a range of tannin content that
were essentially isogenic comparisons. Data from
this study are shown in Figure 3.
The protein digestibility and nutritional quality as
measured by rat weight gain are significantly
reduced by increasing levels of tannins in the diet.
We can conclude from Butler's (1982) data that
tannins bind specifically to proline residues in
proteins and thus to the storage proteins (the
prolamines) which are rich in proline but poor in
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