JOURNAL OF THE SCIENTIFIC AGRICULTURAL SOCIETY OF FINLAND

188

Maataloustieteellinen Aikakauskirja
Vol. 49: 188-191, 1977

The carbohydrate composition and nutritive value of forages

Maija-Liisa Salo
Department of Animal Husbandry, University of Helsinki, 00710 Helsinki 71

Abstract. The carbohydrates of forages, their digestibility, the relationship between
composition and the development stage in grasses and legumes, and the dependence
of nutritive value on composition are examined.

Carbohydrates and their digestibility
The carbohydrates of forages can be divided into two main classes: the

cellular carbohydrates and the structural or cell-wall carbohydrates. The divi-
sion is an apt one in regard to nutrition physiology, since animals are able to
digest the substances of the first category with the aid of their own enzymes,
but those of the second only with the aid of microbes.

The cellular carbohydrates are: a) Monosaccharides (glucose and fructose),
b) Sucrose, c) Oligosaccharides, which occur in forages only in very small amounts,
d) Fructosans, water-soluble polysaccharides, which are common in grasses
but are not found in legumes, e) Starch, which occurs in forages only in traces
or not at all.

The structural carbohydrates are hemicellulose and cellulose. Hemicellulose,
a collective term for the non-cellulose structural polysaccharides, is composed
of various polysaccharides differing in composition and digestibility. In general,
hemicellulose is more susceptible to acid and alkali hydrolysis than cellulose.
The main components of hemicellulose are xylose in grasses and uronic acids
in clovers. Other consituents are arabinose, galactose and glucose. According to
the main component one speaks of xylans, arabans, etc. though monosacchari-
des seldom form pure polymers.

Hemicellulose is digested better than cellulose, except for xylans, which
are the least digestible polysaccharides in forages. Hence it would be desirable
to analyse forages also for xylans. No simple method is known, however.

The term pectin is used in defining the fraction of hemicellulose that is soluble
in a slightly acidic solution and is precipitated from it by ethanol. The compo-
nents are galacturonic acid, arabinose, glactose and glucose. The amount of
pectin in forages is fairly low.

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Lignin, an aromatic substance, is commonly determined along with the
structural carbohydrates. It occurs in cell-walls together with polysaccharides
and is very poorly or not at all digestible. As the plant ages, the content of
both lignin and xylan increases, thus decreasing the digestibility of forages.
The digestibility of grasses is also thought to be lowered by the presence of
silica.

Relationship between composition and the development stage
in grasses and legumes

The composition and consequent nutritive value of forages depends primarily
on the growth stage. Other important factors are the plant species, climatic
conditions and application of fertilizers. Figure 1 shows the changes in the
composition of timothy and red clover as the plant develops from the leaf stage
to the flowering and straw stages. The values shown are for the latitude of
Helsinki, and are rounded-off averages. It may be noted that the sugar
fructosan content, in particular, varies greatly with climate.

The xylan and lignin content of timothy nearly doubles as the plant develops
from the leaf stage to the flowering stage, and thus the proportion of xylan in
hemicellulose is increased still further. The content of cellulose does not rise as



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sharply. At the same time the protein content decreases markedly, while the
percentages of the other cellular substances remain virtually unchanged. An
almost total absence of cellular content and a high percentage of lignin cha-
lacterize grasses at the straw stage.

The composition of red clover differs significantly from that of grasses.
Fructosans are absent and the content of water-soluble caibohydrates is there-
fore fairly low. On the other hand, the percentage of plant acids mainly
malic and glycerolic acids is more than double that of grasses. In both spe-
cies the acids occur primarily as salts. The hemicellulose of clovers differs
greatly in composition from that of grasses: nearly half of it consists of uronic
acids and the amount of xylans is low. Consequently, the hemicellulose of
clover is more digestible than that of grasses. Furthermore, the content of
cellulose is half that of grasses, while that of lignin is equal in the twc species.

Dependence of nutritive value on composition

The energy value of forages depends on the utilization rate and the voluntary
intake by animals. The net energy value of feeds is well illustrated by the di-
gestibility coefficient. Because the determination of digestibility in vivo is
complicated, various laboratory methods have been developed to take the
place of in vivo trials. At present the in vitro method the incubation of samples
in the rumen fluid is considered the best way of predicting the digestibility.
The cellulase method is a simplified and standardized variation of it, where
a commercial enzyme preparate is used instead of rumen fluid. The results of
this simplified method are also found to correlate well with in vivo results,
although the digestive effect of the enzyme preparates is not as good as that of
the rumen fluid. The common in vitro method is, however, better established
and therefore more reliable.

Of the chemical entities, the digestibility is best predicted by the amount
of lignin although not as well as by the two biological methods mentioned above.
Moreover, the determination of lignin is laborious and different variations of
the method do not give the same results. Various fibre complexes crude
fibre, neutral detergent fibre (NDF), and acid detergent fibre (ADF) are
poorer than lignin as parameters of digestibility, although better than cellulose
and hemicellulose. The percentages of various cellular substances, on the other
hand, vary so much that they cannot be used in predicting the digestibility.
When the material is homogenous there may be a good correlation between the
crude protein content and the digestibility.

The high nutritive value of the feed implies further that the voluntary intake
is copious. The voluntary intake is determined to a great extent by the diges-
tibility, but the bulk factor is also of some significance. Thus the voluntary
intake of clovers is known to exceed that of grasses, though the digestibility is
the same. This difference has been explained by the faster digestibility of the
cell-wall polysaccharides of the clovers in the rumen, and hence the smaller
bulkiness. As Figure 1 shows, the cell-wall compositions of these two plant
species are very different.



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Probably the cow also appreciates the texture and taste of the feed. The
clover is softer than grasses, because it contains hardly any of the silica that
makes the leaves of grasses coarse. The sugar, in turn, makes the feed palatable,
and fresh forages up to the flowering stage contain plenty of sugars. In sugar
content the clovers take second place to the grasses, but at the same time they
have a very high content of plant acids, which also may increase the palatability.

REFERENCES

Salo, M.-L. 1965. Determination of carbohydrate fractions in animal foods and faeces. Acta
Agr. Fenn. 105; 1 102.

—» 1965. The composition of pasture grasses at the University Farm Viik. J. Scient.
Agric. Soc. Finl. 37: 135-147.

—* 1973. Analytisk bestämning av vallväxternas fodervärde. NJF:s symposium Tune
29. —3O. Okt. Repr. 18 p.

» & Kotilainen, K. 1969. Determination of free and combined plant acids. J. Scient.
Agric. Soc. Finl, 41:277-289.

—» Nykänen, A. & Sormunen, R. 1975. Nurmikasvien koostumus, pepsiini-HCI-liukoi-
suus ja in vitro-sulavuus eri kasvuasteilla. J. Scient. Agric. Soc. Finl. 47:480—490.

Terry, R. A., Osbourn, D. F., Cammell, S. B. & Fenlon, J. S. 1974. In vitro digestibility
and the estimation of energy in herbage. Proc. sth Gen. Meet. Eur. Grassl. Fed.
Uppsala.

Ms received January 3, 1977

SELOSTUS

Korsirehujen hiilihydraattikoostumuksesta ja rehuarvosta

Maija-Liisa Salo
Yliopiston kotieläintieteen laitos, Helsinki

Kasvuaste vaikuttaa oleellisesti korsirehujen koostumukseen ja rehuarvo vuorostaan riip-
puu koostumuksesta. Erityisesti ligniini- ja ksylaanipitoisuuden nousu alentaa sulavuutta
ja edelleen rehuarvoa. Korsirehujen sulavuus voidaan määrittää varsin tarkasti in vitro-
menetelmällä ja saadun tuloksen (D-arvo) sekä raakaproteiinipitoisuuden perusteella voidaan
laskea energia-arvo märehtijöillä. Heinäkasvien ja puna-apilan sulavuus on samalla kasvu-
asteella suunnilleen sama, mutta apilan täyttävyysvaikutus on pienempi; apilan ksylaani-
ja selluloosapitoisuus on näet suhteellisen alhainen ja sen vuoksi apila sulaa ja kulkee etu-
mahojen läpi heinäkasveja nopeammin.