Farm Animal Metabolism and Nutrition

Matching Enzyme Activity to

Diet Composition

Role of fibre-degrading enzymes

Fibre in its various forms is a major cause
of poor feed digestion especially in young
animals. Even within a group of feedstuffs
such as cereals, there is considerable varia-
tion in the amount and type of NSP they
contain (see Tables 19.1 and 19.2).
The two main types of NSP found in
cereals are the arabinoxylans (or pentosans)
and the
-glucans. Arabinoxylans are made
of a linear backbone of xylose substituted
with arabinose (Bedford, 1995).
-Glucans
are made up of a polymer of glucose which
has kinks that are responsible for the anti-
nutritional properties of barley and oats
(Bedford, 1995).
Given the typical arabinoxylan com-
position of wheat (Table 19.2), the addition
of a
-glucanase to a wheat-based diet will
have significantly less effect than if it were
added to a barley-based diet. Yet the addi-
tion of a xylanase (or pentosanase) to a
wheat- or rye-based diet will have more
effect than if it were added to a barley-
based diet.

The amount of soluble arabinoxylan

and 
-glucan in a feedstuff is important

because it is this fraction of the cereal

which is believed to increase the viscosity

of digesta (important especially in poultry).

Increased digesta viscosity slows the rate

of digestion and absorption, increases

microbial activity, reduces feed intake and,

especially in birds, increases faecal mois-

ture content. Xylanases and 
-glucanases

reduce the size of the molecules of

arabinoxylan and 
-glucan, and in turn

reduce digesta viscosity.

Two points emerge from the foregoing:

● It is important that the feed enzyme

activity is matched to the ingredient

composition of the diet.

● Feed enzyme supplementation is more

likely to improve digestibility when 
-

glucanases are added to barley diets and

xylanases are added to rye or wheat diets.

Role of phytase

Phosphorus is a major mineral required by

animals in bone and is used in nerve and

embryo development. Inorganic forms of P,

although generally highly available, are

Feed Enzymes 407

Table 19.1.Cell wall (NDF), starch, fat (acid ether extractives) and metabolizable energy (ME) for poultry
content of selected feed grains (g kg^1 dry matter basis).

Cereal Cell wall Starch Fat ME (MJ kg^1 )

Barley 201 562 26 14.5
Maize 117 700 42 16.1
Oats 315 469 51 14.8
Naked oats 114 590 101 16.7
Triticale 119 517 22 14.4
Wheat 124 674 21 15.5

After Chesson (1993).

Table 19.2.Arabinoxylan and
-glucan content of cereal grains (g kg^1 ).

Arabinoxylan
-glucan
Grain Endosperm Grain Endosperm
Cereal Grain total water-soluble (% of total) Grain total water-soluble (% of total)

Barley 56.9 4.8 22 43.6 28.9 99
Oats 76.5 5.0 12 33.7 21.3 47
Rye 84.9 26.0 44 18.9 6.8 71
Wheat 66.3 11.8 35 6.5 5.2 48

After Chesson (1993).