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V. The Voluntary Intake of Forages

V. The Voluntary Intake of Forages

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and extrinsic factors, which depend on the method of presentation of the

forage, on the effect of processes such as ensilage and dehydration, and

on environmental factors. The intrinsic factors determine how much of a

forage a ruminant animal could eat under ad libitum conditions; the extrinsic factors determine how much the animal is able to eat under the

particular feeding conditions imposed upon it.





It is first necessary to emphasize the great importance of rigid standardization of the conditions under which intake measurements are made,

if comparisons are to be made between results from different centers

(Chalupa and McCullough, 1967). Voluntary intake is generally defined

as the amount animals will eat when an excess of 15 percent is offered

(Blaxter et al., 1961). But fresh feed should be presented at least twice

daily and uneaten residues removed to avoid soiling this fresh feed, so that

limitation of intake by extrinsic factors is avoided. Ruminants are also

found to vary much more in their capacities for feed intake than, say, in

their digestive capacities, and adequate numbers of animals must be

used to obtain reliable intake data. Heaney et al. (1968) summarized

CV’s* of individual animal intakes ranging from 10.5 percent (Minson

et al., 1964) to 16.4 percent in their own experiments. Complicating

factors are also the species and size of the experimental animals, and

most intake results are now reported in terms of the metabolic weight of

the animal, expressed as liveweight to the power 0.75 (although agronomists must have detected that some of their colleagues in ruminant

nutrition still prefer 0.73). In this way the voluntary intakes of both

sheep and cattle could be expressed on a comparable basis (Blaxter and

Wilson, 1962). But it must be accepted that intake cannot be a precise

parameter of a forage, because the amount that an animal eats depends

on the individuality of the animal (its species, sex, physiological status,

health, etc.) as well as on the intrinsic features of the forage.

1 . The Relation between Forage Digestibility and Intake

It is now accepted that the major factor limiting the amount of forage

eaten by ruminants is the capacity of the rumen and digestive tract.

Ruminants are able to eat much more of highly digestible forages than of

less digestible forages, because the latter occupy more volume and are

within the rumen for a longer time and because from them more indigestible residue has to be passed down the hind tract (Balch and Campling,

1962). A decrease in voluntary intake as forage becomes more mature,

*Coefficient of variation.



and so less digestible, has been shown in many experiments both with

temperate forages (Crampton et a / . , 1960; Minson et al., 1964; Demarquilly, 1966b; Osbourn et al., 1966; Heaney et al., 1966) and with

tropical forages (Grieve and Osbourn, 1965; Milford, 1967; da Silva and

Gomide, 1967). These results appeared to confirm the concept that

rumen fill controls voluntary intake. Unfortunately this led to the more

generalized concept that the voluntary intake of a forage could thus be

predicted from its digestibility, and that a single forage parameter, related to digestibility, would be adequate to estimate both digestibility

and intake, the two main components of nutrient intake. There is now increasing evidence that this is a too simplified concept. While with most

forage species intake decreases as the forage becomes less digestible, the

relationships between intake and digestibility can differ markedly between different forages. Therefore, different forage species at the same

level of digestibility may be eaten in quite different amounts, a fact that

is in conflict with the earlier concept.

This first became apparent with the observation by several workers of

higher intakes from legumes than from grasses of the same digestibility

(e.g., Van Soest, 196%; Osbourn et a / . , 1966; Milford, 1967; Weston

and Hogan, 1967) and subsequently of different levels of intake between

grass species. Thus, low intakes have been reported with timothy

(Phleum prarense) (Minson e f al., 1964; R. L. Reid and Jung, 1966;

Miles and Walters, 1966), with tall fescue ( F . arundinacea) (Van Soest,

19641, and with Phalaris arundinacea (0. N. Andrews and Hoveland,

1965; O’Donovan et al., 1967) compared with other grass species. With

timothy the rate of fall in intake with decreasing digestibility was found

to be less than with ryegrass (Minson et al., 1964), and Heaney et a / .

( 1966) found only a negligible change in intake with decreasing digestibility of Phleum nodosum.

Within a species, differences in intake characteristics have also been

shown. Thus Osbourn et al. ( I 966) found the intake of a diploid variety

of Italian ryegrass to be 16 percent greater than that of a tetraploid variety

during first growth in the spring. Large differences in intake between different lines of Phalaris arundinacea have been shown by Roe and

Mottershead ( 1962) and O’Donovan et al. ( 1967).

2 . Differences in Intake between Forage Species

Observations of different intake-digestibility relationships for different forages are of considerable importance. As long as voluntary intake was accepted as being determined mainly by level of digestibility,

there appeared to be little prospect of improving the nutritive potential

(intake X digestibility) of forages except by an improvement in digesti-



bility. The evidence that factors in forage other than digestibility can also

affect intake now offers a much wider scope for the improvement of

forage nutritive value. Two possible lines of development are indicated

in the work of Osbourn et al. (1966) and O’Donovan et al. (1967),

noted above. Thus Osbourn et al. (1966) and Osbourn (1967) showed

marked differences in voluntary intake, in the order lucerne > ryegrass > timothy, at a given level of digestibility. Chemical analysis of

these forages showed that the “digestible” fraction in lucerne contained

a higher proportion of pepsin-soluble material, and a lower proportion of

digestible fiber than the “digestible” fraction in timothy, with the levels

in ryegrass intermediate between those for lucerne and timothy (Fig. 3).





Voluntary intake



S 24

5 48



FIG. 3. The composition of the digested fraction, and the voluntary intakes of lucerne,

S.24 ryegrass, and S.48 timothy forages of the same dry matter (D.M.)digestibility. (From

Osbourn, 1967.)

Van Soest ( 1 965c) reported a similar conclusion, that lucerne contains a

higher proportion of cell contents (material soluble in neutral detergent)

and a. lower proportion of cell wall constituents, than grass of the same

level of digestibility. While the grass and lucerne forages are of the same

digestibility, it is likely that the lucerne will reach the “digested” stage

more rapidly than the ryegrass, and the ryegrass than the timothy. The

digestible fraction of the lucerne could thus occupy less volume X time

within the rumen; as a result the animal could eat more of it than of the

grasses. Osbourn ( 1 967) also reported that, in the experiment described

by Osbourn et al. (1966), the diploid ryegrass had a higher content of

pepsin-soluble material than the tetraploid ryegrass, which could have

been associated with the higher intake of the diploid variety.



The observations illustrated in Fig. 3 offer one possible explanation as

to why different forages of the same level of digestibility may be digested

at different rates, and so have different intake levels. But there is also the

possibility that rate of digestion, and in turn the rate of intake, may be

affected by conditions within the rumen. As Tilley et al. ( 1 964) have

shown (Table 1) the rate and extent of cellulose digestion decreases as

the pH of an in vitro system (and by analogy of the rumen in vivo) diverges from the physiologically normal level of about pH 6.8. Some

forages, particularly highly buffered, low-sugar, forages such as the

legumes, are found to give a characteristically higher rumen pH (6.6 to

6.8) than perennial ryegrass (<6.5), and Raymond (1966a) has suggested

that these differences in rumen pH could lead to a higher rate of digestion

of the cell wall fraction of lucerne than of ryegrass, and that this could

partly account for the higher level of intake of lucerne and of other

legumes. This introduces the possibility of developing feeding regimes

which, by maintaining rumen pH close to the optimum level of 6.8, could

ensure the most rapid rate of digestion of the cell wall content in the feed,

with a resulting high level of voluntary intake.

The results of Roe and Mottershead (1962) and O’Donovan et al.

(1967) have indicated that other factors may also modify the simple

relationship between forage digestibility and forage intake. These

workers showed that, within the species Pharlaris arundinacea, sheep

showed a marked preference for certain strains compared with others.

An extract from one low-preference strain, sprayed on a higher-preference strain, made it unacceptable to the sheep (Roe and Mottershead,

1962). O’Donovan el al. (1967) also established differences in intake

between lines of this grass, and suggested that it might be possible to

breed lines of improved intake characteristics. No intake problem

appears to have arisen with P . tuberosa (0.N. Andrews and Hoveland,

196% but it is of interest that the Aberystwyth variety S.230 ( P . tuberosa

X P. arundinacea) was found to be very unpalatable to sheep (U. I.

Jones, 1959).

Considerable differences have been found between the intake-digestibility relationships for different forage species but different workers are

not in agreement about the form of these relationships. Blaxter and

Wilson (1962) found a curvilinear relationship, intake increasing less

rapidly with digestibility at higher levels of forage digestibility. J. B.

Hutton ( 1963) and Conrad et al. ( 1 964) found no increase in intake above

a forage dry matter digestibility of about 7076, and Baumgardt ( 1 967)

found that the intake of a roughage-concentrate mixture did not increase

when the energy digestibility was above 67 percent (ca. percent dry



matter digestibility). In contrast Osbourn et al. (1966, and unpublished)

have found no divergence from linearity with a wide range of grasses and

legumes up to 80 percent dry matter digestibility.

The results of J. B. Hutton (1963) and Conrad et al. ( 1 964) suggested

that at high levels of forage digestibility intake begins to be limited by

metabolic factors (blood levels of organic acids, glucose, etc.) rather than

by gastrointestinal fill. That is, the ruminant begins to behave, in intake

terms, like a nonruminant (Conrad, 1966). Freer and Campling (1963)

also showed that the intake of (highly digestible) concentrated feeds was

not limited by bulk. Van Soest ( 1965c) has suggested that this is likely to

occur when forage dry-matter contains less than 5 5 to 60 percent of cell

wall constituents.

However, the level at which forage digestibility no longer limits intake

must to some extent depend on the physiological condition of the animal

being fed, i.e., on the critical level of blood metabolites at which it ceases

to eat. The experiments of Osbourn et al. ( 1 966) were all carried out with

“thin” sheep, with a high growth potential, which may therefore have had

a higher threshold intake level than some other experimental animals,

such as the mature sheep used by Blaxter et al. (1 961). Critical studies

to compare the voluntary intake levels of the same forage by ruminants

in different physiological states are needed: particular examples would

be animals in early or late lactation, or subjected to climatic stress.

3. The Effect of Fertilizers on Forage Intake

In many experiments initial observations of differences between

forages have been made in “preference-ranking” tests in which sheep

were offered a free choice from a number of different forages. In only few

experiments have differences in voluntary intake characteristics then

been established by feeding high and low preference strains separately

to sheep under ad libitum conditions. This illustrates a most important

feature in the testing of forage intake, that differences in preference

ranking must be confirmed in intake experiments before superiority of

the higher-ranking forages can be accepted. Thus Bland and Dent (1 962)

reported a positive correlation between the preference ranking and the

content of water-soluble carbohydrates in a number of varieties of

cocksfoot, and Plice ( 1952) suggested that the low preference ranking

of grass fertilized with nitrogen was due to the lower content of sugar in

such grass. But experiments under ad libitum feeding have shown no

effect on voluntary intake from widely different levels of N fertilization

of grass. Thus Marten and Donker (1964) found no differences in voluntary intakes of Bromus inermis fertilized with no nitrogen (24 percent



crude protein; 7.2 percent ethanol-soluble sugar) or with 300 pounds of N

per acre (28 percent and 5.4 percent). Likewise, R. L. Reid and Jung

(1965) found similar levels of intake from Festuca arundinacea grown

with or without nitrogen fertilizer, although sheep showed a marked

preference for the low-nitrogen grass. Holmes and Lang (1963) and

Hight er al. (1968) have also reported that nitrogen fertilization had no

effect on the voluntary intake of grass fed to either dairy cows or sheep.

Cameron ( 1967) found similar intakes of hay fertilized with 0 or 1 12 kg.

of N per hectare. We cannot conclude from these results that nitrogen

fertilization never decreases the voluntary intake of forage - the reports

of feed refusal and poor animal performance sometimes experienced in

practice cannot be ignored- but experimental confirmation is still lacking. And in other cases, described in Section V, D, there is clear evidence that the intake of forages of very low nitrogen content can be increased by nitrogen fertilization.

Greenhalgh and Reid (1967) have suggested that the relative importance of “rumen fill” and “palatability” in determining the voluntary

intake of forages may be studied by introducing feed directly into the

rumen via a fistula. An equivalent decrease in the intake per 0s would

demonstrate that intake is being controlled by rumen fill. If intake per

0s remains at the original level some other factor, such as palatability

(or more strictly, unpalatability) would be indicated. When ad libitum

chopped straw was fed to sheep per os, and an equal quantity of grass

was introduced through the fistula, total daily intake was 23.5 g. dry

matter per kilogram liveweight to the power 0.73, but when the feeding

regime was reversed, total intake increased to 48.8 g., indicating that

the amount of straw the sheep ate was not limited by rumen fill only.

However, further examination of this technique, and in particular of the

effect of the absence of mastication and salivary secretion on the feed

introduced via a fistula is still needed.

Sonneveld (1965) has suggested that forage intake by cattle may be

related to the moisture content of the forage. Herbage was cut daily and

fed to cattle throughout a complete year, and the voluntary intake on each

day was related to a number of forage parameters by multiple regression.

Intake on any day was slightly related to the level of forage digestibility,

but more closely to day-to-day changes in moisture content. lntake was

found to increase with a decrease in forage moisture content, relative to

the previous day’s feed, when forages were at a low fiber stage, but the

reverse was found when the forage was of high fiber content. Conversely

Holmes and Lang (1963) found that intake did not change when forage

moisture content increased to 84 percent, compared with 80 percent in



the control forage, following nitrogen fertilization. The difference between these observations indicates a need for further study.

While the higher intake of legumes than of grasses appears, at least in

part, to be due to the higher proportion of cell contents in the legumes,

this explanation does not appear to account for differences in intake

which have been found between different legumes. Thus Van Soest

( 1 965a) reported a higher intake of birdsfoot trefoil (Lotus corniculatus)

than of lucerne of the same level of digestibility, and Osbourn et al. (1 966)

found a similarly higher intake of sainfoin than of red clover or lucerne.

In the latter experiment the content of pepsin-soluble material in the red

clover (36.4%)was higher than in the sainfoin (33.5%).

Troelsen and Bigsby (1964) had demonstrated an inverse relationship between the resistance to maceration and the voluntary intake of a

series of hays; part of this relationship is likely to be due to a direct

association of physical strength of plant materials with increasing

fibrousness (and so to decreasing digestibility), but Osbourn et al. (1 966)

suggested that there might remain differences in intake, as between sainfoin and red clover. A similar concept is seen in the fibrousness index of

Chenost ( 1 966), which is measured as the electrical energy needed to

pulverize a 5-g. sample of the dried forage, and which was shown to be

closely related to dry matter intake. Evans ( 1 964) has shown differences

in breaking strength of grass leaves, and Theron and Booysen (1966)

have related breaking strength positively to the acid-insoluble lignin content of 8 grass species. Johnston (1967) has shown that the force needed

to penetrate the stems of kale was closely related to in vitro digestibility

values. L. H. P. Jones and Handreck (1967) raise the interesting possibility that variations in the degree of silicification in grasses may contribute to variations in culm strength. In addition grass may become less

palatable to the grazing animal as a result of heavily silicified hairs on the

leaf surface.

These concepts of the intrinsic factors that may affect forage intake

represent an important advance from the earlier, oversimplified, concept

that forage intake is mainly determined by the level of forage digestibility. The demonstration that forages of the same digestibility can differ

markedly in level of voluntary intake, and the elucidation of some of the

possible causes for this, now indicates the prospect of breeding for improved forage intake. Any argument that the extreme cases noted are

exceptional is surely irrelevant. Thus the observation, in Phalaris

arundinacea, of strains of high and low preference rating suggests that

there are also strains of intermediate preference. More important,

knowledge of the factors causing these differences in preference should



allow the breeding of new strains of even higher preference rating.

“Palatability” may be an imprecise concept, but the observation of extreme unpalatability in some species indicates the likelihood of intermediate unpalatability in other species, which it would be unwise to dismiss. For a difference of only 10 percent in voluntary intake between

two forages could have as great an effect on their relative values for

animal production as the difference of 6 units in digestibility between

S.24 ryegrass and S.37 cocksfoot, about which much has been written.


With the recognition that both voluntary intake and the level of

digestibility of a forage play major roles in determining its nutritive

value, Crampton et al. (1960) suggested that intake and digestibility

should be combined in a single parameter, the nutritive value index. The

concept was possibly complicated by relating the actual intake of a given

forage, expressed per kilogram liveweight to the power 0.75 (kg. LW0.7s),

to a theoretical standard forage of intake 80 g. dry matterlkg. LW0.75,the

relative intake being calculated:

Relative intake =

actual intakelkg. LW0.75


and the nutritive value index:


= relative

intake X

% digestibility of forage energy content


( 1 1)

Crampton er al. (1960) proposed, from initial studies with 9 forages,

that the rate of cellulose disappearance in a 12-hour standard in vitro

digestion ( x ) could be closely related to the NVI:


Donefer et al. ( 1 966) subsequently reported an improved correlation

between NVI and the percentage of the forage dry matter soluble in

acid pepsin (P):



1.3 1 4 ~ 7.8

NVI = 1.60P - 0.75 ( r = 0.95**)


In reporting these results, Donefer er al. (1966) suggested that both of

the measures used, rate of cellulose digestion and pepsin solubility, are

likely to be more closely related to the intake characteristics of forages

than to digestibility. The high correlations with N V I result from the

feature, already noted, that intake changes considerably more with

changes in forage maturity than does digestibility (Ingalls et al., 1965).

The relationship of pepsin solubility to voluntary intake is in line with the



conclusions of Osbourn et al. (1966), illustrated in Fig. 3. However the

relatively poor correlation of pepsin solubility with dry matter digestibility (Terry and Tilley, 1964b; Donefer et al., 1966) indicates that a

more precise estimate of NVI might be obtained by a combination of

analyses for pepsin solubility (relative intake) and in vitro digestibility.

In that case, however, the separate estimates of intake and digestibility

could be of more use than the single parameter of N V I , by indicating

the relative importance of the differences in intake and in digestibility

among the forages being studied.



While nitrogen fertilization generally seems to have little effect on the

intake characteristics of forages, an exception occurs in the case of

forages of very low crude protein content, for which rate of digestion

within the rumen, and so level of intake, seems to be limited by the lack

of nitrogenous substrates for the rumen microorganisms. The critical

level of feed protein depends on the type of forage, but it is commonly

in the range of 4-6 percent crude protein. Thus Minson (1 967) showed a

54 percent higher intake of pangola grass (Digitaria decumbens) fertilized with 250 kg. of urea per acre (7.2 percent crude protein in the

forage dry matter) than at 100 kg./acre (3.7 percent crude protein).

Minson and Milford (1967) showed that the intake of the same species

(at 3.6 percent crude protein) was increased when a supplement of

lucerne was fed, to a maximum intake at a total diet content of 6 percent

crude protein, after which the intake of the grass decreased as it was

progressively replaced by lucerne.

In most experiments the available feed nitrogen has been increased

indirectly by supplementary feeding, particularly with' sources of nonprotein nitrogen, such as urea. This is discussed more fully in Section

V , D; but in relation to forage intake there is evidence that the intake of

low-protein forages can be considerably increased by feeding urea. Thus

Campling et al. (1962) found a 40 percent increase in the intake of oat

straw (3.0 percent crude protein), and Graham (1 967) found a 50 percent

increase with Digitaria decumbens (4.0percent crude protein), resulting

from urea supplementation. However, at a higher level of feed protein

(5 to 7 percent), Kay et al. (1968) found no significant improvement in

the intake of barley straw when supplementary nitrogen was fed. Weston

(1967) showed that at very low levels of crude protein the main factor

limiting the intake of wheaten hay was nitrogen deficiency; when this

was remedied, intake was then limited by the rate of removal of indigesti-



ble residues from the rumen. However, Egan and Moir (1965) have

queried whether the intake of low-nitrogen hay is limited solely by deficiency of feed nitrogen within the rumen, as infusion of casein directly

into the abomasum or duodenum was found to increase the intake of such

hay. This source of nitrogen could have benefited bacterial digestion

within the rumen only indirectly, and these authors suggested that the

amount of protein reaching the duodenum may itself affect voluntary





The use of nonprotein.nitrogen compounds is one aspect of the larger

subject of feed interactions, including the effect of other components of

the total ration on the amounts of different forages that the ruminant will

eat. Most studies to date have been essentially practical and have recorded mainly the changes in forage intake when supplements are fed.

As with the effects of supplementary feeding on forage digestibility,

noted in Section IV, C, 3 , an understanding of the biological basis of

these interactions is essential if supplementary feeding is to be other

than an empirical procedure.

As might be expected, the most general observation is that, as increasing amounts of supplementary concentrates are fed, the ruminant animal

eats less forage. This decrease in forage intake appears to be more

marked with forages of high digestibility (i.e., of initially high intake)

than with forages of lower digestibility and intake (Blaxter and Wilson,

1963; Campling and Murdoch, 1966).

The considerable reduction in the voluntary intake of forages of high

digestibility may well be associated with the observations, noted in

Section V, B, 1, that such forages tend to behave like concentrated feeds,

so that their voluntary intake may be determined by levels of blood

metabolites, rather than by rumen fill; in that case level of forage intake

might be expected to decrease as a result of the enhanced levels of blood

metabolites following concentrate feeding.

The reduction in intake of forages of lower digestibility may in part be

related to the effects of supplementary feeding on rumen pH, noted in

Section IV, C, 3. As the level of concentrate feeding increases, rumen

pH tends to decrease (P.K. Briggs et al., 1957; Topps et al., 1965) and

it is possible that this causes a reduced rate of digestion of the cell wall

constituents in the forage (Tilley et al., 1964), and so a reduced level of

voluntary intake. The depression in rumen pH could perhaps be greater

with concentrates of low protein content (containing mainly cereals)

than with those of higher protein content, and this may partly account for



the greater reduction in forage intake when low-protein concentrates

are fed.

However, the most important feature is evident, that in many cases

supplementary feeds partly replace rather than supplement the forage

with which they are fed; the significance of this in determining the economic response to supplementary feeding emphasizes the urgent need for

more detailed studies of these food interactions.

VI. The Efficiency of Utilization of Digested Nutrients

The previous sections have examined the factors that determine the

quantity of digested nutrients (intake x digestibility) made available to

the ruminant; the following sections consider the efficiency with which

it utilizes the various components within these digested nutrients, in

particular the energy, protein, and minerals, as well as other components

in forages which may modify their potential as practical ruminant feeds.



1 . Digestible Energy

Most results of digestibility experiments with forages have been reported in terms of the digestibility of the dry matter or of the organic

matter in the forage (Eq. 1) or of its content of total digestible nutrients

(TDN), which effectively measures the percentage of digestible organic

matter in the forage dry matter (Minson et a f . , 1960). Digestibility is

primarily an index of the energy value of a feed. Crampton et al. ( 1 960)

have proposed that all results should be reported in terms of digestible

energy by measuring C (Eq. 1) in kilocalories; TDN data already published could be converted on the basis of 4.4 kcal. of digestible energy

per gram of TDN. The latter factor can only be approximate, as it

varies with the crude protein and ether extract contents of the forage

(Heaney and Pigden, 1963), and direct calorific determinations on feed

and feces samples provide the only reliable method.

2 . Metabolizable Energy

All the expressions of digestibility based on Eq. (1) measure the

“apparent” digestibility, rather than the “true” digestibility of the feed

energy, for the feces contain metabolic residues not derived from the

feed (Section 111, C ) . But these are in practice more useful measures than

“true” digestibility, because these metabolic residues represent an

inevitable energy loss associated with the processes of digestion and

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V. The Voluntary Intake of Forages

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