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IV. The Digestibility of Different Forage Species

IV. The Digestibility of Different Forage Species

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forage species harvested during first growth during the spring could be


% digestibility of dry matter = 85.0 - 0.48X


where X = number of days to harvest from April 30.

These reports stimulated further investigations that have greatly increased our understanding of forage digestibility. Clearly the “date of

harvest” in Eq. (9) can only be relevant in the area close to Cornell

(Ithaca, New York), where the forages used in these in vivo digestibility

determinations were harvested, but the basic principle, that digestibility

is closely related to forage maturity, with “date of cutting” being used

to describe stage of maturity in a particular location, was soon confirmed

by other workers (Kane and Moore, 1959, and others; reviewed by

Blaser, 1964). However, several divergences from the original concept,

that all forages are of similar digestibility at a given date, became evident.

Thus the digestibility of timothy (Phfeumpratense) (Mellin et a f . , 1962;

Minson et a f . , 1964), of lucerne (Medicago sativa) (Demarquilly, 1966a),

and in particular of white clover (Trifofium repens) (Harkess, 1963)

were shown to decline less rapidly with advancing maturity than the 0.5

unit per day indicated in Eq. (9). Small differences in the digestibility of a

given forage variety cut on the same date in different years may be due to

the delayed onset of active spring growth in a “late” season (Minson

et af., 1960), or to differences in leaf percentage in the forage grown in

different years (R. H. Brown et a f . , 1968). But the most important observation has been that there can be large and consistent differences in

digestibility between forage species and forage varieties. In a detailed

series of in vivo experiments, Minson et a f . ( I 960, 1964) found that

certain species (Lofium spp. and Festuca pratense) were considerably

more digestible than others such as cocksfoot (Dactylis gfomerata) and

tall fescue ( F . arundinacea); within a species late-maturing varieties

(e.g., S.23 ryegrass) maintained a high level of digestibility to a later date

than early-maturing varieties (e.g., S.24). These differences, illustrated

in Fig. 1 , were considered large enough to be of agronomic and nutritional

significance. Figure 1 also shows that the digestibility of each grass

studied did not fall at a uniform rate as it matured. There was an initial

period of almost constant digestibility before digestibility began to decrease. In the case of Lofium and Dactylis this change to a more rapid fall

in digestibility, at a rate very similar to the 0.5 unit per day recorded by

Reid et a f . ( 1 959), was closely associated with the first emergence of

flowering heads; the digestibility of timothy (S.48), however, decreased

well before this stage, and this species also showed the much slower

rate of fall in digestibility noted above.



In establishing these novel digestibility patterns, Minson et al. (1964)

had two main advantages compared with their colleagues in North

America-in the more general use of the genus Lofium in the United















date of first cuttinq

FIG. I . The percent digestibility of the organic matter in grass varieties during first

growth in the spring. S.23 and S.24, ryegrass; S.37 and Germinal, cocksfoot; S.215, meadow

fescue; S.48, timothy; S. 170, tall fescue; O.M., organic matter. 0 indicates date of first

ear emergence. (Data from Minson et al., 1964.)

Kingdom than in North America; and in the availability of widely differing maturity types, within this genus, developed at the Welsh Plant

Breeding Station. These results stimulated other in vivo studies, which

have in general confirmed the important conclusions implicit in Fig. 1.

Thus Castle et a f . (1962) and Harkess (1963) have found consistently

higher digestibility of ryegrass than of cocksfoot, and Lowe et af. (1962)

reported that late-maturing varieties of grasses were more digestible

than early-maturing at a given cutting date in the spring. These in vivo

studies, initially on first spring growth, have also been extended to regrowths during the rest of the growing season, and consistent patterns

have again emerged. The digestibility of regrowth cocksfoot and tall

fescue is always lower than that of the corresponding regrowth of ryegrass, and the rate of fall of digestibility with time is much less in these

later, largely vegetative, regrowths than in the first, reproductive, growth

(Minson et af., 1960, 1964); a similar slower rate of fall in digestibility

has been shown with regrowths of lucerne (Demarquilly, 1966a).



However, the digestibility of regrowth forage is likely to be less predictable than that of first growth. The latter comprises largely reproductive

tillers which develop from the start of active growth in the spring; regrowths can contain both reproductive and vegetative tillers, the relative

proportions of each depending on the management of earlier harvests.

Thus the regrowth from a tiller whose reproductive growing point (ear)

is harvested will comprise mainly leaf, which will decrease only slowly

in digestibility at ca. 0.1 percent per day. In contrast, a tiller which is

harvested before the ear has reached the height of cutting or grazing will

continue to develop, and its digestibility will decrease at the 0.5 percent

per day characteristic of first growth. Harvesting up to the time of first

ear emergence will remove only some of the developing ears, and the digestibility of the regrowth will decrease at an intermediate rate, e.g.,

0.3 percent per day. A later harvest, which removes most of the potential

ears, will give a leafy regrowth which decreases more slowly in


Under a cutting regime regrowths are mainly leafy, and of relatively

predictable digestibility (Minson et al., 1960, 1964). Under a grazing

situation the prediction of the digestibility of regrowths is less precise.

The grazing animal generally leaves some of the herbage on offer, and

this continues to decrease in digestibility, so that the combined regrowth

and remaining herbage is of lower digestibility than the corresponding

regrowth from a cut sward (Tayler and Deriaz, 1963). As noted in Section IV, C , 2, the digestibility of a regrowth from grazing will also depend

considerably on the moisture and nitrogen status of the sward, which

determines the proportion of the next harvest that is composed of new

growth of high digestibility.

Limited information indicates that the digestibility of forage mixtures

can be calculated from the proportions and digestibilities of the constituent species at the time of harvest (Harkess, 1963). An exception may be

the case where the digestibility of one species is low because it is deficient in protein content (as has been shown with some tropical forages;

Smith, 1962). Such a species, grown with another species of higher protein content, could have an enhanced digestibility, so that the digestibility

of the “mixture” would be somewhat higher than predicted.

The patterns of forage digestibility discussed above were all based on

in vivo experiments; they have been considerably extended by the use of

laboratory in v i m techniques. The validity of the two-stage in vitro

technique was indicated by Terry and Tilley (1964a), who showed that

the basic patterns of digestibility shown in Fig. 1, and in particular the

nonlinear fall in digestibility with time and the consistently higher diges-



tibility of ryegrass than of cocksfoot, could be closely reproduced by

in vitro measurements. With this method many more forage samples

can be examined than would ever be possible by in vivo experiments,

but the need to confirm, in vivo, the more important conclusions indicated

from in vitro studies must be emphasized. Thus Dent and Aldrich (1968)

have measured the digestibility, in both first growth and regrowths, of

numbers of varieties within several forage species. They have shown consistent differences in digestibility between species, and between maturity

types within species, at different centers and in different years. Their

work indicated that different varieties of the same maturity type within a

species might differ in digestibility. Thus a small number of varieties of

cocksfoot (e.g., Roskilde I1 and Scotia) appeared to be more digestible

than the varieties Germinal and S.37, shown in Fig. 1, and Reveille

tetraploid ryegrass was more digestible than S.24 ryegrass. These in

vitro results, subsequently confirmed in in vivo experiments (Osbourn,

unpublished) have particular relevance to the possibility of breeding

more digestible forage varieties, discussed in Section VIII.




The use of in vitro techniques to measure the digestibility of different

plant fractions has also provided a more logical understanding of these

patterns of forage digestibility. These are in many cases contrary to what

would have been predicted in terms of the earlier concepts of forage

nutritive value, that leaf was more digestible than stem, and that forage

fractions high in protein (N) content would be more digestible than those

of lower N content. The results of Minson et al. ( 1 960) showed that these

concepts might be incorrect. Thus in first growth, S.24 ryegrass forage cut

in 1959 on May 1 , with a nitrogen content of 2.62 percent and 53 percent

of leaf lamina, was of exactly the same digestibility as that cut on April

20, which had contained 3.66 percent N and 77 percent leaf, S.37 cocksfoot cut on May 5 , with 2.78 percent N and 58 percent leaf, was 4.5 units

less digestible than the ryegrass cut on May 1 . In later experiments

Tayler and Rudman (1966) harvested a S.24 ryegrass sward in two

horizons, a top fraction cut above 13.5 cm., and a bottom fraction from

6 to 13.5 cm. The digestibilities of the organic matter in the two fractions, in vivo, were 84.0 and 82.6 percent, respectively.

This indirect in vivo evidence has been followed up in detail by in vitro

digestibility determinations on forage samples separated into fractions

of leaf, leaf sheath, stem, inflorescences, and dead material. Terry and

Tilley ( 1 964a) analyzed these fractions from the forages fed in vivo by

Minson et al. ( 1 960, 1964) (Fig. 1). They showed that in all species the



digestibility of the leaf fraction fell only slowly with advancing maturity

(0.13 percent per day) whereas that of the leaf sheath (0.4 percent) and

stem fractions (0.7 percent) fell much more rapidly. In the immature

forages the stem was always more digestible than the other components.

On any given date each fraction in S.24 ryegrass was more digestible than

the corresponding fraction in S.37 cocksfoot, of equivalent maturity

type. A typical set of results is shown in Fig. 2. The digestibility of the

whole plant

leaf sheath


leaf blade




FIG.2. The digestibility in vifro of the dry matter in the whole plant, and in the leaf blade,

leaf sheath, and stem fractions of S.37 cocksfoot during first growth in the spring. Figures

in parentheses are the percentage of stem in the whole plant. (Data from Terry and Tilley,

I 964a. )

whole forage material, calculated from the proportions and digestibilities

of the constituent fractions, changed with maturity just as in the in vivo

experiments-that is, with a slow fall in digestibility up to the time of ear

emergence, followed by a more rapid fall as the stem and leaf sheath

fractions, by now less digestible than the leaf, comprised an increasing

proportion of the total forage. The slower and more steady fall in the digestibility of S.48 timothy could be explained by the much higher proportion of leaf sheath in this species than in ryegrass or cocksfoot.



These conclusions have been confirmed in considerable detail by

Pritchard et al. (1963), Wedin et al. (1966), Walters et al. (1967), and

Dent and Aldrich (1968); Mowat et al. (1965) found similar results with

timothy and bromegrass, but were unable to show higher digestibility

of cocksfoot stems than of leaves even in immature forage.

Similar logical patterns of digestibility have been shown with legume

forages (Terry and Tilley, 1964a; Mowat et al., 1965). With lucerne

( M . sativa), red clover ( T . pratense), and sainfoin (,Onobiychis viciifoliu) the older leaves tend to senesce and fall, so that the digestibility

of the leaf fraction decreases very little as the plant matures. By separating the stem fraction into 6 inch subfractions, measured from the top

of the plant, it was shown that digestibility decreased down the stem, but

that the digestibility of any given fraction changed relatively little with

maturity. A similar result has been found with the stems of brassicas and

forage maize (Zea mays), the stem tip of marrow-stem kale being highly

digestible (81.8 percent dry matter) and the stem base of lower digestibility (62.3 percent) (Dent, 1963).

Within a particular forage species, Walters et al. (1967) and Dent and

Aldrich (1968) have shown, at a similar stage of morphological development, that “late” varieties tend to be less digestible than “early” varieties

because both the stem and leaf fractions are somewhat less digestible,

and because they contain a higher proportion of senescent and dead material. Small differences in digestibility of a given variety at ear emergence

in different years can be attributed to differences in leafistem ratios,

stem at this stage being less digestible than leaf (R. H. Brown et al., 1968).

Of particular importance is the observation, already noted, of differences in digestibility between varieties of similar maturity type within a

species (Dent and Aldrich, 1968). The higher digestibility of REVEILLE

ryegrass than S.24 was not accounted for in terms of leafstem ratio, but

because both the leaf and stem fractions in REVEILLE were more digestible than the same fractions in S.24. Differences in digestibility have also

been shown between the individual plants (genotypes) within a variety

(Cooper et al., 1962; Walters et al., 1967; Mowat, 1969), resulting from

differences in digestibility of the plant fractions rather than from different leaf stem ratios.

However, while the in virro techniques used in these studies can

describe the changes in digestibility within and between different forage.

species, they cannot explain them; for this the newer chemical techniques (Section 111, C ) are needed, to analyze digestibility measured

in vitro into its component parts. In detailed studies by Tilley, Terry,

and Outen (unpublished) the forage sample is separated into a cell con-



tents fraction, soluble in acid pepsin, and a cell wall fraction (analogous

to the fraction soluble in neutral detergent and the cell wall residue of

Van Soest, 1967). The digestibility of the cell wall fraction is also measured by in vitro (rumen organism) digestion. Consistent differences have

been shown between S.24 ryegrass and S.37 cocksfoot, harvested at the

same stage of maturity: (a) the content of pepsin-soluble material is

higher in the ryegrass than in the cocksfoot; (b) as a result there is a

higher content of cell wall fraction in the cocksfoot; and (c) this fraction

in the cocksfoot is less digestible than in the ryegrass, so that the content

of “digestible cell wall material” in the two species is very similar. As a

result the digestibility of the ryegrass (cell contents X 0.98 digestible

cell wall fraction) is higher than that of the cocksfoot.

Within different plant fractions, the small decrease in digestibility of

the leaf fraction as the plant matures (Fig. 2) is accounted for by the consistently high level of cell contents and high digestibility of the cell wall

fraction in the leaf. Young stem material contains an even higher proportion of cell contents than the leaf (hence its higher digestibility); but as

the stem matures the cell content fraction decreases rapidly and is replaced by cell wall material which becomes less digestible with increasing lignification, so that the digestibility of the stem decreases rapidly

with advancing maturity.

Extension of these more detailed studies of the components of digestibility to the genotypes within a species which have been found to be of

higher digestibility (Section VIII) may provide a more objective basis

for selection for improved digestibility than the in vitro techniques that

have so far been used.







1 . Environmental Effects

The difference in digestibility between forages cut on the same date

at Cornell (J. T. Reid et al., 1959) and in Maryland (Kane and Moore,

1959) appeared to be due to the forages in these two locations being at

different stages of physiological maturity. It is of course possible that,

even at the same stage of maturity, the digestibility of a forage may differ

between locations; thus Aldrich and Dent (1 967) have found an indication

of higher digestibility at a northern than at a southern latitude in the

United Kingdom in cocksfoot cut 10 days after 50 percent ear emergence.

Deinum et al. (1 968) measured the in vivo digestibility of perennial ryegrass grown under high and low light intensities, and with low and high

levels of nitrogen manuring. Considerable differences in chemical com-



position of the grass were found between treatments, but these had no

significant effect on dry matter digestibility at any one sampling. These

experiments showed lower levels of digestibility on all treatments during

the summer when temperatures were higher, confirming earlier results of

Deinum, based on chemical analysis of ryegrass grown in controlled environment cabinets. Deinum et al. ( 1 968) postulated that this effect of

high temperature might partly account for the generally lower level of

digestibility of tropical than of temperate forages. Hiridoglou et al. ( 1966)

have also shown that high summer temperatures were associated with

low in vitro digestibility levels. Forages growing in summer also tend to

contain lower moisture contents than late season forages; however, the

only report found on the effect of water intake on forage digestibility

(Thornton and Yates, 1968) has indicated a small increase in the digestibility by cattle of the dry matter and fiber in chaffed oat straw-lucerne

hay when water intake was restricted.

2 . Fertilizers and Forage Digestibility

The effects of fertilizer nitrogen on forage digestibility have been

studied in numerous experiments; most of these have reported an insignificant effect from the use of widely differing levels of application

(summarized by Blaser, 1964): thus Minson et al. ( 1 960) found no effect

on the digestibility of ryegrass or cocksfoot from levels of nitrogen application varying from 0 to 175 pounds/acre. However, Raymond and Spedding (1 965) have indicated several situations in which fertilizer nitrogen

is likely to affect forage digestibility: (a) in a mixed grass-clover sward

the use of this fertilizer may reduce the contribution, in the forage

harvested, of the more digestible clover complement: (b) uneaten herbage

left on a sward after stock have grazed will continue to decrease in

digestibility, and by diluting the highly digestible new growth will depress

the digestibility of herbage available at the next grazing; fertilizer nitrogen will increase the proportion of new growth in this harvest, and so may

increase its digestibility; (c) unfertilized herbage may contain an inadequate level of nitrogen for the growth of rumen microorganisms: thus

Smith ( 1 962) found an increased level of digestibility after application of

nitrogen to such forage; 40 pounds of fertilizer nitrogen per acre increased

the crude protein content of late-cut veldt hay from 3.6 to 6.8 percent and

the digestibility of the forage dry matter from 5 1.7 to 59.5 percent.

These effects of fertilizer nitrogen are consistent with the concepts

of forage digestibility already discussed. McIlroy ( 1 967) has summarized

results showing that fertilizer nitrogen increases the crude protein content and decreases the soluble carbohydrate content of herbage. But



there is little net change in the content of crude protein plus soluble

carbohydrate, which largely comprises the cell contents fraction, or in

the composition of the cell wall fraction. Thus little change in the digestibility of the forage would be expected except when the digestibility of

the cell-wall fraction is limited by the low content of protein in the


There is also the possibility that certain forages contain specific components which reduce bacterial activity within the rumen. Hawkins ( 1 959)

suggested that the low digestibility of the protein fraction in Sericea

lespedeza and vetches might be due to the formation of insoluble protein

complexes with the tannin in these forages, and Smart et al. ( 1 96 I ) found

a depression of cellulose activity in vitro by an extract from Lespedeza

cuneata. Schillinger and Elliott ( 1 966) observed differences of u p to 15

percent between the digestibilities in vitro of different lucerne plants.

Low levels of digestibility could be raised by addition of amino acids

(glycine, aspartic acid, glutamine) to the in vitro system, and these

amino acids also increased the growth rate of voles fed on the lucerne

forage. These authors attributed these differences to the presence of

water-soluble antimetabolites in the low digestibility plants.

Such cases are, however, likely to be exceptional, and the digestibility

of most forages appears to be in line with the systems of evaluation proposed by Van Soest ( 1967) and Terry and Tilley ( 1964a).

3. The Effect of Feed Supplements on Forage Digestibility

These systems do not, however, predict adequately the digestibility

of forages fed in mixed rations. Thus when carbohydrate (starch) supplements are fed with forages, there can be a significant decrease in the

digestibility of the fiber (cell wall) fraction of the forage, unaccounted

for by any change in the composition of the cell wall (Eq. 5 ) . This has

been attributed to a preferential digestion of the starch by the rumen

microorganisms, so that the extent of digestion of the plant fibers is reduced, or alternatively that the amylolytic bacteria compete preferentially

for ammonia against the cellulolytic bacteria, so reducing cellulose digestion (El-Shazly et al., 1961).

More recent work has indicated an alternative explanation. First, it is

known that when a starch supplement is fed with a forage there is a reduction in the pH of the rumen contents compared with that when the

forage is fed alone (Topps et al., 1965). Second, Tilley et al. ( 1964) have

shown a marked reduction in the rate and extent of digestion of dry

matter and cellulose in vitro when the pH of the in vitro system is reduced

(Table I); a decrease, with decrease in rumen pH, has also recently been



indicated in the rate of “digestion” of cotton threads suspended within

the rumen in vivo (Wilkins, unpublished). Tilley et al. (1964) have therefore suggested that the lower rumen pH when a starch supplement is fed


The Effect of the pH of the in Vitro Digestion on the Extent of Dry Matter and

Cellulose Digestibility of Samples of Cocksfoot, S.37, by the 2-Stage in Vitro Method“

Sample No.

In vivo

Percent of forage dry matter digested

In vitro (48 hours)

pH 6.8

pH 6.0













“From Tilley et al. ( 1964).

*Figures in parentheses are percentages






67 (12)

63 ( I 1)

50 (10)

49 (10)

41 (7)

pH 5.5











of digestible cellulose in the digestible dry

may provide a less favorable environment for the cellulolytic and other

bacteria that are able to digest plant fiber (see also Head, 1961). In

anthropomorphic terms, the natural rumen microflora has become

adapted to the pH 6.6 to 6.8 characteristic of the rumen contents of the

grazing herbivore; any marked divergence from this pH finds a microfloral population increasingly unable to digest fiber. This hypothesis, if

substantiated, could lead to the development of feeding regimes, aimed

at optimizing rumen pH (and redox potential) to ensure maximum digestion of the cell wall fraction of forages.

As already noted, forages may be of low digestibility because they are

of very low protein content (< 4% crude protein). The digestion of these

forages can be increased by feeding protein supplements, and there has

been much interest in the use of urea for this purpose. Thus Campling

ef al. (1962) measured an increase in organic matter digestibility from

41 to 50 percent when a urea supplement was fed with oat straw of 3.0

percent crude protein, and other examples have been reported (see M. H.

Briggs, 1967). Within the rumen the urea is rapidly deaminated, the

ammonia produced then being used by the celluloytic and other bacteria,

whose digestive activity would have been limited by deficiency of protein in the unsupplemented forage.

The feeding of urea to increase the digestibility of forage is not often

used in practice, because a crude protein level in forage of less than 4



percent is uncommon. The main use of urea is likely to be as a substitute

for protein in productive rations (Section VI, D).

V. The Voluntary Intake of Forages




It was noted in the Introduction that the quantity of forage that ruminant animals eat is in practice seldom controlled in the same way that

most other farm feeds are rationed. Yet the amount of forage that animals

eat is in many cases the major factor determining their level of nutrient

intake and their output of useful products. Increasing emphasis has been

given in the last decade to the study of voluntary intake.

Earlier studies of forage intake were undoubtedly hindered by the confusion of “intake” with “palatability” (Blaxter et al., I96 1 ; Campling,

1964), but it is now accepted that forage intake is mainly controlled by

largely involuntary physiological reflexes within the animal, rather than

by its subjective liking for different feeds. The development of some of

the current concepts on voluntary intake have been reviewed by Balch

and Campling (1962), Conrad ( 1 966), and L. D. Brown (1966). From

these a broad distinction appears between the factors determining intake by ruminants and by nonruminants. Intake by nonruminants is controlled mainly by levels of blood metabolites, the animal ceasing to eat

when these reach a threshold level. Intake by ruminants depends much

more on the capacity of the digestive tract, particularly the rumen, eating ceasing when a certain degree of “fill” has been reached, and starting

again when “fill” has been reduced by digestion and movement of food

residues through the digestive tract: only on feeds of high energy concentration does blood metabolite level, rather than gastrointestinal fill, begin to control the amount of food that ruminants will eat (Conrad, 1966).

However, as with other aspects of forage nutritive value it is essential

to recognize that the amount of forage that animals will eat is likely to be

determined by a complex of factors. Some earlier investigations may

have oversimplified the problem; the observation by Blaxter et al. ( I96 1)

that the newer concepts related to digestibility and rate of passage of

foods are “attributes which are hardly consonant with their acceptability

to the palate or taste” perhaps dismissed too lightly the possible significance of these other attributes. It is evident also that the amount of

forage that animals eat may depend as much on the amount of forage

available as on any inherent characteristics of the forage itself. Raymond

(1966a) has proposed that the factors determining forage intake can be

usefully divided into intrinsic factors (i.e., features inherent in the forage)

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IV. The Digestibility of Different Forage Species

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