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IX. The Effects of Processing on the Components of Forage Nutritive Value

IX. The Effects of Processing on the Components of Forage Nutritive Value

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forages has been considered in Sections IV, C and V, B. Both the digestibility and intake of dehydrated forage have been found to be similar to

those of the fresh forage, but the apparent digestibility of the nitrogen

fraction is reduced if the temperature of the forage is maintained above

105°C. for a significant period during drying (see, e.g., Ekern et al., 1965).

Despite this reduction in nitrogen digestibility, body nitrogen retention

from dried forages is not reduced (Ekern et al., 1965) unless heat damage

is excessive: possible reasons for this were considered in Section VI, C.

In practice the main method of forage dehydration is haymaking. Considerable changes in nutritive value can occur in this process, due not to

dehydration per se, but to deficiencies in the techniques used, leading to

dry-matter losses in the field which may rise to 25 percent or more under

unfavorable conditions. Because of differential loss of leaf the loss of

crude protein from the crop may be as high as 40 percent and dry-matter

digestibility may be reduced.

Shepperson (1960) compared the digestibility of hay made by three

methods (slow field making, rapid field making, and barn drying) with the

digestibility of the herbage cut. The digestibility of all the hay samples

was lower than that of the original crop, the depression in digestibility

being directly related to the time the cut crop remained on the field before it was lifted, the amount of mechanical tedding needed before lifting,

and the extent of leaching by rain. The hay under the least favorable conditions was 7 units less digestible than the herbage cut.

Many other similar experiments (Watson and Nash, 1960, p. 53) have

confirmed the importance of rapid field curing if depression of digestibility is to be avoided; but it is certain that most of the hay made today

still suffers high losses of both dry matter and nutritive value. As Shepperson ( 1960) indicated, nutrient losses during haymaking can be so great

that crop digestibility, based on species and stage of maturity, may give

only a very approximate guide to the digestibility of the resulting hay.

Further deterioration can occur in storage. Even under optimal conditions there is a slow loss of dry matter; thus Melvin (1963) found a 10

percent loss of weight from ryegrass hay stored for 40 weeks, and Greenhill et al. (1961) showed that storage losses increased with increased

moisture content in the hay and at higher temperatures. These losses are

mainly in the sucrose, glucose, and fructose fractions in the hay and may

lead to some decrease in digestibility. More serious is the depression in

digestibility which occurs when hay overheats in storage (Watson and

Nash, 1960), and molding can also lead to reduced intake of hay (Demarquilly, 1966a) and may present a toxic hazard to both animals and

man (Lacey, 1968).







One of the most important advances in the period under review has

been the discovery that grinding and pelleting a dried forage may lead to a

considerable increase in its nutritive potential. Earlier studies were mainly

practical, but recent research has greatly clarified the mechanisms involved (see reviews by Minson, 1963; Beardsley, 1964; L. A. Moore,

1964). Most workers agree (a) that the voluntary intake of pelleted

ground forages is considerably higher than of the corresponding long

forage, (b) that under ad libitum conditions the long forage is more

digestible, but (c) that there is usually an increase in the net intake of

digestible energy on the pelleted forage. The magnitude of these effects

differs, however, among forage species and with the stage of maturity of

a given forage (Heaney et al., 1963).

These results can primarily be attributed to the markedly higher rate of

passage through the digestive tract of the small particles in ground

forages compared with the larger particles in long or chopped forages. The

latter must remain within the rumen until they are broken down by mastication and microbial digestion to a size small enough to pass through the

reticulo-omasal orifice (Campling et a/., 1963). As voluntary intake of

forage depends largely on rate of passage (Section V), the animal is able

to eat more of ground than of chopped forage. However, because the

ground forage passes more rapidly through the reticulorumen there is

much less time for it to be subjected to microbial digestion, and digestibility, particularly of the fiber fraction, is reduced (Campling et al.,


Dehority et al. ( I 962) and Tilley and Terry ( 1963) had shown that the

digestibility of forage in vitru is increased by fine grinding (presumably

by making the fiber structure more accessible to microbial attack), but

in vivo any such effect seems to be outweighed by the reduced time available for digestion. Hinders and Owen ( I 968) have also shown a change in

the site of digestion of fiber, only 60 percent of the fiber which is digested

from pelleted lucerne being digested within the rumen (the remainder

being digested in the hind tract, including the cecum), compared with

over 90 percent with lucerne fed long.

As a given crop becomes more mature, the increase in intake following grinding is greater, however, than the corresponding decrease in digestibility, so that the effect of grinding on digestible intake increases.

The digestibility of legumes is depressed by grinding less than that of

grasses; thus Demarquilly and Journet ( 1 967) found an average decrease

of 9 units in digestibility for grass compared with 4 units for lucerne, and



Buchman and Hemken ( 1964) reported little reduction in digestibility

when lucerne was ground and pelleted. The difference in response between grasses and legumes is in line with the data in Fig. 3. At a given

level of digestibility the “digestible” fraction of lucerne contains much

less digestible fiber than does grass, and so might be expected to be less

sensitive to reduction in fiber digestibility following grinding. Demarquilly

and Journet (1967) also suggested that grinding lucerne led to an increased rate of fermentation within the rumen, in contrast to evidence of a

marked depression in rate of cellulose digestion with grass hay (Campling

et al., 1963).

The stage has now been reached when the terms drying, grinding, and

pelleting may be inadequate descriptions of these processes, and more

detailed specifications, e.g., of temperature and duration of drying, and

of particle size and pellet dimensions, will be needed. Possible effects of

different drying conditions have been noted (Sections IV, C, VI, C).

Demarquilly and Journet ( I 967) found that very fine grinding through a

1.5-mm. screen depressed grass digestibility more than through a 5-mm.

screen, but that there was little increase in intake consequent on the

finer grinding, which may thus be disadvantageous. Kamstra and Jahn

(1966) found that very high pellet pressure may increase cellulose digestibility. The pelleting process itself may also be important, by making

ground herbage (a dusty material) more acceptable to stock: conversely

practical experience confirms a marked reduction in intake when pellet

density is too hard. Ronning and Dobie ( 1962) have reported detailed

studies on the effects of pellet size on voluntary intake, which may be reduced below that of the long forage when unsuitable pellets are made. Pelleting may also have less effect when it is applied to forages of very low

protein content. Thus Minson ( 1 967) found an increase in intake of only

I4 percent when unfertilized Pangolagrass (3.7 percent crude protein) was

pelleted, compared with a 30 percent increase with fertilized grass of 7.2

percent crude protein; intake of the low protein grass may have been

limited by the low nitrogen status of the animal (Egan and Moir, 1965)

rather than by rate of passage, and so have been relatively insensitive to


It also appears that the metabolizable energy value of ground forages

cannot be calculated from Eqs. ( 1 9) and (20), because of the reduction

in methane production consequent on the reduced fiber digestion within

the digestive tract, compared with long forage: net energy values also

cannot be calculated because of the lower heat increment with milled

than with long forages. As a result Blaxter and Graham (1956) found

very similar net energy values for chopped dried grass and finely ground



grass, even though the energy digestibility of the latter was 1 1 . 1 units

lower. Recently Graham ( 1967) has reported that the net energy value of

pelleted rations can be further improved if they are fed at frequent intervals rather than once or twice per day. Using a comparative slaughter

technique, Paladines et al. ( 1964) found very similar levels of digestibility

when equal intakes of chopped o r pelleted hay were fed to lambs, but

lower body energy gains by the lambs on the chopped hay. This they

accounted for in terms of higher heat increment (wastage) on the chopped

hay. Under ad libitum feeding conditions they attributed 78 percent of

the higher energy gains on the pelleted hay as resulting from increased

intake and 2 2 percent resulting from increased efficiency of utilization of

metabolizable energy; the possible role of decreased methane production

(found by Blaxter and Graham, 19S6) could not be assessed with the

experimental technique used.

Numerous experiments (reviewed as above) have shown the effects of

processing dried forages on animal production, and many of these have

related the results to measured intake and digestibility data. These results are often difficult to interpret because of the different significance

of digestibility for long and ground forages. They are also complicated

when concentrates are fed with the forage, and particularly when concentrates and forage are pelleted together. Thus the intake of pellets containing ground forage and concentrates may be less than that of the long

forage and concentrates fed separately (McCroskey et al., 196 1 ) .

There is often a marked effect of grinding and pelleting on rumen

volatile fatty acids, and many experiments have shown higher propionate:

acetate proportions in the rumen liquor of animals fed on pelleted compared with long feeds. Earlier experiments had indicated that this would

only occur with mixed forage-concentrate feeds, but P. L. Wright et al.

( 1963) showed a marked shift from acetate to propionate production when

hay was ground, and Demarquilly and Journet (1967) have found a

similar but smaller effect with dried lucerne. As with other ruminant

feeds, the higher nutritive value of pelleted feeds has sometimes been

attributed to this higher proportion of propionic acid in the rumen acids,

but it appears likely that the lower methane production and the lower heat

increment with ground feeds may be more important factors than the

differences in rumen acids. An important exception occurs with milk

production, where there is clear evidence of a depression in the butter

fat content of milk associated with the decreased acetate production

when ground forage is fed. G. D. Thomas et al. ( 1 968) found a depression in milk fat from 4.6 to 3.9 percent when coarsely ground hay in a

mixed ration was replaced by finely ground hay; at the same time there



was a decrease from 68.3 to 59.2 percent in the molar proportion of

acetate in the rumen acids.

Another quite different effect from diets of ground forages is the

disease of rumen parakeratosis, which is characterized by hardening of

the small papillae on the rumen wall, and which leads to reduced nutrient

absorption and decreased animal production. The problem is most acute

with finely ground feeds, but is reduced when small amounts of long

roughages are fed (Garrett et al., 1961).

Processing forages by grinding and pelleting clearly offers exciting

possibilities for improving levels of ruminant production. Much more

detailed work is needed, however, if such processing is to become an

integral part of a planned program of crop production and animal feeding,

rather than a somewhat empirical operation, as at present. Its implications may then extend beyond purely nutritional advantages, with drying

and pelleting providing efficient conservation of forage crops in a form

suited for mechanized handling and feeding (Raymond, 1968).




Ensilage can be simply described as the storage of wet crops under

anaerobic conditions by preservatives that inhibit microbial or enzymatic

changes in the crop. The most common preservatives are acids-either

mineral or organic acids added to the crop, or organic acids produced by

bacterial fermentation of carbohydrates present in, or added to, the crop.

In general, the higher the moisture content and the buffering capacity

from organic salts in the forage, the lower is the pH needed to ensure

stability (Playne and McDonald, 1966). Crops high in crude protein and

moisture content, such as legumes and nitrogen-fertilized grass, are the

most difficult to ensile; with these, wilting to reduce moisture content,

or the use of additives, is advised, in particular when the ratio of crude

protein:soluble carbohydrate in the forage exceeds 2 (Gordon et al.,


1 . The Voluntary lntake of Silage

The very considerable literature summarized by Watson and Nash

(1960) reveals a remarkable imbalance of work to that date toward

studies of the chemistry and microbiology of the ensiling process compared with studies of the effect of ensilage on the nutritive value of the

product. Deficiencies in silage as a ruminant feed were often noted in

practice, but the first formal definition of the problem appears to have

been by Presthegge ( 1 959) and L. A. Moore et al. ( 1 960), who showed

that the voluntary intake of silage was less than that of hay. This has



been confirmed by many other workers, and Harris and Raymond ( 1963)

have also shown that the intake of silage is generally lower than that of the

fresh crop. In all these experiments products from the same crop were

compared: most previous comparisons of hay and silage had been confounded by silage being made at the “silage stage of growth” and hay at

the “hay stage,” that is from a more mature and less digestible crop.

Presthegge (1959) and L. A. Moore et al. (1960) also showed that the

intake of silage was increased if the crop was wilted before ensiling.

These results appeared just at the time that the significance of voluntary

intake was becoming widely recognized, and as a result particular emphasis came to be placed on the importance of wilting crops before

ensiling so as to ensure high intake levels. As noted above, wilting may

be advantageous in ensuring an efficient ensilage process, and is essential

if silage is to be made in tower silos. But many crops, such as forage

maize and ryegrass, contain sufficient available carbohydrate to give

stable, low pH, silage with minimal wilting. Chemical additives can also

be used with unwilted crops. In these cases wilting, which makes the

silage system more weather-sensitive, appears unnecessary as an essential part of the ensilage process, but might appear advantageous because

it leads to an increase in silage intake. Because of the practical problems

in wilting, recent research on the causes of low intake of silage has thus

aimed to develop alternative methods of increasing intake which will

obviate the need for wilting.

As with forage intake, it is clear that there is no single factor causing

the intake of unwilted silage to be lower than that of the equivalent fresh

or dried forage. It is not due to the high moisture content per se in the

silage, for addition of water to hay or wilted forage does not decrease

voluntary intake (J. W. Thomas et al., 1961). Campling (1964) has suggested that silage within the rumen forms a fibrous dough from which

“digested” feed particles can pass only with difficulty to the hind tract,

so that rate of passage, and as a result level of feed intake, is restricted.

Most workers, however, have considered that during the ensilage process

chemical compounds are produced which limit intake. In the case of

poorly fermented silages with high contents of butyric acid and amines,

this could be a direct effect of unpalatability, but Neumark ( I 962) suggested that protein breakdown products of specific pharmacological

activity might also be present in such silages. Neumark’s original suggestion that the active compound might be histamine was not confirmed by

P. McDonald et af. (1963), who found no decrease in voluntary intake

when histamine was added to silage. More recently, however, Neumark

and Tadmor ( 1968) have indicated that histamine is active only when



acetic or formic acid is also present, and that these compounds may act

within the abomasum rather than within the rumen itself.

More serious is the problem of low intake with the low pH silage which

the efficient preservation process aims to make, and in which there is

minimal degradation of the protein fraction. There is now some evidence

that this is at least partly due to the high content of free organic acids

in such silage (Harris et al., 1966; McCullough, 1966). It is recognized

that some chemical additives, such as mineral acids or ammonium bisulfate may reduce silage intake (Watson and Nash, 1960, p. 641; McCarrick et al., 1965), but this has been attributed to the ruminant animal's

inability to metabolize inorganic anions, in contrast to the organic acid

anions, which comprise its main energy metabolite. However, it has been

found that a number of silage constituents, including lactic, acetic, and

propionic acids and longer-chain fatty acids, may lead to reduced feed

intakes when they are added to silage before feeding, or infused directly

into the rumen (J. W. Thomas et al., 1961; Rook et al., 1963; Ulyatt,

1965). Thus McLeod, Wilkins, and Wilson ( 1968, unpublished) have

shown a linear depression of silage intake by lambs as the pH of silage

was progressively lowered by the addition of lactic acid (Table 11).

Lambs were used because they appear to be most sensitive to the factors

in silage that depress intake. In the reverse direction,' McCarrick et al.

(1965) and McLeod, Wilkins, and Wilson (1968, unpublished) have found

significant increases in voluntary intake by the addition of sodium bicarbonate to silage. In the latter experiments the intake of unwilted silage

by lambs and calves was raised by I5 percent when the silage was partially neutralized from pH 4. I to pH 5.3 before feeding.


The Voluntary Intake, by Lambs, of Silage Acidified to Different

Levels of pH by Addition of Lactic Acid".b

















I .9

S.E. of treatment mean, 0.12


"FromMcLeod, Wilkins, and Wilson (unpublished).

"The original silage, pH 5.4, was prepared by partially neutralizing an acid silage, pH 4.0,

with sodium bicarbonate.

'Dry matter as percentage of liveweight.



The exact mechanism whereby the organic acids in silage may limit

the voluntary intake of silage is still uncertain. I t appears unlikely to

operate directly within the rumen; while R. G. Warner et al. ( 1966) found

a 40 percent reduction in hay intake when rumen pH was reduced to

6.0 by infusions of lactic or citric acids, this depression of hay intake

could have been the result of decreased cellulose digestibility at this low

pH (Section IV, C, 3) which would be quite uncharacteristic of that of

the animal fed on silage, which is generally in the pH range 6.6 to 6.8.

It appears that silage is largely neutralized by saliva before it is swallowed. Thus Lambourne ( 1 965, unpublished) used esophageal-fistulated

sheep to collect the feed bolus swallowed by the animal, and showed

that silage (pH 4.0) was partially neutralized (ca. pH 5.8) before it was

swallowed. Unwilted silage (pH 4.0, 20 percent dry matter) can contain

20 times as much H+ ion per unit of feed dry matter as unwilted silage

(pH 5.0, 40 percent dry matter); thus it may be postulated that the intake

of unwilted silage is lower than that of wilted because of the much greater

quantity of acid to be neutralized by saliva during the process of ingestion.

Orth and von Kaufmann ( 1966) have also suggested that saliva secretion

may be depressed by the high acid content and lack of physical structure

in high-moisture silage. There is as yet no firm evidence that silage intake

is limited via restriction of salivation. But if the apparent effect of high

silage acidity in depressing voluntary intake is further confirmed it may

indicate a possible contradiction between high intake and the requirement for high acidity in the ensilage process.

The comparison of the voluntary intakes of hay, wilted silage, and unwilted silage is an important and legitimate research study. But the results of these studies, which have compared hay or silage when each was

fed as a sole feed, have perhaps been extrapolated too widely in advice on

practical feeding systems, in which hay or silage are most unlikely to

comprise the sole feed of productive livestock. The relevant question

then is how much hay or silage is eaten when both are fed in combination

with the other ration components. Recent information suggests that the

superiority, in intake terms, of hay o r wilted silage compared with unwilted silage, when each is fed alone, may largely disappear when they

are fed in mixed rations. Thus Osbourn (1967) found that the intake of

hay by lambs was 35 percent higher than that of unwilted silage (pH 4.4,

2 3 percent dry matter) made from the same crop. As increasing amounts

of rolled barley were fed, the hay intake decreased by 0.68 g. per gram of

barley fed, but the silage intake decreased by only 0.28 g.; when barley

comprised 40 percent of the total dry matter intake, there was no difference between the hay and silage intakes (Fig. 7). Campling and Murdoch



(1966) also found that the intake of silage by dairy cows was depressed

less than the intake of hay when concentrates were fed, and S. M. Brown

( 1 960) found that the intake of unwilted silage by cows was depressed


q. D. M{ kq.Wo'7J/24hr.



HAY y = - O . b 4 ~ + 5 2 . 9 0

SILAGE y = - 0 . 2 6 ~ + 3 8 . 6 0






q. D.M./ kq.Wo.7y24 hr.



FIG.7. The voluntary intake, by lambs, of hay and silage made from the same crop of

ryegrass and fed with increasing levels of rolled barley. D.M. = dry matter. (Based on

Osbourn, 1967.)

less than that of wilted when both were supplemented by concentrates.

Of particular interest are recent experiments in which supplements of

pelleted dehydrated forages have been fed with silage (Wilkins and

Osbourn, unpublished). S.24 ryegrass was conserved either as silage

(pH 4.3, 20 percent dry matter) or as pellets after dehydration and grinding. The dry matter intakes of the silage and pellets when fed as sole feed

to sheep were 15.3 g./kg. LW and 26 g./kg. LW, respectively. Increasing

amounts of pellets fed with the silage resulted in no decrease in the

amount of silage dry matter eaten until pellets comprised 40 percent of

the total ration: at this point the intake of silage plus pellets was similar

to that of the pellets fed alone, and the intake of digestible dry matter was

in fact higher because of the lower digestibility of the pellets than of the

silage. The nutritional and practical implications of this observation could



be of considerable importance, for instance, in the use of dehydrated

lucerne (high crude protein and minerals) as a supplement to low protein

silage made from ryegrass or wholecrop cereals (Raymond, 1968).

A further interesting development has been the addition of urea to the

crop before ensiling, with the aim of increasing the crude protein content

of silages made from low protein crops, in particular corn silage (reviewed

by M. H. Briggs, 1967; Essig, 1968). An addition of 0.5 to 1.0 percent

urea appears to increase dry matter loss slightly, and markedly to increase

the free ammonia content in the silage; a perhaps unexpected observation

is that simultaneous addition of 0.5 percent ground limestone improved

silage fermentation. Essig ( 1968) reported a 5 percent increase in gains

when treated corn silage was fed to cattle.

In general it seems likely that, in mixed rations, the intakes of wilted

and well-preserved unwilted silage may not differ greatly (S. M. Brown,

1960). This could mean that a decision to wilt a crop before it is ensiled

should be made on the basis that wilting is needed to ensure successful

ensilage, rather than with the aim of increasing voluntary intake.

2 . The Digestibility of Silage

While most attention has been paid to the intake characteristics of

silage, the digestibility of silage is also an important determinant of its

nutritive value. Earlier studies (Watson and Nash, 1960, p. 397) had indicated a rather lower digestibility of silage than of the crop ensiled. In

some cases, however, this may have been because the intake of silage

was based on dry matter determinations made at 100°C. As Harris and

Raymond ( 1963) have shown, samples of silage from digestibility experiments may suffer considerable losses of volatile constituents during

oven-drying. This leads to an underestimate in the amount eaten and in

the measured digestibility (Eq. 2). These authors measured the true dry

matter content of silage by toluene distillation and showed almost

identical digestibilities for silage and the original crop.

Harris ( 1 963) also found that the digestibility of corn silage was very

similar to that of the fresh crop, and Johnson and McClure ( 1968) found

the digestibility of corn silage cut at increasing stages of maturity to be

65.3 percent, 71.9 percent, and 69.8 percent, in line with the expected

changes in crop digestibility. In this experiment, voluntary intake was

closely related to silage digestibility (42.9, 58.9, and 54.0 g./kg. LW0.75,

respectively), but this has not been found with unwilted grass silages

(Harris and Raymond, 1963).

Ensilage under poor conditions can lead to depressed digestibility for instance, when there are high losses of soluble constituents in effluent,



or when overheating and caramelization occur (Watson and Nash, 1960;

Wieringa et al., 1961). A small decrease in dry matter digestibility may

also occur when forage is wilted before ensiling. Thus Harris et al. (1966)

found a mean dry matter digestibility of 63.9 percent for a number of

wilted silages (26 to 54 percent dry matter) compared with 66.3 percent

for unwilted silages from the same crops (16 to 23 percent dry matter);

Schulz and Oslage (1967) found a similar fall in digestibility with some

silages, but concluded that this did not necessarily result from wilting.

The reason for this lower digestibility is not known, but if it occurs generally it must somewhat reduce the advantage attributed to higher intake with wilted silages.

X. The Nutritive Value of Grazed Forage

Grazing, the most common method of using forages, imposes a number

of modifying factors on the basic components of nutritive value already

discussed. Thus “forage grazed” may differ markedly in composition from

“forage on offer,” because animals are able to graze selectively; the

amount of forage eaten may depend as much on extrinsic factors such as

feed availability and environment as on the intrinsic factors discussed

in Section V ; and the nutrient requirements of grazing livestock may

differ from those of housed livestock, with which all basic data on requirements have been determined.





Central to any consideration of these factors is the need for information on the nutrient intake by grazing ruminants. Yet despite much research effort in the last twenty years the techniques now available still

appear inadequate, and they limit the precision with which grazing

studies can be interpreted.

Two distinct types of technique have been used, involving, respectively, measurements on the pasture and measurements on the animal.

The former, which include “IN - OUT” pasture sampling and “browse

unit” estimates, have been fully reviewed by D. Brown ( 1 954) and J. T.

Reid (1962). They are not considered further here, except to note that

the technique of estimating forage intake as the difference between yield

samples before and after grazing appears to be reasonably precise only

when applied to “strip” grazing, where the difference between the yield

estimates is large and pasture growth during the period of grazing is


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IX. The Effects of Processing on the Components of Forage Nutritive Value

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