The Role of Fiber in Ruminant Ration Formulation
Kung, Jr., Ph.D.
Department of Animal & Food Sciences
University of Delaware
Newark, Delaware 19711
Several livestock species have the ability to digest fiber via
microbial fermentation and to obtain useful energy from this process.
However, in ruminants, fermentation of fiber occurs prior to the
small intestine (Figure 1). Thus,
microbial protein that is produced, as fiber is being digested, is
available for digestion to amino acids with subsequent absorption for
use by the animal. Although
horses and pigs can digest some fiber, fermentation of fiber takes place
in the hindgut which, is after the site of active protein digestion and
amino acid absorption. The
objective of this paper is to explain how fiber is digested in the
rumen, to review practical factors that affect fermentation of fiber in
the diet of ruminants, and to explain the basis for recommendations for
fiber in ruminant diets.
What is Fiber?
Fiber is the predominant fraction of the plant cell wall and
primarily comprised of carbohydrates.
The primary components of fiber are cellulose, hemicellulose, and
lignin. On a chemical basis, cellulose is comprised of linear chains
of the sugar. Starch, the
carbohydrate source in grains, is also comprised of glucose molecules.
In cellulose, glucose molecules are linked together in a b1,4
link whereas in starch, it is an a1,4
link. Only microbial
enzymes can digest the b1,4
linked glucose in cellulose (Figure 2).
Hemicellulose is also dependent on microbial enzymes for
digestion because it has a complex structure made primarily of the sugar
xylose that is also in b1,4
links. Hemicellulose is
closely associated with lignin that has a strong negative influence on
Fiber Digestion in the Rumen
rumen is an environment with a diverse population of microorganisms.
Bacteria and protozoa dominate the fermentation both in terms of
numbers and metabolic processes. Of
the bacteria present in the rumen, several general types of bacteria can
be described based on metabolic functions.
Amylolytic bacteria specialize in fermenting starch (from
concentrates) while fibrolytic bacteria ferment fiber.
Different populations of bacteria will dominate the rumen
fermentation depending on the type of diet being fed.
Cattle fed diets solely of forage with high fiber will have a
ruminal bacterial population that is high in fibrolytic bacteria.
Simplistically, the fermentation of fiber (cellulose and
hemicellulose) results in the production of acetic acid that is used by
the cow for energy and is the primary precursor of fat in milk.
In contrast, digestion of sugars and starches yields propionic
acid that is converted to glucose in the liver of the cow and used for
amount and size of fiber particles in the diets of lactating dairy cows
is important to maintaining optimal rumen function.
Long fiber in the rumen forms the rumen “mat”. The mat is where fibers are entangled because they are too
long to pass to the lower gut. Fiber
from the mat is regurgitated and chewed producing large amounts of
saliva that naturally buffers the rumen.
When large feed particles are chewed, the surface area for rumen
bacteria to attach, and then digest the feed is increased.
In order for a feed particle to pass out of the rumen and into
the lower gut, it must attain a size of about 1-mm!
Passage of particles from the rumen is important because without
digestion and passage, food would fill the rumen and depress intake.
There must also be a balance between retention time in the rumen
for microbial digestion and passage.
For example, grinding fiber to extremely small particles may
assist in passage from the rumen but ruminal digestion of that fiber
particle may actually be decreased if it passes too quickly.
The normal process of particle size reduction in the rumen leads
to increased surface area for microbial attachment and digestion.
of fiber in the rumen varies greatly and is dependent on a number of
different factors. For
example, lignin acts as intracellular cement that gives plants rigidity
but unfortunately, it also has negative effects on fermentability.
Theories such as lignin encrusting the fiber, complexing with
other nutrients, or toxic components of lignin have been proposed as
reasons for negative effects on digestion.
Lignification of the plant cell wall generally increases with
increasing plant maturity and within specific forage species, increased
lignification is associated with reduced digestion (Cherney et al.,
1993). (This relationship is not valid if comparisons are made among
different forages.) Thus,
immature forages generally have more digestible fiber than mature
forages. These facts
explain why harvesting alfalfa in the late bud or early flower stage is
more beneficial than harvesting it in the full bloom stage of maturity.
Harvesting alfalfa even earlier than in the late bud stage would
increase digestibility even more, but total dry matter yield would be
reduced. Thus, harvesting
forages at optimum stages of maturity is often a compromise between
yield and high digestibility. Latitude also has an effect on the fiber of plants.
Generally speaking, forages grown in hotter climates closer to
the equator (e.g., southern Arizona) have more lignin and are less
digestible than forages grown in temperate regions (e.g., Minnesota).
In attempts to improve forage quality, forages have been bred to
contain lowered concentrations of lignin.
Brown midrib corn silage (Oba and Allen, 1999) and sorghum silage
(Aydin et al., 1999) have been shown to have less lignin and greater
fiber digestibility than parent stock and have produced more milk when
fed to cows (Table 1).
The pH of the rumen has
profound effects on the growth of rumen microbes and the digestion that
takes place in this forestomach. A
pH of 7 is considered neutral, where the amount of acid and base are
equal. When pH falls below
7, we consider the medium to be acidic in nature.
The lower the pH falls below 7, the more acidic it is. A number of different factors can affect ruminal pH.
Lack of sufficient fiber or fiber that is chopped too finely,
reduces chewing times and thus, reduces saliva production causing a
decrease in ruminal pH. Cows
fed a diet with long fiber particles chew for more than 10 hours,
ruminate for about 6 hours, and can produce as much as 50 gallons of
saliva per day. The ratio
of forage:concentrate in the diet of lactating cows also affects ruminal
pH. As concentrates
increase in the diet, total acid production in the rumen increases,
causing a decrease in pH (Figure 3). In contrast, feeding buffers, especially in corn silage-based
diets, can increase ruminal pH. The
type of concentrate can also affect ruminal pH as the starch in barley
is more readily fermented to acids in the rumen than the starch from
corn. Besides requiring
carbon skeletons, ammonia-N and energy for growth, fibrolytic bacteria
in the rumen grow best when the pH of the rumen is between 6.2 and 6.8. If rumen pH falls below 6.0-6.2, fiber digestion in the rumen
begins to decline. As rumen
pH decreases, fibrolytic bacteria in the rumen become less active and
fiber digestion is decreased. When
ruminal pH falls below 5.8-5.9, the rumen is mildly acidic and fiber
digestion in the rumen ceases completely. When ruminal pH drops below
5.2 to 5.5, animals can succumb to acidosis.
Common signs of acidosis include laminitis (cows with swollen
feet and hocks), low milk fat tests (< 3.1% fat), cycling intakes, a
high incidence of displaced abomasum, loose manure, reduced cud chewing,
and excess consumption of free choice buffers.
Because the digestion of fiber in the rumen leads to acetic acid
production, and because acetic acid is a major precursor for milk fat,
reduction in ruminal fiber digestion leads to a decrease in milk fat
test (Figure 4). Thus, any
factor that helps to maintain ruminal pH above 6.0 is generally
beneficial. For example,
feeding a TMR rather than “slug feeding” grain in the parlor twice a
day, helps to moderate decreases in ruminal pH that occur after feeding.
Fiber Requirements for Dairy Cows
Calves consuming milk replacer and starter have no minimum fiber
consumption and fermentation of grain in the rumen stimulates microbial
development and absorptive functions of the rumen.
Bulk from fiber may help to develop rumen musculature, however
preweaned calves offered bulky and fibrous feeds will have reduced
intake of starter that will delay weaning, thus hay is often restricted
until after weaning.
calves. Once weaned,
growing dairy heifers also have no minimum requirement for fiber if
rumen function is maintained, but diets high in fiber are fed because
they are more economical as long as other nutrient requirements are met.
lactating cows. From a
practical standpoint, only dry and lactating cows have true minimum
requirements for fiber in their diets.
The reason for this finding is primarily due to the high total
dry matter intakes that are found in lactating dairy cows producing
copious quantities of milk. Feedlot steers can be adapted to 100% grain diets but
lactating cows cannot because their total dry matter intake is too high
and consumption of high grain diets lead to acidosis.
Feedlot steers, even on full feed, consume only about 11-13 kg of
dry matter whereas lactating cows can consume +25 kg of dry matter/day.
fiber is an ingredient in diets that is not well defined.
The National Research Council (1989) recommends a minimum dietary
ADF content of 19 to 21% and NDF content of 25 to 28% (dry matter basis)
for lactating cows (NRC, 1989). However,
these fiber requirements do not account for the form of fiber, particle
length, rate or extent of digestibility, or interactions of fiber with
other dietary components. To
account for this lack of knowledge, the NRC suggested that 75% of the
dietary fiber should be from traditional forage sources (e.g. corn
silage, alfalfa or grass hay or silage).
Forage NDF should equal 1.2% of BW. The assumption is that
traditional forage sources should consist of large particles that would
encourage chewing. However,
many nonforage fiber feeds have high fiber contents but because particle
size is small, they are not chewed well, resulting in low production of
saliva. Having 75% of the
fiber requirement in a diet from traditional forage fiber does not
always insure that cows will chew and produce sufficient saliva for good
rumen function because the length of chop may be highly variable.
Corn silage is often chopped too finely at harvest.
Heinrichs et al. (1999) and Rippel et al. (1997) have reported
that forage particle size can also be decreased to undesirable levels
during over-mixing of TMR. Removal
of frozen silage from stave silos can also reduce particle size.
In certain instances, TMR wagons with built in face-shearing
equipment (mainly seen in some places in Europe) also reduce forage
chemical fiber (what is analyzed for and usually balanced for) does not
take into account particle size, the term “effective fiber” has been
used to better define the fiber requirements of dairy cows.
Effective fiber stimulates rumination, chewing, and saliva
production. It also
maintains a normal fat test, normal rumen pH, and normal rumen mat. Balancing diets for effective fiber becomes more important as
cows increase their productive levels and therefore require more
concentrate and less forage in their diets.
Adequate effective fiber may also be an issue in diets that are
high in by-product feeds. Feeds
with small particles usually are low in effective fiber.
Values of effective value for some feeds are shown in Table 2.
As can been seen, feeds with high concentrations of NDF do not
necessarily have a high effective fiber rating.
For example, imagine a stem of alfalfa hay pulled from a bale
that may be 6-8 inches long compared to a piece of soyhull that is
perhaps 1/4 inch or less long. Which feed would stimulate more chewing and saliva
A summary from 32 experiments showed that forage particle length
had no relation to chewing time (Allen, 1997).
At first thought, this finding does not agree with previous
studies that showed reduction in chewing time and salivary flow with
forages of smaller particle size. However,
this finding is explainable because feedstuffs vary considerably in rate
and extent of digestibility thus, confounding the relationship. In contrast, particle size is related to chewing time within
experiments. From a
practical view, this finding is important because in diets where no or
small amounts of by-products are fed, a common fault in effective fiber
is the fact that the primary forage sources have been over-processed.
guidelines have been proposed to maintain adequate effective fiber in
the diets for lactating dairy cows.
Corn silage and alfalfa silage should be chopped at a theoretical
3/8 inch. For kernel
processed corn silage, the theoretical chop length can be increased to
¾ inch. Some guidelines for alfalfa based forage diets is shown in
Table 3. Several methods
exist to quantitatively measure effective fiber in dairy cattle diets.
Most of these methods are based on separation of particles by
size. Recently, Oetzel (1996) compared four particle size separators
and, in general, they gave similar results.
The Pennsylvania State Separator is commonly used in the
Northeast and consists of three boxes with several screens.
After shaking a measured sample, forage is separated into 3
fractions: top screen, middle screen and bottom box.
Suggestions for distribution of samples are shown in Table 4.
Some commercial labs offer particle size determinations as an
option in their forage analyses. Some
county extension agents and nutritionists have separators that can be
used. I believe that the most important time for use of a particle
size separator is before harvesting of forage for silage in order that
proper settings can be made for chopping.
Next, separator boxes are useful for assessing what cows are
actually receiving at the feed bunk.
For consulting nutritionists and extension agents, the separators
are good teaching tools when used on farm.
What can be done if the effective fiber content of your diet is
too low and you are stuck with silage that is chopped too finely?
First, evaluate the ration ADF and NDF content.
Use thumb rules to estimate appropriate forage:concentrate ratios
and NDF from “forage sources”.
Determine the amount of nonforage fiber in the diet and assess
its contribution to over all NDF. If your balancing program allows you
to, calculate an effective fiber value and check for adequacy. Next,
determine if the cause of low effective fiber can be easily remedied.
For example, is TMR mixing time too long? Perhaps increasing the
chop length of processed hay in the TMR will be sufficient.
If there is no quick fix for the problem (for example all the
silage was chopped too fine at harvest, replace finely chopped silage
with some long dry hay. Three to five pounds may do the trick but if the
diet is extremely fine, 8 to 10 pounds may be needed.
Sodium bicarbonate also will help to buffer the rumen.
There is also some evidence that a small amount of long straw (3
to 5 pounds) may help because of its coarse nature but, this has not
been adequately documented in controlled studies.
attention to fiber levels should be given when balancing diets that
contain a large proportion of byproduct or sources because they are
often low in effective fiber. Firkins
(1995) suggested three reasons why this may be so.
First, although the potentially fermentable fiber content of many
of these feeds is high, the rate of fiber digestion is slower than for
most traditional forages. Secondly, the small particle size of many of
these feeds and high- density results in a fast rate of passage, thus
decreasing the time spent in the rumen. Thirdly, replacing traditional
forage sources with fiber from nonforage sources may have further
negative associative effects with other feeds.
For example, soyhulls have a high fiber content but their
effective fiber content is low and thus cannot be used to replace large
portions of dietary forage (Grant, 1997).
Weidner and Grant (1994b) replaced silages with 25% of soyhulls
in a dairy diet and found that soyhulls decreased the dietary particle
size by 33%, reduced ruminating time by half and altered consistency of
the ruminal mat. In another
study, Weidner and Grant (1994a) substituted soyhulls at 15 and 25% for
forage (alfalfa and corn silage 1:1) which comprised 60% of the control
TMR DM for cows in early and mid-lactation. They found soyhull
substitutions reduced cow performance unless coarsely chopped hay was
added to the higher soyhull diet. From this study they concluded that
when high quality forage is limited, the percentage of dietary NDF from
forage can be successfully reduced to 45% with the inclusion of 25%
soyhulls and 20% coarsely chopped alfalfa hay in the diet for lactating
dairy cows. The maximum
rate of inclusion of soyhulls for cows in mid to late lactation is about
20 to 25% of total ration DM. Cows
in early lactation (< 28 DIM) should probably not be fed soyhulls
the many other nonforage fiber sources available for use in ruminant
diets, most have effective fiber values that are less than 40 to 50% of
their total NDF content (Stern and Zirmer, 1993).
The one major exception to this is that whole cottonseed appears
to be relatively good source of effective fiber. Armentano and Clark
(1992) reported that the NDF in whole cottonseed was equivalent to 1.23
times the NDF in alfalfa. Firkins
(1995) reported an effective fiber value for whole cottonseed as 85% (%
of NDF). When using thumb
rules for ration balancing, Shaver and Howard (1991) suggested that
minimum NDF from forage in a diet could actually be decreased if whole
cottonseed was included in the formulations (Table 5).
Fiber Be Too Long?
fiber be too long? From
today's point of view, we seldom think of this question because the
industry has moved to TMR that requires forage processing.
However, if forage is stored as silage, forage particles that are
too long will hamper the exclusion of air from the forage mass, cause
excessive heating during storage, and result in heat damaged protein.
Thus, in this instance, forage particle length can be too long.
Interestingly, Beauchemin et al. (1994) reported that when silage
particle length was increased in a diet that had adequate fiber from
forage, fat content increased at the expense of milk production
suggesting that particle length could be too long in some instances.
Forage particle length may also be considered too long if it
allows sorting of selective pieces of the forage.
dairy cows have obligate requirements for fiber in order to maintain
normal rumination, chewing and saliva production, and normal ruminal
function. In diets for high producing cows, the amount of fiber in the
diet tends to decline as energy density increases.
Rumen acidosis often occurs when there is insufficient amounts of
total fiber or effective fiber in the diet.
Guidelines and particle size separator boxes are available for
use when balancing for effective fiber in diets.
Allen, M. 1997.
Relationship between fermentation acid production in the rumen
and the requirement for physically effective fiber.
J. Dairy Sci. 80:1447-1462.
Armentano, L., and P. Clark. 1992. How to
stretch your forage supply. Hoard’s
Dairyman. July. p. 494.
Aydin, G., R. J. Grant, and J. O’Rear.
1999. Brown midrib
sorghum in diets for lactating dairy cows.
J. Dairy Sci. 82:2127-2135.
Beauchemin, K. A., B. I. Farr, L. M. Rode, and G.
B. Schaalje. 1994.
Effects of alfalfa silage chop length and supplementary long hay
on chewing and milk production of dairy cows.
J. Dairy Sci. 77:1326-1339.
Cherney, D.J.R., J. H. Cherney, and R. F. Lucey.
1993. In vitro
digestion kinetics and quality of perennial grasses as influences by
forage maturity. J. Dairy
Firkins, J.L. 1995. Fiber value of alternative
feeds. In Proc. 2nd Annual
Alternative Feeds Symp. USDA, Allied Industries, and the University of
Missouri-Columbia Extension Service. St. Louis, MO.
Grant, R.J. 1997. Interactions among forages and
nonforage fiber sources. J. Dairy Sci. 80:1438-1446.
Heinrichs, A. J.
1996. There are new
ways to check silage particle size.
Hoard's Dairymen. June.
Heinrichs, A. J., D. R. Buckmaster, and B. P.
Processing, mixing, and particle size reduction of forages for
dairy cattle. J. Dairy Sci.
NRC. 1989. Nutrient Requirements of Dairy Cattle. National Academy Press.
Oba, M., and M. S. Allen.
1999. Effects of
brown midrib 3 mutation in corn silage on dry matter intake and
productivity of high yielding dairy cows.
J. Dairy Sci. 82:135-142.
Oetzel, G. R., 1996.
Application of forage particle length determination in dairy
practice. Wis. Vet. Med.
Assoc. Proc. Madison, WI.
Rippel, C. E. Jordan, and S. Stokes.
particle size in Texas TMR. In
Proc. Mid South Ruminant Nutrition Conference.
Shaver, R. 1993.
Troubleshooting problems with carbohydrates in dairy rations.
Vet. Med. Oct. 1001-1008.
Shaver, R., and W. T. Howard. 1991. Troubleshooting
TMRs. University of
Stern, M. D., and C.
J. Zirmer. 1993. Consider value, cost when selecting nonforage fiber.
Weidner, S. J., and R.
J. Grant. 1994a. Soyhulls
as a replacement for forage fiber in diets for lactating dairy cows. J.
Dairy Sci. 77:513.
Weidner, S. J., and R. J. Grant. 1994b.
Altered ruminal mat consistency by high percentages of soybean
hulls fed to lactating cows. J. Dairy Sci. 77:522.
Effects of feeding low lignin (Brown midrib) forages to dairy
ADF digestion, %2
ADF digestion, %
and Allen, 1999.
of the total mixed ration.
et al., 1999.
2. Comparison of fiber content and digestion and energy density in
of fiber digestion, %/hr
Soyhulls, fine ground
from Firkins, 1995.
Table 3. Recommendations
for particle size of alfalfa silages.
Forage dry matter from:
of particles > 1.5 inches long
50% long hay + 50% silage
25% long hay + 75% silage
Recommended particle-length goals for the Pennsylvania State
left on screen
(with other forage)
(if only forage)
(with more ADF,NDF)
(chopped or rolled)*
particle length sometimes results with kernel processing because
some knives are removed from choppers.
feeding strategy and adequate forage and total NDF levels you can
get by with less “TMR” particles on the top screen.
Heinrichs, A. J.
Minimum amounts of NDF in the diet for lactating dairy cows based
on type of forage fed with or without whole cottonseed.
haylage based forage program
than 50% of forage as corn silage
NDF from forage (DM basis)
from Shaver and Howard, 1991.
Figure 1. A
comparison of the digestive tracts of various animals.
(AA = amino acids).
è Sm. Intestine è Colonic è
Microbial fermentation Ô
Intestinal digestion Ô
Figure 2. Digestion
of cellulose and starch by ruminants. G = glucose.
Figure 3. Effect of
increasing forage:concentrate ratio on ruminal pH.
The relationship between ruminal pH and milk fat content.
Adapted from Allen (1997).