The Role of Fiber in Ruminant Ration Formulation

 Limin Kung, Jr., Ph.D.
Department of Animal & Food Sciences
University of Delaware
Newark, Delaware 19711 

Introduction           

            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. 

Fiber Digestion in the Rumen 

The 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 energy. 

The 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.           

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.  Calves consuming milk replacer and starter have no minimum fiber requirement.  Instead, 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. 

Weaned 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. 

Dry and 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. 

Unfortunately, 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 particle size. 

Effective Fiber 

Because 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 production?    

            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.  

General 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.  

Nonforage Fiber Sources 

Special 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 (Grant 1997). 

            Of 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). 

Can Fiber Be Too Long? 

Can 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. 

Conclusions 

            Lactating 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. 

References 

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 Sci. 76:790-797. 

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.  pp 221-231. 

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. p. 438. 

Heinrichs, A. J., D. R. Buckmaster, and B. P. Lammers.  1999.  Processing, mixing, and particle size reduction of forages for dairy cattle.  J. Dairy Sci. 77:180-186. 

NRC. 1989. Nutrient Requirements of Dairy Cattle. National Academy Press. Washington D.C. 

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.  p. 153. 

Rippel, C. E. Jordan, and S. Stokes.  1998.  Evaluating particle size in Texas TMR.  In Proc. Mid South Ruminant Nutrition Conference.  Pp. 20-30. 

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 Wisconsin Handout. 

Stern, M. D., and C. J. Zirmer. 1993. Consider value, cost when selecting nonforage fiber. Feedstuffs. 65(2):11. 

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.

Table 1.  Effects of feeding low lignin (Brown midrib) forages to dairy cows. 

Item	Control	Brown midrib
Corn silage1       ADF, %       Lignin, %       ADF digestion, %2 DM intake, kg/d Milk, kg/d	21.2 2.5 31.8 23.5 38.9	19.9 1.7 34.9 25.6 41.7
Sorghum silage3       ADF, %       Lignin, %       ADF digestion, % DM intake, kg/d Milk, kg/d	34.7 9.5 38.8 21.5 21.5	36.5 7.5 42.8 22.7 24.3

¹Oba and Allen, 1999.

²Digestion of the total mixed ration.

³Aydin et al., 1999.

Table 2. Comparison of fiber content and digestion and energy density in feeds.

 

Feed	NDF, %	Effective NDF, % of NDF	Rate of fiber digestion, %/hr	NEL, Mcal/kg
Alfalfa hay	44	95	.052 - .165	1.32
Corn silage	45	95	.029 - .082	1.69
Corn cobs	89	40-80	---	1.91
Oat hulls	90	80	.035 - .043	1.98
Cottonseed hulls	78	60	---	1.94
Wheat middlings	37	50	.042 - .144	1.56
Soyhulls, fine ground	67	20	.011 - .077	1.94

Adapted from Firkins, 1995.

 

 

 

 

 

Table 3.  Recommendations for particle size of alfalfa silages.

 

Forage dry matter from:	Theoretical length of cut	Percent of particles > 1.5 inches long
50% long hay + 50% silage	3/16 inch	< 7
25% long hay + 75% silage	¼ inch	7-10
All silage	5/16-3/8 inch	15-20

Shaver, 1993.

Table 4.  Recommended particle-length goals for the Pennsylvania State Separator.

 

Corn Silage                            Hay Silage                    TMR
	percent left on screen
Top screen	2-4 (with other forage)	8-12 (bottom unloader)	6-10 (minimum)
	5-10 (if only forage)	10-15 (top unloader)	3-6** (with more ADF,NDF)
	10-15 (chopped or rolled)*	15-25 (bunker,bag)
Middle screen	40-50	30-40	30-50
Bottom pan	40-50	40-50	40-60
*Large particle length sometimes results with kernel processing because some knives are removed from choppers. **With careful feeding strategy and adequate forage and total NDF levels you can get by with less “TMR” particles on the top screen.

Heinrichs, A. J.  1996.

Table 5.  Minimum amounts of NDF in the diet for lactating dairy cows based on type of forage fed with or without whole cottonseed.

 

Item	Alfalfa haylage based forage program	More than 50% of forage as corn silage
	% NDF from forage (DM basis)
Without whole cottonseed	21	23
With whole cottonseed	19	21

Adapted from Shaver and Howard, 1991.

 

Figure 1.  A comparison of the digestive tracts of various animals.  (AA = amino acids).

 

 

Diet èGastric è Sm. Intestine è Colonic è Feces                          Dog, human

                              (AA absorption)                            

 

 

Diet èGastric èSm. Intestine è Colonic èCecal è Feces              Horse, pig

                              (AA absorption)                  (fermentation)

 

 

Diet èRumen/Reticulum/Omasum

(fermentation)

      ê

Gastric èSm. Intestine  èColonic èCecal èFeces  Ruminant

                                       (AA absorption)                   (fermentation)

 

b1,4 link     ¯

a1,4 link      ¯

Microbial fermentation Ô       Acetate Ô           Fat synthesis                                                                                                     Energy                Microbial fermentation Ô       Propionate Ô      Energy                                                                                                                          (Glucose)                   or Intestinal digestion Ô             Glucose Ô          Energy

G---G---G---G---G---G---G  Ô Cellulose               G---G---G---G---G---G---G    Ô Starch

Figure 2.  Digestion of cellulose and starch by ruminants. G = glucose.

 

              Feed                                              Digestion                               End product       Fate                                                

 

 

 

Figure 3.  Effect of increasing forage:concentrate ratio on ruminal pH.

Figure   4.  The relationship between ruminal pH and milk fat content.  Adapted from Allen (1997).