Effective Fiber

What is Effective Fiber:

The term “effective fiber” can be defined as the ability of a fiber source to stimulate rumination. Various metabolic disorders can result if minimum effective fiber levels are not met. At low fiber intake, rumination decreases and thus, saliva production decreases, rate of passage from the rumen slows down and rumen motility can be decreased. These factors can results in rumen acidosis which will lead to decreased feed intake, decreased dry matter digestibility, laminitis and rumen parakeratosis. Milk fat depression is also associated with insufficient rumination. The three major characteristics of a feed source that contribute to its effectiveness at stimulating rumination are:
1) chemical composition (fiber level), 2) functional specific gravity and 3) particle size.

Chemical Composition of Fiber:

The most commonly accepted and used measurements of chemical fiber in feedstuffs are either acid detergent fiber (ADF) or neutral detergent fiber (NDF) as described by Van Soest (1982). As diagramed below, ADF constituents are cellulose and lignin, whereas NDF contains the additional component of hemi-cellulose. Because it contains the least digestible components, ADF has commonly been associated with digestibility predictions for forages while NDF is more commonly associated with the “fill factor” of fiber within the rumen. While these descriptions are fairly general, they do have merit when analyzing forage analysis results and when formulating rations intended for on-farm use. 

The NRC for Dairy Cattle (1989) recommends a minimum dietary NDF level of 25%. Furthermore, it is recommended that 75% of this NDF come from forage, resulting in minimum forage NDF level of 18.75% in the ration. Recent research resulting from both in situ and in vitro analysis procedures continues to demonstrate that forage fiber has a wide range of both extent and rate of degradation within the rumen environment. As will be discussed below, digestibility differences can affect both particle size and specific gravity of forage particles that in turn impacts their effective fiber value. As a result, generalizations about minimum NDF and forage NDF values should serve only as initial guidelines for ration balancing

Differences in extent and rate of digestion can be attributed to a variety of factors including but not limited to: maturity, variety, weather conditions during growing, processing, quality of fermentation and rate of passage through the rumen. Particle size of forages has a direct impact upon rate of passage due to the impact it has upon ruminal mat formation and density of particles within the rumen.  Particle size also affects digestion of forages; reducing forage particle size will increase digestibility of the forage.

In the rumen, forage fiber degradation will depend on both the actual digestibility of the fiber component and/or the rate of passage of fiber. These factors will, in turn, impact the ability of dietary fiber to serve as effective fiber and stimulate chewing. On the farm, it is not easy to determine whether rate of digestion or rate of passage is the issue that needs to be addressed. However, recent advancements in digestibility analysis combined with on-farm tools and cow observations can be used to evaluate physical particle size to help sort out these issues.

It must also be noted that there is a difference in the fermentation dynamics of forage fiber when comparing legumes and grasses. It can be generally stated that legume fiber will have a more rapid kd when compared to grass fiber, but that the total extent of digestibility will be more complete in grasses.  This must be accounted for when formulating for total and effective fiber levels in rations as the dynamics of kd and kp relative to ruminal fermentation will be different depending upon the fiber source.

Effect of Chemical Composition on Rumination:

Rumination activity is directly related to the NDF content of a feed. The NDF component is made up of three major constituents: cellulose, hemicellulose and lignin. Of these three, cellulose and hemicellulose are potentially degradable by the rumen microorganisms. Lignin provides rigidity to the cell wall of plants and is the most limiting factor to nutritional availability and digestibility of fiber by rumen microorganisms. As plants mature, the proportion of lignin relative to the total fiber content increases. This means that the rumen bacteria are able to digest less fiber and the cow will have to spend more time ruminating feed to allow it to pass out of her rumen. 

Diets containing high fiber forages are likely to inhibit intake. In ruminants, the volume of the rumen limits feed intake.  Furthermore, a cow will only ruminate for approximately 11 hours per day. If the cow is unable to reduce the particle size of the ingested fiber sufficiently over a 24 hour period, passage out of the rumen will be slowed.  As a result, intake will be depressed until the fiber can be ruminated to a size that will pass out of the rumen. 

The protein content of a feed is also a factor to consider. The rumen available protein is degraded to ammonia, which can affect the buffering capacity of the rumen and alter rumen pH. In addition, enough protein needs to be available to maintain high microbial growth rates. If degradable protein is not available to the microbes, ruminal fiber digestion may be depressed because of decreased microbial growth. 

Effect of Specific Gravity on Rumination:

Functional specific gravity is another important characteristic of forage particles that contribute to their effectiveness. At an early growth stage, the size of the plant cell becomes fixed. As maturation occurs, the cell wall will thicken, become  more rigid and intracellular space will decrease. This thickening will cause an increase in the specific gravity of the plant cell (i.e. cell becomes more dense). The specific gravity of a forage particle can only be changed when the cell structure is collapsed. This occurs during mechanical grinding, pelleting or rumination. 

Particles in the rumen tend to be stratified in relation to their specific gravity. The lighter particles float at the top and probably get caught up in the rumen mat. The heavier particles sink straight to the bottom of the rumen and are very unlikely to be ruminated. 

There are many factors that influence the functional specific gravity of a particle. Particle size is a major factor.  As particle size of a forage decreases, its specific gravity increases. Part of the function of rumination is to reduce the particle so that it has an appropriate size and specific gravity to pass out of the rumen. This is how long hay escapes the rumen and does not remain floating at the top forever. 

Particle Size

Rumination is the main source of feed particle size reduction. Mastication during eating and microbial digestion also reduce feed particle size, but to a much lesser extent. The average particle size leaving the rumen via the reticulo-omasal orifice is less than 1mm. As the majority of the feed entering the system is much larger than that, it is easy to see the great importance of rumination.  Recently attention has been focused on optimal feed particle size for two major reasons. One, TMR mixing frequently results in particle size reduction of forages. Secondly, the smaller the particle size of a forage, the more likely it is to undergo an optimal ensiling fermentation. Another issue with forage particle size is that very fine particles will increase rate of passage through the rumen thus increasing intake, at least initially. However, this is accomplished at the expense of digestibility, since residence time and microbial exposure are shortened. A strong direct relationship between particle size and rumination time has been well documented. There is also strong evidence to indicate that particle size plays a significant role in the maintenance of rumen health. Many studies have demonstrated the relationship between reduced particle size and the increased incidence of milk fat depression. As particle size decreases, rumen pH and acetate:propionate ratios tend to decrease as well, suggesting rumen health and productivity will be compromised. The particle size at which rumen health is maintained is widely discussed but remains undiscovered. Varying research suggesting critical particle sizes of anywhere from 6.4 mm to 9 mm to between 10 and 20 mm illustrates how little is currently known. 

Impacting particle size and particle size impacts:

Because particle size is more easily manipulated than either the chemical composition or the specific gravity of forages, most recommendations and discussions about effective fiber center around particle size. Particle size of forages can be impacted by a variety of scenarios within the forage and feeding management programs. In many cases the issues that lead to improper particle size can be traced back to management flaws that can be corrected in a short time frame but in some cases may have to be dealt with long term until other forages are available for ration formulation.

            The most common particle size issues seen in the field are:

The most common reasons for undesirable particle size are:

In most cases, reduced particle size is the issue of concern rather than particles that are too long.  However, the impact and effects of either can be of a similar nature. Observations in the field show us that reduced particle size in TMR’s is a primary factor in reduced cud chewing activity, increased incidences in rumen displaced abomasums, lowered butterfat tests and decreased ruminal pH. If ruminal pH is severely compromised (<5.6) consistently it can result in being one of the primary factors leading to sub-clinical or clinical laminitis. 

The other extreme is when fiber particles are too long within a TMR and/or when offered as separately fed forage. Within a TMR the impact of long particles can many times be one that leads to particle separation and refusal of the longer particles contained in the TMR. When this scenario takes place it becomes the ultimate paradox between what has been formulated on paper and then offered to the herd versus what is truly consumed by the herd. Many herd observations have highlighted this situation where the physical form of the diet offered looks sound, but upon review and measurement of the TMR refusals it is found that long fiber particles are what is left. This results in the formulated fiber level and physical form not reaching the ultimate destination of consumption by the cow and entry into the rumen. In these cases many of the same “symptoms” of reduced particle size are experienced, as true consumption by the herd is that of mid to small particles with the long being left in feed refusals. This can be shown in many cases by doing particle size evaluation at timed intervals following the offering of the original TMR mix. If it can be demonstrated to a producer that during the course of a feeding period the remaining mix available for consumption contains a greater percentage of “long” particles than what the original mix did then particle separation can be established as a potential management problem.

A ration with extremely long particles can also have a negative impact upon potential dry matter intake (DMI) due to simple physical fill of the rumen. This will of course be impacted to a great degree by the chemical make-up of the long fiber particles. If highly digestible then physical fill will be less of an issue, but if the forage is of a lower digestibility than desired then rumen retention will be increased and total DMI will be compromised.

Measuring particle size:

Ideally, particle size of forages is measured and adjusted at harvest.  Some common guidelines for cut length at harvest are listed here:

• Haylage: 3/8” TLC (theoretical length of cut)
• Corn Silage (unprocessed): 3/8” TLC
• Corn Silage (processed): 3/4”   TLC

            note: at an equal TLC, processed CS will have a 15-25% smaller particle size than unprocessed CS. 

Keep in mind that the sharpness of the blades, ground speed of the tractor, etc. will affect the actual length of cut. As a result, many consultants and producers recommend using a particle size separator (discussed below) to directly measure the size of the forage particles at the chopper over the entire harvest season. However, many times a particle size problem is suspected after forage harvest is complete. In this case, the problem must be either confirmed or dismissed using a particle size evaluation system.

Visual observation of a TMR is at times a valid index for evaluation of the particle size of forages that are available for ingestion by a herd. A common rule of thumb is that 20% of forage particles should be greater than 1.5”.  However, visual particle size determinations are often misleading and more quantitative measurements are required. 

Mertens (1997) developed a system for evaluating the physical effectiveness of fiber (pef) that when combined with chemical NDF values gives a physically effective NDF (peNDF) value for a feedstuff. Table 2 shows the pef factors that he developed for various feedstuffs based upon their physical form.

Table 2: Physical effectiveness factors per kg of NDF from various sources and physical form classifications that indicate total chewing activity relative to long grass hay.

Classification	Grass-Hay	Grass-Silage	Corn-Silage	Alfalfa-Hay	Alfalfa-Silage	Conc-entrates	By-Products
Long	1.00			.95
Coarse chopped	.95	.95	.90	.90	.85
Medium chopped	.90	.90	.85	.85	.80
Fine chopped		.85	.80	.70	.70
Rolled HM corn						.80
Rolled barley						.70
Rolled, coarse grd						.60
Medium ground	.40			.40		.40	.40
Ground/pelleted							.30

Mertens (1997)

Based upon the above factors Mertens reported that approximately 19.7% peNDF was required to maintain a milk fat percentage of 3.4% in Holstein cows and 22.3% peNDF was required to maintain a ruminal pH of 6.0. This proposed approach by Mertens can be utilized in the development of ration formulation programs and approaches to the numerical description of “effective” fiber in rations. 

The question though still remains as to how to evaluate the “effective” fiber length within a TMR at the farm site. Current field practice for this determination is through the use of forage particle separators and/or screens.  Ruppel at Cornell was one of the first to propose this approach in evaluating chop length at harvest and has since applied it to TMR evaluation. Heinrichs and co-workers at Penn State University have further developed and refined procedures and equipment for particle length evaluation. The two most commonly used pieces of equipment are shown in Figures 1 and 2. There is continued debate within the nutrition and academic communities as to the merits or de-merits of each of these system, but regardless of the “issues” they are both tools that can be effectively used in the field for evaluation of particle size for either individual forages or TMR mixes.

The “Ruppel/Cornell” screening system utilizes three screen sizes, ½”, ¼’ and 1/8”. Field experiences by one of the authors (Holtz) and others in New York and the Northeast have shown the following guidelines to be particle size goals that result in sound rumen and overall cow health along with maximizing dry matter intake.

Table 3.

Percent remaining By weight	Corn silage unprocessed	Corn silage processed	Haylage	TMR
Top screen (1/2”)	15-20 %	20-25%	20-25%	10-16%
Middle screen (1/4”	30-40%	25-30%	30-35%	30-40%

Heinrichs and co-workers have developed the following guidelines for the Penn State separator.

Table 4.

	Corn Silage	Haylage	TMR
Upper sieve(>.75”)	2-4 % if not  sole forage 10-15% if chopped and rolled	10-15% in sealed silo   15-25%, bunker silo, low DM	6-10% or more if 3-6% must focus on TNDF and FND
Middle sieve(.75-.31”)	40-50%	30-40%	30-50%
Bottom pan(<.31”)	40-50%	40-50%	40-60%

(Heinrichs and Lammers. 1997)

Regardless of the system used, the key is to evaluate a herd on a consistent basis. Much like body condition scoring, no two individuals are going to have exactly the same results but a consistent monitoring program used on an on-going basis can effectively evaluate current herd status and any changes over time. As mentioned earlier, a part of using these tools is to evaluate the TMR refusals and to then make comparisons between the original TMR offered and that which is remaining at feed clean-out. The goal, of course, is that the “refusal TMR” has the same particle separation profile as that of the original mix. If not, this indicates that sorting of the TMR has taken place and true effective fiber intake needs to be questioned.

Another key to using these systems is to evaluate the results relative to overall feeding management in the herd. Is the feeding management such that consistent and multiple meal intake occurs over a 24 hour period or is the herd meal pattern such that cows slug feed? In general, shorter particle size can be tolerated when meals are consistent and regular, whereas slug-feeding programs will most times require a longer particle size in order to maintain proper rumen pH. 

Dado and Allen (1994) investigated the feeding behavior of cows when offered an ad lib TMR over a 24 hour period. Table 5 is a summary of their observations.

Table 5.

Variable	First calf hiefer	2nd+ lactation cows
Milk production lbs./day	63.3	82.7
DMI, lbs./day	44.1	54.7
NDF intake, lbs./day	13.7	16.8
Meal size, lbs. DM/meal	4.0	5.5
Meals/day	11.3	10.8
Eating time/meal (min.)	25.9	31.1
Eating time, min./day	284	314
Ruminating time, min./day	453	460
Total chewing time, min./day	737	774
Ruminating, min/lb.	33.6	27.6
NDF Water intake, gal/day	16.7	23.6
Drink size, quarts	5.7	7.6
Drinking times	13.0	14.9
Drinking time, min/day	17.7	19.1

   (Adapted from Dado and Allen via C.J. Sniffen et al.)

This is in contrast to data from the University of Maine (Sniffen, et al.) that showed when the TMR offering was restricted during a 24-hour period the number of meals was reduced to 5-7 per day.

Questions to ask and observations to make relative to the above data are such things as:

What are the cows saying?

The ultimate judge of particle size adequacy is the dairy herd. As mentioned previously, the reason for providing effective fiber is to maximize ruminal health and digestible DMI. Not only does maximal rumen health lead to overall cow health, but also production of both milk and milk components tend to be more efficient. Microbial protein production is an end goal in both ration formulation and feeding management. In order for ruminal microbes to thrive, grow and supply both lower tract protein needs and volatile fatty acid production for energy needs, the rumen pH needs to be maintained at approximately 6.0 (5.8-6.2 range). Grant et al. (1990) showed that when rumen pH was below 6.0 the growth of cellulolytic bacteria was significantly depressed. This of course led to an increase in propionate producing organisms and a decrease in the acetate (precursor to butterfat):propionate ratio.

The simplest observation to make within a herd is the cud-chewing activity of cows. This is a sound index relative to fiber mat and effective fiber status.  Cows to be observed for this activity should be those that are in a resting phase (either standing or laying) following a meal activity. The minimal goal is that 40% of these cows are actively and aggressively chewing. In herds where effective fiber is “ideal”, the common observation is 50-70% of cows actively and aggressively chewing. Remember that cows have a physiological drive to ruminate and will do so even with an inadequate rumen fiber mat and low effective fiber intake. As a result, simply observing an acceptable proportion of cows ruminating isn’t necessarily a sign that effective fiber intake is adequate; the cows must be ruminating vigorously as well.

Other observations that can be made within a herd relative to effective fiber status are such things as:

Rumenocentesis:

Another tool used by many in the nutrition and veterinary communities to evaluate ruminal health and factors that may be leading to undesirable pH levels is rumenocentesis. This is not a universally accepted practice, but one that the author has used for troubleshooting, and a practice that many feel is based upon sound science and does have field application. Nordlund, Oetzel and co-workers at the University of Wisconsin have been the leading researchers of this topic over the past few years. An excellent review of the logistics, evaluation and herd testing procedures for rumenocentesis is found in the Journal of Dairy Science, 1999. 82:1170-1178. Key values summarized in this review are:

Nutritional consultants and veterinarians can use rumenocentesis as a quantitative tool to show producers that rumen health is being compromised and that ration and/or feeding management changes need to be implemented. As with many consulting “tools” the use of this information can allow for a “teachable” moment with a producer and prod a producer to take action.

Exceptions:  (there are exceptions to most every issue)

The above discussions, observations and recommendations have been in the context of traditional silage and silage plus dry hay forage programs. Also, they are for the most part based upon dealing with effective fiber via the feeding of a TMR mix. There are certainly exceptions to these traditional ration formulations; particularly rations that contain high levels of fermentable fiber from by-product feed sources (i.e. beet pulp, soy hulls, citrus pulp, almond hulls, brewers grains, etc.). In contrast to the traditional rations that typically are high in starch and other rapidly fermentable NSC sources, these by-product rations ferment to less lactic acid production and thus are not as detrimental relative to rumen pH declines. 

Guidelines for “effective fiber” values for these types of rations are not as clear as those of traditional rations. Nutritionists and producers who have experience and success with fermentable fiber by-product rations still find that the inclusion of a minimum level of long “effective” fiber is necessary.  The “rule of thumb” used by feed manufactures and nutritionist in the Midwest and Northeast has been to include a minimum of 2 lbs. of palatable/digestible dry hay to these TMRs.

Figure 1. Ruppel/Cornell forage particle size screens.

Figure 2.  Penn State University forage particle size separator.

References:

Balch, C.C. 1971. Proposal to use time spent chewing as an index to the extent to which diets for ruminants possess the physical property of fibrousness characteristics of roughages. Br. J. Nutr. 36:383.

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desBordes, C.K. 1981. Influence of specific gravity on rumination and passage of indigestible particles through the gastrointestinal tract. Ph.D. Diss., Univ. Vermont, Burlington.

Erdman, R. 1988. Dietary buffering requirements of the lactating dairy cow: A review. J. Dairy Sci. 71:3246

Garrett, E.F., M.N. Pereira, K.V. Nordlund, L.E. Armentano, W.J. Goodger and G.R. Oetzel. 1999. Diagnostic methods for the detection of subacute ruminal acidosis in dairy cows. J. Dairy Sci. 82:1170.

Grant, R.J., V.F. Colenbrander and D.R. Mertens. 1990. Milk fat depression in dairy cows: role of particle size of alfalfa hay. J. Dairy Sci. 73:1823.

Grant, R.J., V.F. Colenbrander and D.R. Mertens. 1990. Milk fat depression in dairy cows: role of silage particle size. J. Dairy. Sci. 73:1834. 

Heinrichs, J. Evaluating particle size of forages and TMRs using the Penn State Particle Size Separator. Penn State DAS 96-20. University Park, PA. 

Heinrichs, A.J. and B.P. Lammers. 1997. Particle Size Recommendations for Dairy Cattle. Proceedings of NRAES-99: Silage Field to Feedbunk. p. 268

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Related Links:

Penn State Particle Size Separator