1999 Colorado State University Dairy Nutritional Conference
Particle Sizing: A Tool for Evaluating
Forages and Total Mixed Rations
JERRY D. OLSON, D.V.M., MS
Diplomate, ACT
E-mail: Olson013@tc.umn.edu
Department of Clinical and Population Science
College of Veterinary Medicine
University of Minnesota
St. Paul, MN 55108

1. Cows have a minimum dietary requirement for fiber. That requirement can best be measured as the neutral detergent fiber (NDF) content of the ration. The effectiveness of the NDF in maintaining normal rumen function is dependent upon the size of the forage particles. A minimum portion of the forage particles need to be large enough to stimulate adequate chewing activity. Sieves can be used on-farm to characterize and monitor the distribution of forage particles by size in total mixed rations (TMR) and in individual forages used in a TMR to physical effectiveness of the fiber.

2. Sieves can be used to characterize the distribution of forage particle by size in a TMR and evaluate their potential role when problems of low milk production, low milk fat test, and disease problems associated with ruminal acidosis are seen in a herd.

3. Sieves can be used to identify problems associated with cows sorting out and avoiding forages from TMR's resulting in low milk production, low milk fat test, and ruminal acidosis.

4. Sieves can be used to characterize and identify problems associated with characteristics various particles in a TMR. The characteristics of forage particles on the top sieve may identify problems with inadequate forage processing leading to excessive feed sorting by cows. The maturity and proportion of unbroken corn kernels on the middle sieve may explain poor performance of corn silage.

5. Sieves can be used to the uniformity of the mixing process in a TMR by a mixer.

Introduction
Nutritionists frequently refer to the three rations that exist on dairies: 1) The ration formulated on paper; 2) The ration offered to the cow; 3) The ration eaten by the cow. When there are large differences between the ration formulated on paper and the ration eaten by the cow, milk and milk fat production are frequently less than expected and health problems in the herd often increase. I am amazed at the frequency with which I see large differences between the ration formulated for the cows and the ration which the cows actually eat. One of the major reasons for the large differences between these two rations is the result of cows sorting out hay and silage particles from the total mixed ration (TMR).

The 1989 NRC, Nutrient Requirements of Dairy Cattle, provides recommendations for the minimum fiber requirements of the cow measured as neutral detergent fiber (NDF). To ensure adequate fiber intake, the minimum recommendation is 25 to 28% dietary NDF with 75% of it supplied from forage. Therefore, the minimum recommendation for forage NDF on a DM basis is about 19% (25% x 75% = 18.75%) of the ration. The ration eaten by the cows when forage has be sorted out may be deficient in forage NDF.. The effectiveness of NDF in maintaining milk fat and rumen health is dependent of the ability of the forage particles to stimulate chewing activity and stabilize rumen pH. Rations that are formulated with adequate "chemical fiber" or NDF may not provide adequate amounts of physically effective fiber because the forage particles are too small to provided adequate effective "physical fiber" to stimulate adequate chewing activity to maintain milk fat production and optimum rumen fermentation. Recently sieves for particle-sizing have become available as a tool which can be used on the dairy by nutritionists, veterinarians and dairy producers to objectively measure feed particle distribution in the TMR on the dairy.

Sieves for particle-sizing can be used in several ways to evaluate particle-size distribution in TMR's:

1) Sieves can be used to evaluate the distribution of feed particles by size in TMR's. Forages usually need to be processed in some manner so they can be used in TMR's. How forages are processed and the degree to which they are processed affect the mean size and distribution of the sizes of forage particles. Some factors which affect size and distribution of particle lengths include the settings for chop length on the forage chopper for forages fed as silage. The size of the openings in the screens of hay grinders affect particle size of ground hay. Sieves can be used as a tool to determine if there is an adequate proportion of larger forage particles in the TMR.

2) Sieves can be used to detect feed sorting by cows in a TMR. The sieves can be used to measure the distribution of particles by size in TMR's when freshly delivered to the feed bunk and in the TMR that has been left in the feedbunk several hours after the cows have picked and sorted through the TMR. If significant sorting is occurring, there will be a marked increase in the proportion of larger particles left on the top sieve of the particle-sizer.

3) Sieves can be used to evaluate the uniformity of the mixing the TMR. Numerous factors affect the ability of a TMR mixer to uniformly mix and deliver the components of a TMR. By taking samples of TMR at various points in the feedbunk as the TMR is delivered, the uniformity of the TMR can be accessed and monitored. One of the more common problems contributing to poorly mixed TMRs is over filling the TMR mixer. This particular occurs problem when mixers are over-filled with dry hay which tends to "float" above the mixing augers.

4) Sieves can be used to characterize the material on the top and middle sieve. Frequently when the material on the top sieve is examined, a large portion of the material is characterized by pieces of corn cobs and coarse stems of hay. Much of this material ends up as material cows have sorted out of the TMR and will become material cleaned out of the bunk and discarded. Much of this material would have been more likely eaten if the particles were smaller and more difficult for the cows to sort from TMR. Whole corn kernels from corn silage usually accumulate on the middle sieve. If the majority of corn kernels on the middle sieve are mature and unbroken, a large portion of the kernels may pass through the cow undigested. The decreased production of cows fed corn silage with mature, unbroken corn kernels is readily identified by examining the characteristics of the kernels on the middle sieve and the amount corn kernels found in the manure.

Signs of Inadequate Effective Fiber in Ration

The dairy cow has a fiber requirement which can be measured by the NDF content of the total ration. The symptoms of inadequate effective fiber in a TMR can result when either there is an absolute deficiency of "chemical fiber" or NDF in the ration or when there is adequate "chemical fiber" but there is inadequate quantity of larger forage particles to stimulate adequate chewing activity. Not only does the cow need an adequate level of NDF in the ration but the NDF needs to be in a form that will stimulate adequate chewing. Effective fiber refers to the ability of the fiber in a feed to maintain an adequate milk fat test. Physical effective physical fiber refers to the ability of fiber in a feed stimulate chewing acitivity to maintain normal milk fat test. It becomes apparent that some rations can have adequate chemical "fiber" but are unable to maintain normal milk fat test because they lack effective physical fiber. Neutral detergent fiber (NDF) is the appropriate measure of "chemical fiber."
The symptoms of inadequate effective fiber in rations of dairy cattle include:
1) Low milk fat test.
2) Decreased milk yield.
3) Erratic dry matter intake (DMI).
a. Erratic DMI-large variations from day to day
b. Excessively high DMI.
c. Low DMI when ruminal acidosis is a component of the problem.
4) Health problems.
a. Ruminal acidosis/Laminitis
b. Displaced abomasums
c. Ketosis
The effectiveness of NDF in maintaining milk fat production and optimizing ruminal fermentation are based on the particle size and inherent characteristics of NDF that affect chewing activity, ruminal pH, and milk fat production. The amount of forage, NDF content of the TMR, and size of forage particles in the ration affect the amount of total time cows spend chewing when the ration is initially eaten and later when ruminating on the feed. When the amount of forage in the ration is held constant, the grind of the forage affects the amount of time the cows spend chewing (Table 1). As the grind of the hay in the TMR is changed from a coarse to fine grind, the amount of time spent chewing by the cows decreases.

The consequence of reduced chewing time is the cow produces less saliva to buffer rumen fermentation. Since saliva is an important rumen buffer, cows that spend less time chewing, produce less saliva, and have a more poorly buffered rumen and are at greater risk of developing ruminal acidosis. When the rumen pH becomes more acid, the growth of cellulolytic organisms, the bacteria which digest fiber, are depressed. The primary product of bacterial digestion of fiber is acetate which is used by the cow to make milk fat. With depression of fiber digesting bacteria in the rumen or an inadequate amount of fiber in the diet, depressed milk fat test can occur. If rumen pH falls below 5.5, acid production in the rumen shifts to lactic acid and a ruminal lactic acidosis can develop. The severity of the ruminal lactic acidosis can range from an acute clinical life-threatening acidosis to a chronic, subacute ruminal acidosis. The type of health problems that develop depend upon the severity of the ruminal acidosis. Cows that develop a subacute ruminal acidosis may become lame as a consequence of laminitis, sole abscesses, and/or sole ulcers subsequent to the acidosis. The effectives fiber may also affect DMI. For cows with mild ruminal lactic acidosis, DMI may be erratic. As DMI increases, the rumen may reach a point where a ruminal acidosis develops. At this point DMI decreases and the ruminal acidosis self corrects and the cycle is repeated. Cows with slightly less effective fiber may be in chronic subacute ruminal acidosis and have depressed DMI.

Not all forage NDF's are created equal with respect to the ability to stimulate chewing activity. . Merten, 1997, proposed creation a system for comparing the NDF of various forages by comparing the physical effectiveness of the NDF in a test forage. The proposed test forage would be compared to the physical effectiveness of NDF in grass hay. Table 3 summarizes the physical effectiveness factors (pef) for some common forages. The most interesting observation is that the pef for finely chopped grass hay, grass silage and corn silage is .85 to .80 compared to the pef of alfalfa hay and silage which was .70. Grasses typically have NDF's 20 percentage points higher than their ADF's, compared to a 10 percentage point difference for alfalfa. For grasses and alfalfas with similar values for ADF, the grasses obtain a distinct advantage in contributing more total pef compared to alfalfa. For example, the total pef NDF for a finely chopped corn silage with an NDF of 50% would be 40% (0.80 x 50% = 40%). In comparison, the total pef NDF of a finely chopped alfalfa hay with an NDF of 40% would be 28% (.70 x 40%=28%). The example points the importance of having a coarse to medium chop for alfalfa to maintain pef in the ration. The example also points out how little penalty there is reducing the pef of corn silage by finely chopping it.

Guidelines for Particle Size in TMR's and Forages

Recently separators have been developed for on-farm use to determine particle size distribution in TMR's and forages. Some commercial analytical laboratories offer particle size separation analysis as part of their services (Dairyland Laboratories, Arcadia, WI). The on-farm separators consist of a series of stacked screens or sieves designed to characterize and standardize the distribution of particles in individual forages and TMR's. A particle separator commercially available for on-farm use is the Nasco Forage Particle Separator (C19524N; Fort Atkinson, WI). A particle separator can also be built in the home shop by drilling 3/4" holes in a 14" X 14" sheet of material for the top sieve and 5/16" holes in a 16" X 16" sheet of material for the middle sieve. The top sieve which retains material greater then 3/4" in length, identifies the larger forage particles that contribute to the fiber mat in the rumen and stimulate cud chewing and saliva production. The material that is less then 5/16" in size represents material that is rapidly digested and/or may be removed from the rumen in the fluid out flow. Approximately a half pound or 250 grams of grab sample of the forage or TMR are placed on the top sieve of the particle separator. The separator is shaken back and forth in an arms-length shake for 10 times horizontally in one direction and then rotated one-quarter turn. This is repeated 3 more times until the separator has made a complete rotation. To much material on the top sieve can interfere with the ability to effectively sieve material. The portion of the sample remaining on each sieve can be weighed with a digital scale (Sunbeam Digital Deluxe Scale, Nasco, WA12267N, Ft. Atkinson, WI). The percent of sample on each is screen calculated. The guidelines for TMR and forage particle distribution are presented in Table 2.

Factors Affecting Forage Particle Size and Particle Size in a TMR
Numerous factors affect the length and size distribution of forage particles in a TMR beginning with the harvesting and processing of forages. The selection of chop length for ensiled forages is affected by type of storage structure (bunkers vs vertical silos) , moisture content of the crop and type of forage. Dry hay has always presented challenges for controlling length of particle size in a TMR. If dry hay is ground before it is added to the TMR, the size of screen openings in tub grinders, moisture content of the hay, and hay quality interact to affect the size of ground hay particle sizes. The size of forage particles in TMR is affected by the type of TMR mixer and various hay processing packages that are available as add-ons to the mixer, the moisture content of the forage and other ration ingredients, the sequence in which ingredients are added to the TMR, the total mixing time of the batch, and size of the batch.

Table 1. Effect of forage particle size on rumen function and milk production.

Grant, et al., 1990. JDS 73:1823
1 Represents the grind of hay. Fine = hay groud through a .24" screen; Medium = 1:1 mix of fine and coarse ground hay; Coarse = hay ground through a 3" screen.
a, b, c Means in the same row with different superscripts differ (P<.05).

Table 2. Guidelines for TMR and forage particle sizes.

Table 3. Physical effectiveness factors for NDF based on chewing activity

1Physical effective fiber
Adapted Mertens, 1997, JDS 80:1463

Selected bibliography:
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
Heinrichs, J. 1997. Usingfeed particle size in ration formulation. Tri-State Dairy Nutrition Conf. May 1997. pp 137-143.
Lammers, B.P., D.R. Buckmaster, and A.J. Heinrichs. 1996. A simple method for the analysis of partilce sizes of forage and total mixed rations. J Dairy Sci 79:922-928
Mertens, D.R., 1997. Creating a system for meeting the fiber requirements of dairy cows. J Dairy Sci. 80:1463
Rippel, C., E. Jordan, and S. Stokes. 1997. Evaluating particle size in Texas TMRs. 1997 Mid-South Ruminant Nutrition Conference. pp 20-30.
Varga, G.A., 1997. Fiber in the ration: How effective should it be? Proc.1997 Cornell Nut Conf, pp 117-128.

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