Particle Size and Ration Uniformity:

Is it Important to the Cow?

Sandra R. Stokes

Texas A&M University, Route 2, Box 1,
Stephenville, TX, 76401, U.S.A.

Take Home Messages


With the genetic potential of today's dairy animals, early lactation rations require high levels of energy for peak performance. Formulating diets to contain adequate energy for high milk production often results in diets with high levels of grain; combine this with the lower intakes of early lactation cows, and there is little room left in the diet for fiber. Furthermore as diets may include processed forages and a range of by-products the physical nature of the fiber may be altered, thus leaving it's ability to stimulate rumination and saliva flow reduced. The effects of inadequate fiber in lactation rations can be noticed through erratic dry matter intakes, decreased milk yields, lowered milk fat production, and health problems (laminitis, ketosis, displaced abomasums). The importance of adequate fiber in the ration to maintain rumen health is typically recognized by the dairy producer. Subacute acidosis can cause significant losses to the dairy producer (lowered production, health problems, higher culling rates) and effects may be felt long-term. Laminitis is acknowledged as the primary contributor to lameness in dairy cattle and can cost the dairy producer as much as $627/case in delayed reproduction, body weight loss, and decreased milk production (12). The incidence of laminitis in confinement operations is thought to average 35% and while the causes of laminitis are several, lactic acidosis appears the primary culprit (personal communication, J.K. Shearer). The potential for these losses has aroused the feed industry's interest in developing an on-farm assessment of ration effectiveness in sustaining high levels of performance while maintaining good rumen health. There is a rising interest in particle size evaluation of dairy rations to assess effective particle length. Particle size evaluation is an attempt to identify the proportions of the ration that are rapidly digestible, moderately digestible, and effective in stimulating cud chewing and buffer production.

It is difficult to think about particle size distribution without keeping mix uniformity in mind, as the ultimate mixing goal is to have a uniform ration with as little particle destruction as possible. With the diversity of feedstuffs and mixers available in commercial dairying today, defining the process of mixing on a specific operation that will achieve this goal is more of an art than science. The function of the mixer is to uniformly distribute ration ingredients into a final product that will serve the intended purpose. In dairy rations, the final "intended purpose" is a combination of several measures; evaluation of ration performance may include animal measures such as level of milk production, milk composition (butterfat and protein content), rumen function, and general herd health. Many times, these factors may work against each other, i.e., in the case of milk production and rumen function in the fresh or high producing cow. In the case of fresh or high producing dairy cows, nutritionists are continually struggling to reach an acceptable balance between the energy and fiber components in the ration. And certainly it has been the struggle between supplying adequate energy intake while maintaining adequate fiber in the diets of these groups that has stimulated the interest in measuring effective fiber. While ration formulation is the initial factor in achieving optimum performance, feed management can also have a significant impact on ration performance. In many cases, feeding management can override or mask the true potential of the ration. Nutritionists often refer to three rations on the farm: the ration formulated on paper, the ration offered to the cow, and the ration consumed by the cow. This phrase was traditionally thought of in terms of nutrient uniformity; however, now we must think about it in physical respects as well.

In evaluating ration performance, the obvious and most common starting point is the ration on paper. But ultimately, cow performance is supported by the ration consumed by the animal and this may be the more accurate starting point when investigating problem rations. Identifying the ration consumed and potential differences from the ration formulated should be the first step in problem solving farm situations, although identifying reasons for these differences is not always an easy or precise task.

Field Conditions Affecting Ration Form

There are many steps between the ration on paper and the ration consumed by the cow that can cause a difference. These would begin with loading and delivery accuracy, but may also include mixer design, loading sequence, mixing time, and animal sorting.

Mixer Design

There are several types of feed mixers available for commercial dairying. A crude breakdown of these would begin with separating out the horizontal and vertical mixers. Within these categories, there is a range of types including horizontal ribbon and paddle mixers, vertical screw mixers, drum mixers, and mobile mixing boxes. Many attempts have been made to model the mixing process through a series of engineering equations for equipment design. However, most mixing equipment has evolved from past successes. For example, most horizontal mixers have a length approximately three times their diameter and have a rotational tip speed of 250-300 rpm regardless of the diameter, with the inside ribbon usually 2.5 times the thickness of the outside ribbon to balance the directional forces applied due to ribbon diameter (1). With the decision between a horizontal or a vertical type mixer, experience suggests it is not necessarily the choice of one type or the other, but more importantly it is the management of the chosen type that affects the final ration presentation. Several problem incidences with vertical mixers have initiated serious debate over the use of this type; however, there are a number of successful users that can justify their consideration. In discussions with these producers, they attribute the success of these mixers to modifications in their mixing sequence or time and these modifications are based on initial experience. Another important point that recurrently comes up in these discussions is their maintenance schedule for the mixers. Worn or broken mixing components do not allow the mixer to function uniformly. I have heard numerous producers mention that they cannot afford to ignore mixer maintenance as they appreciate it's contribution to ration uniformity. One producer even worked out a schedule for replacing or sharpening knives on a rotating basis to avoid drastic changes in forage particle breakdown when all knives are sharpened or changed at one time. In addition to mixer maintenance, mixer cleaning cannot be overlooked. Build-up of wet feedstuffs can also impair mixer function and inhibit uniform distribution of micronutrients.

Mixing Time

If insufficient mixing time is given, final ration composition can be altered considerably. This will be even more important if the load is split between two or more groups. Even if the load is delivered to only one group, consider the impact of an inadequately mixed ration on individual cow consumption. Appropriate mix time can only be determined through testing and most mixer wagons come with manufacturer suggestions (some manufacturers supply better information than others). Manufacturer's recommended mixing times range from three to six minutes. The question then arises: Is the mixer running while loading and does this time count? With larger mixes and a variety of separate commodities, it is common for ingredient loading to take 15 to 20 minutes. Should the mixer be active while loading? If this is the case, then total mixing time will now be greater than 20 minutes. Physical form of dry hay may alter mixer strategy and processing of long forage may require additional time prior to addition of other feeds. In a survey of actual mixing times in Wisconsin, the average time mixing was 16 minutes, with a range of 2 to 60 minutes (11). While striving to achieve a uniform nutrient mix, the concern with over-mixing is the physical breakdown of fiber particles.

Loading Sequence

Several ingredient properties can influence mixing: particle size, particle shape, density, hygroscopicity, static charge, and adhesiveness (1). From this list, particle size, shape, and density appears to have the greatest impact on mix uniformity. With respect to particle size, the addition of forage and the level of forage inclusion in dairy rations presents a unique challenge to targeting adequate mixing times. The differences in forage and concentrate particle size alone will present a challenge. Furthermore, differences in particle density between ingredients adds other considerations. On a dry matter basis, corn silage and haylage are fairly equal in bulk density (kg/m3); however, on an as-fed basis, corn silage tends to have a 33% greater bulk density than haylage (7). In addition, mineral density can be two to three times greater than that of grain and protein, making them difficult to hold in a random distribution. As a general rule, lighter and larger particles tend to move upward while the smaller, more dense particles gravitate downward. It has been traditionally advised to load larger particle size ingredients first (forage) and heavier, smaller particles last; however, with the use of individual commodities and with rations containing many ingredients with a large variation in size, shape, and density, determination of loading sequence has become a method of trial and error.

Measuring Mix Uniformity in Dairy Rations

The basic assumption of all rations is that each bite taken by the animal matches that balanced by the nutritionist. This is very critical in all species, but the dairy cow provides a unique challenge due to her requirement for daily milk production. The importance of ration uniformity is perhaps better accepted in meat animal production (feedlot, swine, and poultry) than it is in dairy cattle production; thus on- farm tests to evaluate mix uniformity have been developed with high grain diets. Due to the inclusion of forages, dairy rations present a different degree of difficulty in assessing uniform mixing. Forage type and inclusion level can affect marker analysis. Basic considerations in ration uniformity testing include uniformity assay, sampling technique, and interpretation of results.

Uniformity Assays

Several methods that have been investigated for determination of ration uniformity include a chemical assay for a selected nutrient, a marker, and a ion test.

*Chemical assay for nutrient. The traditional assay included a chemical, quantitative measurement of a selected nutrient. The limitation to these assays tend to be the cost and time for analysis. They are not supportive to on-farm periodic evaluation.

*Markers. One marker that has been used is iron filings. This process includes adding a sufficient quantity of iron filings marked with a soluble dye to achieve a 16 to 25 count per 50-100 gram sample. The filings are removed through magnets onto paper, and water is sprayed on this paper to allow counting of colored spots. The limitations to this method may be practical use with rations that contain heavy, wet feeds.

*Ion analysis. The Quantab® (Environmental Test Systems, Inc., Elkhart, Indiana) is a method for determining the chloride ion concentration of solutions. The procedure involves extracting salt by means of a hot water soak. Titrators, consisting of a thin strip laminated with a capillary column impregnated with silver dichromate, are used for a color reaction, thus allowing calculation of chloride ion concentration. This method is relatively fast (10 to 15 minutes), requires minimal lab equipment (hot water, filter paper, measuring device, and paper cups), and is relatively inexpensive ($25 US for 50 tests). However, this procedure is sensitive to acidity and may pose a problem with dairy rations including fermented feeds.

Sampling Technique

Accurate sampling presents a challenge. Remembering the diversity of particle sizes between ingredients used in dairy rations, collecting a sample representative of the total mixed ration can be difficult. Care must be taken when collecting traditional "grab samples" that samples are representative, especially if decisions will be made based on analysis. Benhke (1) described a more accurate sampling technique involving plastic sheets placed in the feed lane and then using a quartering technique to reduce the sample size to a workable size. The question of sample size is also unanswered. With the diversity of ingredients in a dairy ration, what size of a sample accurately represents an entire mixer wagon? Also, how many samples from a mixer are necessary to represent the entire wagon? Another question remaining is does the sample taken from the initial mixer wagon represent the ration being eaten by the cow. If not, then how should the initial sample be adjusted for feed refusals?

Interpreting Uniformity Tests

Assuming accurate sampling techniques and proper marker choice, interpreting results in a decision making manner is the next challenge. Research at Kansas State University reports the mean, standard deviation, and coefficient of variation as indicators of mixing tests. In short, these values identify the distribution of values and condense them into one measure of the mix. Again, work from KSU identifies a coefficient of variation less than 10% to represent a good mix.

Particle Size and Ration Uniformity: Is it Important to the

Dairy Cow?

Particle Size of Dairy Rations

The effects of forage, fiber levels, and particle size on ruminal function has been studied by several groups (2, 4, 5, 10, 15). Reducing forage particle size has been shown to improve the dry matter intake potential of forage diets (3), especially with poor quality forages. This may occur through several factors, including a physical reduction in ration bulk, an increase in surface area for microbial attachment, and/or an increase in passage rate. In actuality, it is probably some combination of all three. Woodford and Murphy (15) evaluated the effect of particle size on intake and milk production by feeding a control (40% alfalfa haylage) or two levels of alfalfa pellet (30% of forage or 70% of forage). These workers reported a 5% increase in milk production (over control) when 30% of the forage supply was in pelleted form, while intakes remained similar between these two diets. However, as inclusion of the pellet increased from 30 to 70% of the forage, both dry matter intake and milk production were decreased (Table 1). In a trial comparing cereal silages with higher NDF levels to an alfalfa haylage control, Okine et al. (10) reported that dietary NDF elicited significant negative effects on dry matter intake (Table 2).

Adequate particle size in the ration appears necessary to avoid low milk fat syndrome. Cows require fiber and forage to stimulate chewing activity and saliva production, both being necessary to maintain a proper ruminal pH and a healthy rumen. Grant et al. (4) examined the effect of particle size of dry hay on chewing activity and milk fat production by including a finely (0.6-cm screen) or coarsely (7.6-cm screen) chopped hay into the ration of cows at three weeks postpartum. These workers reported that dry matter intake and milk production were not affected by particle size of hay and that NDF intake, expressed as a percentage of body weight, was similar between diets (1.2%). However, reduced forage particle size was associated with depressed ruminal pH, decreased acetate:propionate ratios, and lower milk fat synthesis (Table 3). These effects were further supported in a subsequent trial with alfalfa haylage (5). Beauchemin et al. (2) compared diets at two levels of forage fiber (35% and 65%) and chop length of forage (5 mm and 10 mm theoretical chop length) within each level of forage. This group reported cows fed diets containing 65% forage fiber produced more milk when the theoretical chop length (TCL) of the forage was reduced (5 mm). Conversely, with low forage fiber (35%), cows produced more milk on coarser particle size forage (10 mm). Milk fat production was substantially higher for cows fed the higher forage fiber diet (65%) and fat synthesis also tended to increase with coarser chop length.

Table 1. Effect of particle size on feed intake and milk production1.

Diet 2
Dry matter intake, kg/d 23.2a 23.1a 18.8b
Milk production, kg/d 33.7a 35.5b 31.8c

1 Woodford and Murphy (15).

2 Concentrate:haylage:pellets on a DM basis were A) 60:40:0; B) 60:28:12; C) 60:12:28.

a,b,c Means within rows with unlike superscripts differ (P<.05).

Table 2. Effect of ration NDF on feed intake1.
Alfalfa silage Barley silage Oat


Triticale silage
Dietary NDF, % of DM 32.2 35.4 37.9 36.5
Dry matter intake, kg/d 19.7a 19.1a 16.0b 17.4a,b

1 Okine et al. (10).

a,b Means with different superscripts letters in the same row differ (P<.05).

Table 3. Effects of forage particle size on rumen conditions and milk fat synthesis1.

Fine Medium Coarse
Ruminal pH 5.40c 5.80b 6.25a
Ruminal acetate:propionate ratio 2.08c 3.20b 3.89a
Milk fat synthesis, % 3.2c 3.5b 3.8a

1 Grant et al. (4).

a,b,c Means with different superscripts letters in the same row differ (P<.05).

Literature reports of cow response to effective fiber (particle size, forage level) led to an interest in developing a diagnostic tool to evaluate effective fiber. As a result, particle size separators have been developed to measure particle size distribution in feeds. These tools consist of a series of stacked screens designed to separate out a ration sample into various particle sizes. The intention of this process is to have a visual, quantitative assessment of particle distribution as it will occur in the rumen. Some commercial laboratories offer particle size separation analysis as part of their available services. There are also separators available for on-farm demonstration analysis, such as the Penn State Particle Size Separator (6). This tool separates out the particles into three groups: particles greater than .75", between .31" and .75", and those smaller than .31". The top screen (retaining particles greater than .75") identifies those particles that will be included in the rumen mat and will stimulate cud chewing and saliva production; the middle screen (particles between .31" and .75") represents the portion of the TMR that is moderately digestible; while the bottom pan (particles smaller than .31") represents particles that are rapidly digestible and/or may be removed from the rumen in the fluid outflow (8). Use of the separator is fairly simple and can be used on-farm to monitor changes in forage harvesting procedures or feed mixing schemes. Table 4 gives recommendations for forages and TMR's.

Ration Uniformity

There is very little published data on the affects of nutrient uniformity on animal performance. What little work that has been done has focussed on the monogastric species (9, 13, 14), thus there are few benchmarks for using this technique with dairy rations. With respect to nutrient levels, if mixing is poor enough to alter nutrient intake of certain animals, performance may be altered. The animals receiving nutrients in excess will be inefficient in feed conversion; in extreme cases, such as with urea inclusion, this situation may even become toxic. On the other side, animals receiving rations deficient in nutrients will have performance compromised. Feed intake and body size influences susceptibility to ration imbalances in that smaller animals consuming smaller meals are more likely to be influenced by improper ration mixing than are larger animals consuming more dry matter. The influence of dry matter intake is most evident in transition and fresh cows with lower levels of intake, and perhaps it is this group that ration uniformity is of most importance. Instinct would suggest that variation in the final ration delivered to this group should be as minimal as possible. Feeding management style again plays a role, in that the same uniformity may not be required when feeding along a fence line feeder to loose-housed cows as compared to a herd housed in a tie-stall barn.

Table 4. Recommended forage and total mixed ration particle sizes for the Penn State Separator1.

Sieve Corn silage Haylage TMR
Upper sieve2(>.75") 2-4% if not the sole forage

10-15% if chopped/rolled

10-15% in sealed silo

15-25% bunker silo, wetter mixture

6-10% or more

3-6% focus on


Middle sieve2 (.75-.31") 40-50% 30-40% 30-50%
Bottom pan2 (<.31") 40-50% 40-50% 40-60%

1 Heinrichs (6).

2 Portion remaining on the screen.


Transition and high producing cows are most susceptible to ration imbalances. In addition, as performance in early lactation has a significant effect on total lactation performance, ration quality should be emphasized. Most producers and nutritionists would agree that there is a connection between ration quality, herd productivity, and profitability. The challenge then becomes to define ration quality. Furthermore, the question of application of these measures to the dairy cow ration needs to be defined. The literature would suggest that some level of effective fiber is necessary to maintain a stable rumen fermentation and avoid metabolic upsets. The optimal level of effective fiber may vary considerably between individual farms as ingredients and feeding management styles differ. Research suggests that particle size distribution in the ration will vary with forage level. Field experience of producers and consultants would suggest that level of forage, carbohydrate source, and feeding management may also affect a herd's tolerance level of minimum effective fiber. Accurate measures of effective fiber and the impact of mix uniformity in dairy rations on lactation performance still needs to be defined. This is not a clear picture and appears to be influenced by a myriad of farm factors. Tools, such as the particle size separator, can give an idea of the distribution of particle size in a ration. However, information gleaned from the use of these tools needs to be used carefully as it is only one more piece of the puzzle.


  1. Behnke, K.C. 1996. Mixing and nutrient uniformity issues in ruminant diets. In: MidSouth Ruminant Nutrition Conference Proceedings, p. 6-11.
  2. 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.
  3. Campling , C.C., and M. Freer. 1966. Factors affecting the voluntary intake of food by cows. 8. Experiments with ground, pelleted roughages. Br. J. Nutr. 20:229.
  4. Grant, R.J., V.F. Colenbrander, and D.R. Mertens. 1990a. Milk fat depression in dairy cows: Role of particle size of alfalfa hay. J. Dairy Sci. 73:1823.
  5. Grant, R.J., V.F. Colenbrander, and D.R. Mertens. 1990b. Milk fat depression in dairy cows: Role of silage particle size. J. Dairy Sci. 73:1834.
  6. Heinrichs, J. 1996. Evaluating forages and TMRs using the Penn State Particle Size Separator. Penn State Cooperative Extension Service. DAS 96-20.
  7. Kammel, D.W., R.T. Schuler, and R.D. Shaver. 1995. In: Alternative Feeds Symposium Proceeding. p. 271-297
  8. Lammers, B., J. Heinrichs, and D. Buckmaster. 1996. Method helps in determination of forage, TMR particle size requirements for cattle. In: Feedstuffs, Vol. 68, Number 41, p. 14-20.
  9. McCoy, R., K.C. Behnke, J.D. Hancock, and R.R. McElhiney. 1994. Effect of Mixing Uniformity on Broiler Chick Performance. Poultry Sci. 73:443.
  10. Okine, E.K., G.R. Khorasani, and J.J. Kennelly. 1994. Effects of cereal grain silages versus alfalfa silage on chewing activity and reticular motility in early lactation cows. J. Dairy Sci. 77:1315.
  11. Possin, I.R., D. DeCorte, R.D. Shaver, and R.T. Schuler. 1994. Survey of particle length and metabolic disorders on commercial dairies. Dairy Science Department, Univ. Wisconsin-Madison.
  12. Shearer, J.K. 1996. Lameness in dairy cattle: laminitis, claw disease, digital dermatitis, and foot rot. J. Dairy Sci. 79:189(Suppl.).
  13. Stark, C.R., K.C. Behnke, R.D. Goodband, and J.A. Hansen. 1991. On-Farm Feed Uniformity Survey. Swine Day Report of Progress No. 641, AES, Kansas State University, Manhattan, KS.
  14. Traylor, S.L., J.D. Hancock, K.C. Behnke, C.R. Stark, and R.H. Hines. 1994. Uniformity of Mixed Diets Affects Growth Performance in Nursery and Finishing Pigs. Proc. Midwestern Section - Am. Soc. Animal Sci. (Abstracts) Des Moines, IA. March 21-23, 1994. p. 59.
  15. Woodford, S.T. and M.R. Murphy. 1988. Effect of forage physical form on chewing activity, dry matter intake, and rumen function of dairy cows in early lactation. J. Dairy Sci. 71:674.