Methods Used to Measure Forage and Ration Particle Size

Particle size evaluation tools are described and demonstrated in a series of brief videos.
Methods Used to Measure Forage and Ration Particle Size - Articles


There are a variety of methods used to measure forage and feed particle size. Each has its advantages and disadvantages and each can be useful for measuring components that help us fully understand the effects of particle size on the dairy cow, from forage and total mixed ration particle size to refusal analysis and measuring rumen digesta and fecal particles. All of these pieces contribute to better understanding the nutrition of the high producing dairy cow. Particle size evaluation tools are described and demonstrated in a series of brief videos.


Forage and TMR particle size impact the health and production of the dairy cow. Adequate forage particle size is necessary to maintain cow and rumen health by buffering rumen pH. In addition, it is generally accepted that increasing forage particle size decreases dry matter intake (DMI) due to gut fill. Measurement of physically effective NDF (peNDF) has become widely used in dairy cattle nutrition and research. The original peNDF system was developed by Dr. Dave Mertens at the US Dairy Forage Research Center (1997). Mertens described peNDF as the ability of a feed to stimulate chewing and maintain the rumen mat. Although this measure is commonly used, the particle separation method that best measures ration peNDF has not been well defined.

Problems can be created by using peNDF values determined with different sieving methods interchangeably. Many of the systems attempting to estimate physically effective fiber are based on the theory that there is a critical size threshold for particles leaving the rumen, and that particles above this threshold are effective because they stimulate chewing to promote particle size reduction and rumen escape. This article will describe the methods currently used for diet and forage particle analysis and compare differences and similarities between methods.

Mertens' peNDF procedure used 1.18 mm as the critical size at which feed particles are considered physically effective for dairy cows. This number originated from earlier research with both cattle and sheep (at maintenance intakes) that determined 1.18 mm was a threshold particle size for greatly increased resistance to particles leaving the rumen and that less than 5% of fecal particles were generally retained on a 1.18-mm sieve.

Researchers from Penn State as well as Canada and Japan have studied particle size of diets and their impact on rumen metabolism, and have shown in recent studies that the critical threshold for feed particles escaping the rumen of high producing cows is greater than 1.18 mm and more in the range of 4 mm. While there is no single, perfect sieve size to measure particles for all diets and all forages, data from three independent labs show that a 4-mm sieve is more accurate for estimating peNDF for the high producing dairy cow.

Penn State Particle Separator

The Penn State Particle Separator (PSPS) has become a standard particle separation technique used in the dairy cattle nutrition industry. The PSPS is manually operated and separates as-fed forage and TMR samples via horizontal shaking. Lammers et al. (1996) first developed the PSPS as an easy to use, practical, on-farm tool to mimic Standard S424.1 of the American Society of Agricultural and Biological Engineers (ASABE), which is the standard method of determining the particle size distribution of chopped forages. The first PSPS consisted of 3 particle fractions: >19.0, >8.0, and <8.0 mm. The PSPS was later refined by Kononoff et al. (2003) by adding a 1.18-mm screen to allow for more accurate characterization of TMR and forages that have a large portion of particles <8.0 mm. The realization that the critical size for peNDF was greater than 1.18 mm lead to the introduction of the newest sieve, which is found in the 2013 PSPS and has 4-mm openings. (The new screen can be purchased individually and added to the original PSPS or used in place of the 1.18-mm screen.)

The top 19-, 8-, and 4-mm screens have circular holes, and the screen depth is varied to provide a 3-dimensional barrier to prevent particles larger than the hole sizes from falling through. The 1.18-mm sieve is composed of a stainless steel wire cloth that has a nominal screen size of 1.18 mm and a diagonal screen size of 1.67 mm. Recommended sample size for the PSPS is 3 pints or 25% of the ASABE standard sample size, because the PSPS has approximately one-fourth of the surface area of the ASABE separator. The recommended shaking procedure includes placing the PSPS on a flat surface, shaking the separator horizontally 5 times at 1.1 Hz with a stroke length of 7 inches, then rotating the separator a quarter turn and repeating these steps. A total of 8 sets of 5 shakes should be completed for a total of 40 shakes in 2 full turns. Lammers et al. (1996) determined that there was no difference in the results of the PSPS and the ASABE separator in predicting fractions of particles <19.0 and <8.0 mm in 21 of 36 statistical tests. Use of the PSPS is demonstrated in this brief video. 

Several studies have used particles retained on the 1.18-mm sieve of the PSPS to determine peNDF of TMR. Also, studies have been conducted that used the 8-mm screen of the PSPS to determine peNDF. However, the PSPS uses a very different particle separation technique from the one specified by Mertens' peNDF procedure. In addition, it should be noted that when using the 1.18-mm sieve in the PSPS to measure peNDF there may be no significant differences in peNDF of TMR found, even though there are significant differences in particle size distribution and even cow response. This shows a lack of sensitivity when using older configurations of the PSPS to measure peNDF.

The new 4-mm sieve was designed to allow estimation of peNDF using the PSPS. It should be noted, however, that many feed ingredients and byproducts will also be trapped on this sieve. This is obvious and must be handled with the judgment of the operator. In some situations, this fraction must be discounted in its amount when using the PSPS for determining peNDF. The peNDF can be estimated by adding the amount of feed on the top three sieves (all ≥ 4 mm) and multiplying by the NDF content of the feedstuff. This is an estimated value, as the NDF content and digestibility of each fraction are unknown. In addition, some portion of the contents on the 4-mm sieve will likely contain grain or rapidly digested carbohydrates.

Advantages of the PSPS are its portability, low cost (approximately $300; Nasco, Fort Atkinson, WI), ease of use, quick results, use of as-fed samples, and good repeatability. For these reasons it has become popular with dairy farmers and nutritionists worldwide. The PSPS can easily be used in a field or barn whenever it is needed without the need for time-consuming drying of samples. Some disadvantages of the PSPS are that it determines fewer particle fractions than other methods and requires manual operation. Anytime a procedure requires manual manipulation it induces a certain amount of human error; however, the ability to rest the PSPS on a smooth, steady surface effectively limits human error. In addition, moisture content of samples and shaking frequency can affect the particle size distribution and mean particle size measured with the PSPS. Small loses of moisture cause only minor changes in particle size distributions, whereas complete drying causes large differences by increasing the amount of particles passing through each sieve. Therefore, it is important to standardize the shaking procedure and consider the effects on moisture when utilizing the PSPS.

ASABE Particle Separator

The ASABE or "Wisconsin" separator is the standard method for determination of particle size distribution of chopped forages (S424.1; ASABE, 2007). It is a very large (>500 pounds) particle separator that is mechanically operated and uses a horizontal shaking motion. The ASABE separator consists of a pan and 5 square-hole screens with sizes of 19.0, 12.7, 6.3, 3.96, and 1.17 mm when measured nominally or 26.9, 18.0, 8.98, 5.61, and 1.65 mm when measured diagonally; each screen has a frame of 22 x 16 x 2.5 inches (length x width x depth). All of the screens are made of aluminum of varying thickness, increasing with increasing screen size, except the smallest screen, which is wire mesh. The thicknesses of the screens from top to bottom are 12.7, 9.6, 4.8, 3.1, and 0.64 mm. The recommended procedure for the ASABE separator is to use a sample size of 2.5 gallons of uncompressed forage and operate the shaker for 2 min. Several advantages of this separator are it is mechanically operated, has a moderate number of particle fractions, uses as-fed samples, and has screens with more surface area (longer and wider) than the PSPS. These advantages help to reduce human error, more accurately describe particle distribution, eliminate the need for sample drying, and allow for better separation of extremely long particles, respectively. Maulfair et al. (2010) found that when using rations of extremely long particle size, the PSPS did not adequately separate the particles. Extremely long (>2 inches) hay particles bound together and did not allow particles to fall through the top screen when shaken with the PSPS. The larger screens and more vigorous shaking of the ASABE separator allowed enough movement of the longest particles for the smaller particles to fall through the screens. This situation would not be realized very often in a field setting because these diets were very extreme.

The disadvantage of the ASABE separator is that it is the least portable of all separators; it is very heavy, large (40 x 25 x 57 inches; length x width x height), and requires electricity to operate. Plans can be purchased from ASABE and systems must be custom manufactured. The results of the ASABE particle separator are also susceptible to variation with sample moisture content. The disadvantages of this particle separator strictly limit its use to laboratory use. This video demonstrates the use of the ASABE separator. 

Ro-Tap Particle Separator

The Ro-Tap particle separator (RTPS; W.S. Tyler, Mentor, OH) uses a dried sample that is placed on a series of stacked sieves (same sieves used in wet sieving) and shaken horizontally while simultaneously a metal arm repeatedly taps the top of the sieve stack (holds 8 to 16 depending on sieve height) to incorporate a vertical shaking element as well. This shaking system could probably be considered obsolete, except it was used for much of Mertens' research. Mertens (1997) developed the concept of peNDF and used the RTPS to develop the laboratory assessment of peNDF, where the particles retained on a 1.18-mm sieve after shaking are multiplied by the sample NDF content. The Mertens (2005) RTPS procedure specifies a sample size of 1.5 pints, sieve sizes of 19.0, 13.2, 9.5, 6.7, 4.75, 3.35, 2.36, 1.18, 0.60, and 0.30 mm, and a 10-min operation time. A major factor that creates a difference between the RTPS and many other methods is that vertical shaking tends to separate particles by their minimum cross-sectional dimension (usually width in forage particles), whereas horizontal shaking tends to separate particles by their length. This difference is amplified by the fact that the RTPS uses wire screens that have a minimum screen thickness versus the large thickness of the PSPS and ASABE separator screens.

Because the RTPS uses vertical shaking and dried samples, it produces results that can be very different from techniques (PSPS and ASABE separator) that use horizontal shaking and as-fed samples. Which shaking technique is optimal may depend on the samples being separated and the hypothesis being tested. For instance, separating particles based on their smallest diameter may be more similar to how particles attempt to leave the rumen. The other divergence of the RTPS from most other techniques is that samples are dried before they are separated. Drying forage samples makes particles smaller and more fragile, making them more likely to break during the separating process; both of these factors can artificially decrease the resulting particle size distributions. Drying samples also makes this technique more time consuming because samples are usually dried for at least 24 h. Other disadvantages of the RTPS are that it is not very portable, is expensive ($2,300 to $2,500 plus the cost of sieves; Thermo Fisher Scientific, Waltham, MA), requires electricity, and is extremely loud to operate. Some advantages of the RTPS are that it is mechanically operated, many screens can be used (up to 8 or 16 depending on sieve height), and the screen sizes can be customized for the intended use. The RTPS is used for research and by commercial forage testing labs. See a demonstration of the Ro-Tap method in this video. 

Wet Sieving

There are 2 types of wet sieving reported in the literature. The first type consisted of a series of stacked sieves being completely submersed in a vat of water and moving vertically in the water for a period of time. This type of wet sieving was used by Poppi et al. (1980, 1981, 1985) when 1.18 mm was first suggested as the critical particle size for particles leaving the rumen of cattle and sheep. This type of sieving seemingly has not been used for several decades and would likely be considered obsolete. The other method of wet sieving is the type of procedure used by Beauchemin et al. (1997) and improved upon by Maulfair and Heinrichs (2010). In this procedure a series of stacked sieves of decreasing size have water sprayed onto the top screen and in the middle of the sieve stack. While the water is being sprayed onto the samples in the sieve stack, the entire stack is vibrated via vertical oscillation. The bottom pan in the sieve stack is drained to allow water and soluble matter to flow out. Soluble DM (DM that passes through the smallest sieve) can be determined by calculating the DM lost during the sieving process. Six different sieve sizes of the many available sizes can be used at once (up to 12 if half-size sieves are used) and the sizes can be customized to suit the intended uses of the separation. See this separator in action in this video. 

This technique lends itself very well to separating samples that have high moisture contents (rumen digesta and fecal samples) because these samples will not separate well using other techniques without drying, and drying can change the physical properties of samples. Wet sieving is valuable for research because it probably most accurately mimics conditions in the rumen as particles pass through the omasal canal. Particles in the rumen are completely water saturated and suspended in fluid when they pass though the omasal canal, and this is the only particle separating method that closely resembles this action.

However, there are many disadvantages to using this method. The procedure is very time consuming; even with the modifications to increase processing time made by Maulfair and Heinrichs (2010), at least 30 min is required to process a single sample. Wet sieving equipment is expensive ($2,900 to $3,500 plus the cost of sieves; Thermo Fisher Scientific, Waltham, MA), not easily portable, and needs running water and electricity to operate. The characteristics of this method make it very valuable for research but impractical for field use.


Because peNDF is described as the ability of a feed to stimulate chewing and maintain the rumen mat, the best separator should be the one that best correlates to chewing activity. An as-fed sample may correlate better to chewing because that is the form feed is in when presented to the cow. Horizontal separation may correlate better to chewing because it separates on longest diameter, and the cow likely chews until the longest diameter of forage particles is below a certain size. Additionally, repeatability of the separator is extremely important, and portability, ease of use, and cost must also be considered if the separator is to be accepted for field use.

Wet sieving is most likely the best technique when studying particles passing out of the rumen. Rumen digesta and fecal samples can be separated without changing their physical conformation. The separating action of wet sieving also more closely mimics actions that occur in the rumen: separating on smallest diameter while suspended in fluid.


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