Forage particle size

Effective fiber has enough length to stimulate rumination. Rumination results in the production of saliva to buffer the rumen and increases the rate of forage digestion. Effective fiber adequacy can be evaluated visually, using a forage particle size separator, or by observing cow cud-chewing activity.

Cows Need Long “Effective” Forage Fiber (NDF):

Effective fiber is that fiber which stimulates rumination and results in the production of saliva. Long particles, especially long forage particles, are regurgitated by the cow and re-chewed. This increases the rate at which forages are digested in the rumen because it creates more surface area for the rumen bacteria to attach to.

Cud-chewing produces saliva. It has been estimated that cows make over 6.6 pounds (3 kg) of sodium bicarbonate each day. That’s a lot compared to the 0.5-0.75 pound (0.23-0.34 kg) of sodium bicarbonate that we usually add to high production rations! Saliva buffers the acids that are produced from the rumen fermentation. Saliva is vital in controlling rumen acidity, especially in today’s high-producing cows consuming significant amounts of grain. However, even cows consuming only 15 pounds (7 kg) of grain per day can experience rumen acidosis if forage particle length is inadequate.

We need to always have enough Forage NDF in the diet and that Forage NDF needs to have a long enough particle size to stimulate rumination. Rations without enough Effective Forage NDF will upset the function of the rumen, reduce dry matter intake, and reduce milk production. A significant lack of Effective Forage NDF will result in acidosis, laminitis, and displaced abomasums.

NRC (1989) recommends a minimum of 25% NDF with 75% of the NDF coming from forages (18.75% Forage NDF). This minimum level of Forage NDF is common in regions where forages are limiting or forage quality is limiting, such as, southeastern and southwestern U.S. In more temperate climates that rely more heavily on quality forages for milk production, it is more common to have 21-26% Forage NDF in diets fed to early lactation cows.

Remember that forages can also be too coarse. Long forage particles may not pack and ferment well. Also, long forages can more easily be sorted out of the TMR by cows, especially if the ration is dry.

Evaluating Particle Size:

Assuming that forage analysis has been conducted and the ration has been balanced for an adequate amount of Forage NDF, it is important to evaluate the particle size of the ration.

1. Visual Observation of the Ration:

This method involves simply picking up a sample of the cow’s total ration and looking at it. A general recommendation is to have 15% of the particles in the ration measuring over 1.5 inches (3.8 cm) in length.

2. Use of a Forage Particle Size Separator

Researchers at Penn State University have designed a Forage Particle Size Separator for on-farm evaluation of forage particle length. It has two stacked screens as well as a bottom pan. The top screen has 0.75 inch (1.9 cm) openings and the second screen has 0.31 inch (0.79 cm) openings. The screens are made of thick plastic (not thin wire) so that it is less likely for long particles slide through the holes diagonally.

A sample of the ration is taken and placed on the top screen of the stacked Forage Particle Size Separator. The Separator is then shaken horizontally for two minutes until only long particles remain on the top screen and no more is going through it. The weights of each of the three fractions (coarse, medium, and fine) are taken and calculated as a percentage of the total sample weight (or total of the three fractions).

The coarse material will be buoyant in the rumen upon ingestion and will be ruminated by the cow. Medium particles will be buoyant only for a short time and will be ruminated only to a limited extent.

General Recommendations for Particle Size:

  Coarse, >0.75 inch Medium Fine, <0.31 inch
Processed Corn Silage 20-25% 30-40% 35-50%
Unprocessed Corn Silage 10-15% 35-45% 35-45%
Hay Crop Silage 20-25% 30-40% 35-50%
TMR 10-15% 30-50% 40-60%

Note: Some researchers suggest that only 6-10% of the TMR needs to remain on the top screen. The exact recommendation depends in part on the make up of the NFC’s in the diet and feeding management. The author has had the most success across diets with the above recommendations.

The Forage Particle Size Separator can also be used to determine if overmixing in the mixer wagon is a problem. You can hand mix a TMR together, seive it to estimate particle size, and then compare with the seived TMR from the mixer wagon. You also can compare different mixing times.

3. Visual Observation of the Cow:

Evaluate cud-chewing activity of the cows that aren’t eating a few hours after feeding time. Cows need to ruminate about 8 hours per day. They also eat about 5 hours per day. They usually spend 3-4 hours per day being milked. Thus, you should see at least 50-60% of the cows cud-chewing when you observe them.


Dado, R. and M. Allen. 1992. The seven eating habits of highly effective cows. MSU Animal Science Newsletter.

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

Firkins, J.L. 1992. Effectiveness of fiber from non-forage sources. Proceeding of the Tri-State Dairy Nutrition Conference. Fort Wayne, IN.

Heinrichs, A.J. and B.P. Lammers. 1997. Particle size recommendations for dairy cattle. Proceedings of the Silage:Field to Feedbunk North American Conference, Hershey, PA.

Related Links:

Penn State Particle Size Separator

Site provides the instruction manual, as well as several other reference articles, related to the use of the Penn State Particle Size Separat


Mary Beth de Ondarza

Mary Beth de Ondarza
45 articles

Nutritional consultant for the dairy feed industry at Paradox Nutrition, LLC.

Look to Paradox Nutrition, LLC for providing:

  • Controlled research studies on commercial dairy farms
  • Computer modeling with complete nutritional assessment and statistical analysis
  • Complete literature review of a nutrition topic
  • Nutrition writing for company technical bulletins or popular press
  • Dairy nutrition seminars with new, practical, and understandable information for consultants and dairy producers
  • On-farm nutritional consultation

Dr. de Ondarza received her Ph. D. from Michigan State University and her Masters Degree from Cornell University, both in the field of Dairy Nutrition.

Read more »

Paradox Nutrition

Paradox Nutrition

Paradox Nutrition, LLC is a nutritional consultation business for the dairy feed industry. Mary Beth de Ondarza, Ph.D. is the sole proprietor.

Read more »