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Optimizing forage particle size for rumen health and performance

A row of black and white dairy cows eating feed from a trough at a farm. The cows have yellow ear tags and are positioned behind metal fencing.

Utilizing the “Goldilocks’ principle”

The term “Goldilocks’ diet” is often referred to a controlled energy for dry cows. This diet gives just the right amount of energy—neither too much nor too little. This concept can also be applied to the length of cut during harvest or the size of forage particles.

It suggests that forage particle size should be just right—not too long or too short—for optimal rumen health and performance. Longer particles can lead to longer eating times and less resting time. Reduced resting time has been associated with lower milk production. There is also more potential for sorting behavior (Leonardi and Armentano, 2003). Sorting behaviour can have possible rumen health problems, such as sub-acute ruminal acidosis (SARA). It can be exaggerated with diets that are high in starch and low in NDF or there is competition at the bunk due to poor feeding management or over-crowding.

Headshot of Bill Woodley, Woodley Dairy Direction

William Woodley, Woodley Dairy Direction

Conversely, short particles may reduce rumen motility and increase the passage rate of forage, reducing nutrient absorption. Retaining the forage particles in the rumen environment will improve forage digestibility. Finally, balancing particle size helps improve forage digestibility and overall cow health.

How to find the right balance in forage particle size?

There is a wide variation in length-of-cut recommendations for corn silage around the world. These can range from very fine (4mm) to very long (>30 mm).

Why are there so many variations?

Many producers believe that:

Fine chop produces extensive kernel damage; thus, decreasing the reliance on proper kernel processing.

Chopping the corn finely will help pack it better in the bunk. This is especially true for drier corn silage, which has more than 40% dry matter.

Long particle size will improve cud chewing

In many scenarios when producing fine corn silage, the haylage (grass and alfalfa) have been increased in particle size to compensate.

This leads to a less homogenous total mixed ration (TMR) and potential for an increase in sorting behavior. Smaller particle sizes can increase the passage rate. This may cause a loss of digestible fiber in corn silage.

What does the research say about corn silage harvesting practices?

A meta-analysis of corn silage harvesting practices was published in The Professional Animal Scientist 28 (2012):141–149 by L. F. Ferraretto and R. D. Shaver, (Department of Dairy Science, University of Wisconsin–Madison).

The meta-analysis reviewed 24 scientific papers and examined the impact of corn silage harvest maturity (dry matter content), kernel processing, and length of cut on DMI, digestion and milk production.

The theoretical length of cut (TLC) was categorized as:

The theoretical length of cut (TLC) of corn silage

In this meta-analysis the TLC of the corn silage had little effect on production and digestion parameters.

Dry matter intake was unaffected (P = 0.68) by length of cut
Milk and milk fat were unaffected by length of cut
Diet dry matter digestibility (P = 0.69), organic matter digestibility (P = 0.93) and NDF digestibility (P = 0.74) were unaffected by length of cut
The total tract starch digestibility (TTSD) was unaffected (P = 0.22) by length of cut

The effect of starch digestibility is interesting. Many producers believe that fine chop corn silage can reduce the need for kernel processing. But the authors concluded that “kernel processing or the interaction between kernel processing and length of cut are more important than length of cut alone with respect to TTSD”.

More recent research by Filho et al. (2022) examined the physical effectiveness of corn silage particles retained on the 19-mm and 8-mm sieves of the Penn State Particle Separator (PSPS). Researchers designed a control diet with low forage NDF levels (17% NDF). The study diets increased forage NDF from 17% forage NDF to 26% NDF with the inclusion of:

Fine material (less than 8 mm)
Medium material (8-18 mm)
Long material (more than 19 mm)

What are the results?

Rumination per kilogram of dry matter and rumen pH show that forage NDF is somewhat effective. This is true even when corn silage particles are smaller than 8 mm.
Total chewing for each kilogram of NDF and blood LPS indicates something important. NDF from corn silage particles that are smaller than 8 mm is not very effective. In contrast, NDF from corn silage particles between 8 mm and 19 mm is more effective. This is based on the 8 mm sieve of the PSPS.
Corn silage particles larger than 19 mm reduced milk and component yields. They also decreased total chewing and encouraged sorting against long particles. This was compared to corn silage particles that stayed on the 8 mm sieve.

In a nutshell, optimizing forage particle size

To create a homogenous TMR that reduces sorting, ensure the forage has enough physical structure. It should allow for good chewing, but you should not chop it too finely to prevent rumen problems. Generally, harvest forages at a TLC between 10 and 22 mm, ideally between 12 and 20 mm. This range may vary based on species, fragility, maturity, and dry matter content of the crop (Figure 1).

Figure 1: Optimal length of cut depending on forage type & quality

A nutrition expert can help you find the right balance. They can ensure good rumen function and diet digestibility for your specific conditions.

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Reference

Physical effectiveness of corn silage fractions stratified with the Penn State Particle Separator for lactating dairy cows Journal of Dairy Science Volume 106, Issue 9, September 2023, Pages 6041-6059. F.A. Piran Filho, J.M. Bragatto, C.S. Parra, S.M.S. Silva, P.J. Roco, L.F. Ferraretto, M.N. Pereira, J.L.P. Daniel

Published Apr 14, 2025

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