A cow spends a significant amount of time during any given day chewing on forage particles. The ability to chew down forage particles into a manageable size for digestion is foundational in the utilization of forage nutrients for milk.

The response to chew is determined by both the physical and chemical characteristics of forages, says Rick Grant, director of the Miner Agricultural Research Institute in Chazy, N.Y.

Physically, these forage characteristics include particle size, fragility, and the speed of forage breakdown. From a chemical standpoint, moisture content of the particles assists in swallowing, while neutral detergent fiber (NDF), lignin, and crosslinking determine the length of time a cow must spend chewing.

In Miner’s July Farm Report newsletter, Grant cites Italian research showing that dairy cows only chew enough to be able to swallow the bolus or cud. Researchers used wet sieving and image analysis to find that the swallowed bolus was around 10 to 11 millimeters (mm). Although the particle length of the various test forages ranged from 9 to 44 mm, the mean bolus size was ultimately the same for ryegrass hay, grass silage, corn silage, and a total mixed ration (TMR).

“Feeding long forage to dairy cows does not necessarily boost particle size in the rumen beyond the size of the swallowed bolus of feed,” says Grant.

Too-coarse forage can be easily sorted, and forages higher in NDF or longer forages also lengthen the time it takes to consume the feed. Grant states that the same Italian research group found that chews per gram of NDF varied from 0.4 to 3.5, depending on NDF content and particle size.

Basically, a long forage particle size extends the time needed for eating. Having longer forages may actually put cows at a disadvantage, depending on the competition for feed and bunk management.

“Forages with mean particle length greater than 10 to 11 mm take the cow longer to chew and swallow, and the question becomes whether or not that extra time at the feedbunk is available,” Grant states.

If cows are overstocked and feedbunk space is limited, Grant stresses that adequate time to eat becomes an important challenge, especially for younger cows that aren’t as effective at processing forage fiber as mature cows. He emphasizes the importance of being vigilant in feedbunk management to ensure that cows have sufficient time to effectively ruminate, as NDF quality and particle size varies.

Grant’s silage-based TMR particle size recommendations are presented in the table below. He notes that there may be room for future modification. Following the guidelines, dairy producers will be able to provide a ration that:

• Cows can consume within three to five hours per day

• Doesn’t require excessive ingestive chewing to form and swallow the bolus

• Promotes desirable ruminative chewing based on percentage of particles retained on the 4-, 8-, and 19-mm sieves of the Penn State Particle Separator

• Minimizes sorting of long fiber particles retained on the 19-mm sieve

Recommended particle distributions for as-fed TMR using Penn State Particle Separator

Sieve opening, mm

PSPS 2013

% of as-fed

Revised PSPS 2017

% of as-fed





Sortable particles; may be too long and increase time needed for eating.




Functional based on physical effectiveness factor, more so than 4-mm material; maximize amount of particles retained on this sieve at 50 to 60%.




Functions as physical effectiveness factor sieve for calculating physically effective NDF; total of 4-, 8-, and 19-mm sieve particles is the physical effectiveness factor.




Forty to 50% concentrate diets result in at least 25 to 30% of particles in the pan.

Lauren Peterson

Lauren Peterson is serving as the 2017 Hay & Forage Grower summer editorial intern. She is from Wyanet, Ill., and currently attends Kansas State University where she is pursuing a degree in agricultural communications and journalism. While at school, Lauren works at the KSU dairy farm and is an active member of the Horseman’s Association.