The author is president of the William H. Miner Agricultural Research Institute, Chazy, N.Y.

More and more I get questions about the proper chop length for silages and what is a desirable particle distribution for a total mixed ration (TMR) when using the Penn State Particle Separator. Recommendations for TMR particle size have changed substantially in recent years, as we have better understood how particle size affects both eating and ruminating times. To better answer this question, let’s focus on key concepts to bear in mind as we seek to optimize the particle distributions in TMR.

Don’t overdo the top

Let’s consider the role of particles that are retained on each sieve of the Penn State Particle Separator. Probably everyone reading this article has shaken out forages and TMR (total mixed ration) samples with the Penn State boxes, but how often do we really think about what the feed particles retained on each sieve do?

Analysis of TMR particle distributions begin with the top sieve, which has openings of 0.75 inches (19 mm). The longest particles in a ration or forage are retained on the top sieve. These particles can be sorted by cows if too long, and ideally, most should not exceed 2 inches or so in length.

Traditionally, it was common to see 10% to 15% or even more of the TMR particles on this top sieve. But we now realize that a target of 2% to 5% is better for the cow. Rations with too much coarse material, especially if it exceeds 10% on the top sieve, can extend eating time beyond the natural three to five hours per day. This challenge becomes more difficult as the forage ratio in the diet goes up.

Research tells us that cows will typically chew forage and TMR particles while eating to a fairly uniform particle size of 8 to 11 millimeters (mm) before swallowing the bolus of feed. We have measured as much as a sixfold reduction in the longest TMR particles during eating in our studies. This takes time and effort for the cow to accomplish, and the net result can be lengthy eating times that cut into resting and ruminating times.

The key point to remember is that the particles retained on the top sieve of the Penn State Particle Separator can influence eating time at the feedbunk as much or more than they influence rumination. Cows take the time and expend the effort to reduce TMR particle size prior to swallowing while eating, and so they populate the rumen with a particle distribution that can be much more uniform than the TMR itself.

Second sieve chewing time

The second sieve of the Penn State Particle Separator has openings of about 0.3 inches (8 mm), and this is the fraction of coarser particles that we really need to focus on. These particles, although shorter than those retained on the top sieve, are still long enough to be effective at stimulating rumination. These particles are also quite similar in size to the swallowed feed bolus while eating. Consequently, they are more easily processed and swallowed by the cow while eating. A TMR should contain about 50% to 60% of the particles on this second sieve.

Here is where the cow’s daily behavioral time budget is affected. The balance between the long particles retained on the top sieve and the particles on the second sieve will affect the balance between eating time and resting and rumination time for the cow.

Hitting the targets for these two sieves (2% to 5% for the top sieve and greater than 50% for the second sieve) should translate into three to five hours per day eating time and about 12 hours per day resting time . . . with the majority of the eight or so hours of ruminating occurring while the cow is lying down. That combination of chewing behaviors promotes greater dry matter intake, higher rumen pH, and milk with greater fat and protein content, according to recent research.

The third sieve is often called the “pef,” or physical effectiveness factor. This sieve has openings of 0.16 inches (4 mm). The pef is the fraction of forage or TMR particles that are coarse enough to stimulate chewing, and the pef is multiplied by the neutral detergent fiber (NDF) content of the forage or TMR to obtain a physically effective NDF (peNDF) value.

About 10% to 20% of the TMR should be retained on this pef sieve. Even though the particles are smaller, most are still effective at stimulating chewing. Adding the fraction of particles together from the top three sieves of the Penn State Particle Separator gives us the pef for the forage or TMR.

For example, if 5% of a TMR is retained on the top sieve, 55% on the 8 mm sieve, and 15% on the 4 mm sieve, then the pef to use in calculating peNDF would be 0.75 (0.05 + 0.55 + 0.15). So, for a ration containing 30% NDF, the peNDF would be 22.5%.

Back to the beginning

In the original system developed by Dave Mertens in the 1990s, the pef was measured with dry samples and vertical sieving in a laboratory using a 1.18 mm screen. Since then, we have found that, with common silage-based TMR, the 4 mm sieve of the Penn State Particle Separator often gives pef values similar to the standard dry sieving method.

Measuring the pef of forages and TMR has become a routine part of ration formulation and TMR assessment. With typical rations, we often seek to have a peNDF content of about 19% to 21% of dry matter when the NDF has been measured using amylase and reported on an ash-free basis.

When assessing a TMR for particle size, a pef can be calculated by adding together the top three sieves, but we also need to make sure that these physically effective particles are optimally distributed. For best total chewing response that includes both eating and ruminating, there should be 10% to 20% of particles on the third sieve, 50% to 60% on the second, and only 2% to 5% on the top sieve.

Ration particle size

When taking advantage of these recommended TMR particle distributions, we also need to remember that a ration is always fed as part of an overall feeding system. The first step is mixing a ration with a particle distribution that minimizes the risk of sorting long particles and allows a healthy balance between eating time and resting rumination.

Don’t forget that feed needs to be kept in front of the cow 24 hours a day, seven days a week, and it must be within easy reach. Feeding frequency, feed push-up timing, and amount of feed refusals all play a role. Likewise, the TMR needs to be paired with comfortable stalls so that once cows are done eating, they can easily lie down and ruminate. Remember that in a cow’s ideal world, 80% or more of daily rumination occurs while lying down. Rumination while resting results in healthier rumen pH.

Balanced chewing

The best TMR feeding system includes the proper ration particle distribution to optimize the cow’s total chewing response (eating and ruminating) coupled with unhindered access to feed and ample opportunity to lie down in comfortable stalls. Research tells us that this will boost dry matter intake, rumen health, and milk components. The bottom line is that balanced chewing means more milk components.