Due to the emphasis on rising milkfat percent, total milkfat production, and the value of milkfat, many dairy producers are looking for ways to capture better feed margins and greater production efficiency. While diet changes are important in this quest, some new research is compelling our dairy industry to rethink the impact of ration particle size. This is important to forage production as a majority of the desired particles are contributed by silages and hay. Chop length, forage maturity, and forage moisture content are critical factors that need to be considered.

The Penn State Particle Separator has been utilized for a couple of decades to evaluate the particle size of dairy rations. It has been recommended that 2% to 8% of the ration feed particles should be trapped on top of the 19-millimeter (mm) screen. This has been thought to provide enough long particles to stimulate adequate rumination without causing issues with diet sorting.

Feed particles on the top screen should be 1 to 2 inches in length, as longer lengths result in more sorting of the diet. However, new research is showing that the amount of feed particles retained on the second screen (8 mm) is actually more strongly correlated with rumination than the amount of feed particles retained on the top screen. The percentage of particles retained on the top screen is associated with lower intake, less energy-corrected milk, and lower milkfat percent. Therefore, data is showing that herds achieving high milkfat content are being fed diets that have more than 50% of the feed particles retained on the 8-mm screen.

Dairy animals need to spend three to five hours each day consuming the diet. It is critical that the diet particle size is not too fine or too coarse, as diets with a coarse particle size will result in longer feeding time and reduced feed intake. The hours spent ruminating are also critical to enhancing milkfat content. New research on cow behavior has shown that the highest correlation with better milkfat production is rumination time while lying down. Cows with greater rumination time while lying down have higher rumen pH, consume more feed, and produce milk containing greater concentrations of milkfat and protein. This may be because rumination results in an increase in saliva production, and saliva provides essential buffers to maintain a higher rumen pH, resulting in better fiber digestion, and thus, milkfat production.

Support more milkfat

Today, many of our Holstein herds are producing milk with greater than 4.2% milkfat. This has been a growing trend over the past 10 years, and it is the result of several factors, including fat supplementation, genetics, diet supplements, and careful attention to forage quality and ration particle size. Due to the economic value of milkfat, we will continue to push for higher milkfat production, and thanks to this advancement and new research, we must reevaluate how we utilized the Penn State Particle Separator for a dairy diet compared to the previous recommendations discussed above.

Current recommendations from the Miner Institute would indicate that only 2% to 5% of the diet particles should be retained on the top screen and that the maximum particle length does not exceed 2 inches. Longer particles tend to extend eating time and exacerbate sorting.

The second screen should retain 50% to 60% of the total diet particles. This will have the greatest impact on rumination time and rumen pH, and these factors are positively correlated with milkfat production.

About 10% to 20% of the feed particles should be retained on the third screen (4 mm). However, when feeding diets containing 40% to 50% concentrate, 25% to 30% of the particles will be found in the pan. Diets with significant amounts of fibrous co-products, such as corn gluten feed, may result in higher amounts retained in the pan and less retained on the second screen. This may reduce the rumination time since the particles in the pan do not stimulate rumination due to their small size. Only the feed particles retained on or above the third screen provide effective fiber to stimulate rumination.

Based on recent research, the amount of particles retained on the second screen is most important to rumination. If you discover that your diets need additional feed particles on the top of the second and third screens, reassess the particle size of your silages and forages. For example, extending the chop length of corn silage to 3/4-inch and adjusting kernel processing can boost the number of particles retained on the second screen.

Other implications

While diet particle size may help set cows up for success, some other items are also required. Animals must have a comfortable resting area that provides adequate space. Diets should be available 24 hours per day with intentional access to water. And during the summer months, providing adequate heat stress abatement is critical to prevent heat-stressed animals from standing rather than lying down. Failure to provide these items will have a negative impact on rumination while lying down, limiting the positive impact that correct diet particle size can have on milkfat production.

If you are striving to push milkfat higher in your dairy herd, you might want to meet with your nutritionist and reevaluate how you can use the Penn State Particle Separator to gain some valuable insight into the relationships between particle size, cow behavior, and milk fat production. Having the correct combination of particle sizes in the ration is critical to maximizing the genetic potential of your herd to produce greater amounts of milkfat.

This article appeared in the April/May 2025 issue of Hay & Forage Grower on page 12.

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