Better digestibility with reduced particle size?

By Wyatt Smith

In the last few years, we’ve taken a more critical look at the relationship between forage fiber particle size and forage digestibility.  Researchers at the Miner Institute in New York have explored the concept of reducing particle size of forages with lower fiber digestibility to improve animal performance.  They evaluated dramatically reducing the particle size of timothy hay.

Before we get to those results, let’s review a couple key forage terms.

peNDF and uNDF240
Forage passage rate is a paradox.  Forages that pass faster help drive dry matter intake (DMI).  Forages that pass slower or, in effect, are retained in the rumen longer, have greater digestibility.  As a result, we need forage passage rates that are not too slow or too fast to optimize total nutrient digestibility.

• peNDF:  One of the simple measurements used to characterize forage fiber is the particle size of physically effective neutral detergent fiber (peNDF).  This is the portion of NDF that stimulates rumination.
• uNDF240:  Undigested NDF after 240 hours of fermentation (uNDF240) is the primary fiber digestibility measurement currently used to characterize forages and diets.  The fiber residue remaining after 240 hours of in vitro fermentation cannot be degraded by rumen bacteria; thus, it is not utilized by the cow and is deemed indigestible.

What the research showed
One of the key findings of the Miner Institute research was that, when diets contained greater levels of uNDF240, reducing the peNDF of one of the forages of one of the forages in that diet resulted in increased passage rates, allowing for greater intakes.  Before implementing such a strategy on farm, let’s consider some of the research specifics.

In these experiments, timothy hay was the only ingredient to have its particle size altered to adjust peNDF supply in the treatments.  The results indicate that shortening forage particle size in the diets with greater uNDF240 has the potential to increase DMI, but the type of forage may dictate the degree of size reduction needed.

Grasses (such as timothy) and legumes have very different ruminal dynamics.  Michigan State University researchers compared forage passage rates as well as the rumen dynamics of alfalfa silage and orchardgrass silage and found that diets containing the orchardgrass silage had significantly slower passage rate for smaller fiber particles.  They found that more than 55% of the particles within the rumen were less than 2.36 millimeters for both silages, indicating that particle size was most likely not the sole limiting factor of passage.

They attributed this difference in passage to the selective retention of grass forage particles.  This means that the physical form of grass forages has a great effect on rumen retention time.  The thought is that grass forages spend more time in the rumen because their rope-like nature entangles in the rumen mat to a greater degree than alfalfa.  Thus, the particle size reduction of alfalfa may not need to be to the same degree as grass in order to gain the same benefit.

What about corn silage, which generally makes up a significant portion of dairy diets?  Corn silage is a C4 grass, so its NDF fractions differ from timothy, which is a C3 grass.  But, when compared to alfalfa, corn silage will exhibit ruminal dynamics similar to timothy due to its physical form.  Understanding this, we can anticipate that moderately reducing corn silage particle size may increase DMI.

What should I do?
Because particle size reduction research is still relatively new, it is difficult to provide exact recommendations for legume and corn silage particle size reduction.  Data suggest that reducing grass particle size may improve DMI, but that doesn’t give the green light to pulverize forage particle size in the hopes of gaining maximum intake.  This can result in forage fiber that passes too fast and a reduction of utilization by the cow.  Work closely with your nutritionist to dial in the particle size of your specific forages to best match your farm’s unique nutrition program.

This article was originally written for the September 12, 2020, issue of Progressive Dairy.  Click here for the full article. 

About the author:  Wyatt Smith is a Vita Plus dairy specialist in northwest Wisconsin and southeast Minnesota.  He grew up on his family’s 180-cow registered Jersey farm in Minnesota.  He attended the University of Minnesota and earned his bachelor’s degree in animal science with a focus on dairy production.  He then attended the University of Vermont to earn his master’s degree in animal science and dairy cattle nutrition, in cooperation with the William H. Miner Agricultural Research Institute.  His research focused on evaluating the physical and digestibility characteristics of fiber.