The Importance of Chop Length to the Cow

Forage quality is arguably the most important factor in cattle diets. It not only helps determine animal productivity, but forage quality also plays a role in animal health and manure composition. It has been estimated that by 2050, the demand for meat products will increase by 60% and milk products by 30% (Revell 2015). How are we planning to meet this increase with our current cattle populations? One of the answers is by storing our forages properly and presenting them in the most desired way for the cow to be most efficient.

Size does matter when it comes to the particle size of forage entering the cow's rumen. When we think of how the cow’s rumen will "sort" particles, there are different levels. Particles are free to float throughout the rumen, but factors such as density and specific gravity will determine where they go. Dry matter content, fiber type, and surface area also play a role in this. This all leads to something called particle stratification, which is essential for adequate rumen function, rumen motility, and to support a healthy microbial population. The stratification layers start at the top, or what we consider part of the rumen mat. Particles that float to the bottom are considered a part of the ruminal floor, and finally, we have what floats around in the middle of the medium degradation area. I like to think of all these levels working together like scales. Cattle nutrition, after all, is a balancing act. We like to see specific percentages in each of these sections, in order to keep balance and to keep the rumen bugs happy. So, how do we know how much should be at each level?

To answer this question, Penn State came up with a tool called the Penn State Particle Separator that mimics a rumen. We can use this tool to identify the different particle sizes in our rations and which forages have the potential to stay in the rumen, which supports motility, and which supports good pH levels. It's a simple box-shaped design that has multiple sieve layers to sort particles based on length.

What we are looking at in the forage when we perform this test in the Particle Separator is called the Theoretical Length of Cut for the fiber particles. Ideally, Penn State recommends optimal lengths of cut to be: corn silage 3/8 inches to ¾ inches unprocessed or ¾ inch for processed. In addition to the fiber particles, we would like to see greater than 70% kernel breakage after processing in corn silages. For haylage or small grain silage: ½ inches to ¾ inches is recommended. Remember, this is optimal. There are many other factors that may influence the length of cut, such as the type of crop, the moisture at harvest, and the objective during feeding. I also like to remind producers that we can always make forages shorter, but we cannot make them longer! Always air on the side of caution when adjusting your chop length.

So, why is this important for the cow? Well, size matters to the rumen, remember? We want the levels of the rumen to be balanced for healthy function and digestion. When we analyze a diet in the particle separator, we are looking at percentages in each sieve. After a lot of research in this area, Penn State has been able to provide some recommendations. If looking at a TMR, for example, Penn State recommends that the upper sieve should contain 2-8%, the middle sieve should contain 30-50%, the lower sieve should contain 10-20% and the bottom pan should have 30-40%. See the chart below.

Overall, if we have a diet with a particle size that is too small, we can see some negative effects in the cow, such as a lack of rumination and milk fat depression. We can also see issues within the crop being stored, such as excessive seepage and nutrient losses. On the other hand, if our diet contains particles that are too long, poor fermentation in the silo can occur, which could lead to less feed intake and less milk production for the cow. In addition, the long particle size could also negatively affect intakes and promote sorting behavior in the cow.  It is important to highlight that these particle distribution general recommendations are based on typical lactating cow diets that are fed in the Northeast.  Therefore, they may serve as a good baseline to strive for when harvesting this fall. If you are experiencing issues with milk production or animal health, this may be a good area to assess.

After all, meat and milk production start with good forages. What you make today is what your cows will eat for the next year. Good quality forages start in the field and continue into storage. Quality is then dependent on how the forages are presented to the cows. Let’s take the right steps for the cows to become efficient and effective for your business!

References

• Heinrichs, Jud, and Coleen M. Jones. “Penn State Particle Separator .” Penn State Extension, Penn State Extension, 19 Dec. 2022. Accessed 25 Aug. 2025.
• Revell, Brian. “Meat and Milk Consumption 2050: The Potential for Demand-Side Solutions to Greenhouse Gas Emissions Reduction.” EuroChoices, vol. 14, no. 3, Dec. 2015, pp. 4–11, doi.org/10.1111/1746-692x.12103.