What Is Forage Particle Size and Why Does It Matter?
Forage particle size is the measured length of chopped forage, typically after it passes through a harvester, chopper, or TMR mixer. It is commonly described using the Penn State Particle Separator or a similar sieve system that sorts feed into fractions: long, medium, fine, and very fine.
Why it matters:
- Rumen health: Adequate long fiber stimulates chewing and saliva production, buffering the rumen and preventing acidosis.
- Intake control: Very fine forage can pass through the rumen too quickly, reducing rumen fill and potentially causing overeating of grain.
- Sorting prevention: When particle size is inconsistent, cattle often pick out preferred components, leaving behind fine particles or long stems, which unbalances the diet.
- Feed efficiency: Properly processed forage improves digestibility without sacrificing effective fiber.
How Cutting Length Affects Sorting Behavior
Sorting occurs when cattle use their muzzle to push away long, coarse stems or fine powder to selectively eat the tastiest or most comfortable particles. This behavior is strongly influenced by forage particle size distribution:
- Excessively long forage: Cattle tend to toss long stems out of the bunk or leave them behind. This reduces effective fiber intake and wastes feed.
- Overly fine forage: A ration with too many fines (< 4 mm) encourages sorting of grains and pellets while fine fiber is discarded. This can lead to sub-acute ruminal acidosis (SARA) and inconsistent dry matter intake.
- Ideal distribution: A good mix of particle sizes keeps cattle engaged without allowing easy sorting. The ration stays blended during consumption.
Sorting not only wastes nutrients but also creates individual cow variation—some animals get a high-grain meal while others get mostly stems, affecting milk production, growth, and health.
Recommended Forage Particle Size Targets
Below is a general guide for target particle size distributions using a Penn State–style three-screen separator (19 mm, 8 mm, 4 mm) plus a bottom pan. Actual targets may vary by diet, moisture, and animal condition. Always work with your nutritionist to fine-tune recommendations.
| Animal Group | Top Screen (≥19 mm) | Middle Screen (8–19 mm) | Bottom Pan (<4 mm) | Notes |
|---|---|---|---|---|
| Lactating dairy cows (TMR) | 10–15% | 30–50% | ≤40% | Enough long fiber for a healthy rumen mat; avoid >20% on top to reduce sorting. |
| Dry dairy cows | 15–25% | 30–40% | 30–40% | Higher long fiber to maintain rumen fill and function during transition. |
| Beef feedlot cattle | 5–10% | 20–35% | 50–60% | High-concentrate rations require less long fiber, but adequate effective fiber is critical. |
| Sheep and goats | 20–30% particles >4 mm | – | – | Often use a 4-mm screen only; limit fines to prevent acidosis. |
Extension guidelines often recommend that 50–60% of particles in a dairy TMR fall between 4 and 19 mm for optimal intake and rumen health.
How to Check Forage Particle Size on Your Farm
Regular checking takes 10–15 minutes and helps catch processing shifts before animals sort. Use a particle separator such as the Penn State shaker box. Follow these steps:
- Collect a representative sample. Take 1–2 liters of fresh TMR or chopped forage from several spots along the bunk. Do not compress it.
- Place the sample on the top screen. Shake horizontally in a consistent motion (e.g., 5–6 shakes per screen) according to separator instructions.
- Weigh each screen’s contents. A small digital scale works. Calculate the percentage on each screen and the bottom pan.
- Compare to your target ranges. If the top screen exceeds 20% for dairy cows, many will sort. If fines exceed 40%, acidosis risk increases.
- Repeat after any major adjustment to the chopper or mixer, and when changing fields or harvest maturity.
Also, visually check the bunk 1–2 hours after feeding. If you see piles of long stems or fine powder left behind, sorting is happening.
Adjusting Your Forage Harvester or Chopper for Better Particle Size
Several machine settings influence the final particle size of chopped forage. While exact names vary by equipment, the principles are similar:
- Cutting speed (knife velocity): Higher cutterhead speed shortens the cut, but too fast can shatter dry forage and create excess fines.
- Shear bar (ledge) clearance: A tight shear bar gap (e.g., 0.5–1.0 mm) gives a more precise, consistent cut. A wider gap can tear long pieces.
- Theoretical length of cut (TLC): This is set by the feed roll speed relative to the cutterhead. Shorter TLC produces more uniform but potentially finer forage.
- Screen or recutter screen: Some choppers use a screen to recut particles that are still too long. Changing the screen hole size lets you control the top end of particle length.
- Crop maturity and moisture: Overly dry forage shatters into fines regardless of settings. Adjust harvest timing and moisture content for best results.
According to the textbook Forage Conservation and Feeding (5th Edition, Chapter 4, pp. 85–92), the theoretical length of cut should be set based on the type of forage and the animal’s nutrient requirements to balance intake and rumen health.
Common Mistakes When Managing Forage Particle Size
- Cutting too long to “help the rumen.” Overly long stems encourage sorting and reduce actual fiber intake.
- Grinding too fine for convenience. Fine forage can pack in the mixer and feed out well, but it often leads to rumen acidosis and low milk fat.
- Ignoring sorting in the bunk. If you don’t watch what’s left behind, you might miss an obvious problem.
- Not rechecking after harvest changes. A new field, different hybrid, or drier conditions can shift particle size even with the same machine settings.
- Using one setting for all forages. Grass hay, alfalfa silage, and corn silage each behave differently in the chopper. Adjust accordingly.
- Neglecting TMR mixing time. Over-mixing breaks particles down further, while under-mixing can leave long stems intact and poorly distributed.
Final Takeaway: Match Particle Size to Your Feeding Goals
Forage particle size is not about hitting a single exact number—it’s about a distribution that discourages sorting, supports rumen health, and promotes consistent intake. Start by measuring what you have, compare it to target ranges for your animals, and fine-tune your harvester and mixer settings step by step. Even small adjustments in cutterhead speed or shear bar clearance can make a noticeable difference in the bunk. Regular checks keep the ration consistent and help you catch sorting before it becomes a health or performance problem.
References
- University of Minnesota Extension corn silage harvest strategies
- Penn State Extension forage chop length guidance
- Penn State Extension particle separator guide
- Penn State Extension total mixed rations for dairy cows
Frequently Asked Questions
Particle size changes how animals chew, sort, and digest the ration. Pieces that are too long may be sorted out, while overly fine material can reduce effective fiber.
Look for long stems left in the trough, uneven feed refusals, aggressive nose sorting, and changes in manure. Sorting means animals are not eating the ration as it was mixed.
No. Shorter forage can improve mixing, but too much fine material may reduce rumination and fiber effectiveness. The target length should fit the ration, animal type, and mixer performance.
Blade sharpness, feed rate, screen size, moisture level, machine speed, and material type all affect particle size. Wet or tough forage may cut differently than dry straw.
Check when changing forage source, moisture, machine settings, or ration formula. A simple visual check at the trough can reveal sorting before production performance changes.
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- Chaff Cutter Safety and Maintenance Checks for Blades, Feed Rollers and Covers
- Straw Crusher for Cattle Feed: When Fine Crushing Helps and When It Hurts
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