Effect of Different Feed Particle Size on Cud Rumination in Ruminants

1. Overview of Rumination Physiology

1. Regurgitation of a softened feed bolus (the "cud") from the reticulum back to the mouth
2. Re-mastication - grinding the bolus to reduce particle size further
3. Re-salivation - mixing with alkaline saliva rich in bicarbonate and phosphate buffers
4. Re-swallowing - passing finely ground, well-buffered material back into the rumen

2. The Particle Size-Rumination Relationship

2.1 Large/Long Particles - Maximum Rumination Stimulus

• Eating time increases substantially - animals must work harder to initially masticate coarse material
• Cud chewing time increases during rest periods
• Total chewing time (eating + ruminating) is maximized
• Saliva production increases proportionally (50-80 quarts/day in adult cows)
• Rumen pH is better buffered and maintained (typically ~6.0)
• Rumen mat formation - long particles form a physical fiber mat ("rumen raft") that traps smaller particles and slows passage rate, allowing more complete fermentation

2.2 Fine/Short Particles - Reduced Rumination

• Less saliva production and reduced rumen buffering
• Lower rumen pH, predisposing to sub-acute ruminal acidosis (SARA)
• Lower milk fat percentage in dairy cattle

3. The Physically Effective NDF (peNDF) Concept

"Historical research had shown that forage particle size influences chewing activity, ruminal pH, volatile fatty acid profiles, and milk fat percentage... peNDF enabled ration formulation to meet the minimum fiber requirements of ruminants to maintain ruminal pH and milk fat."

• Total chewing time (eating + ruminating) increases linearly with peNDF content
• Rumination alone shows a tendency to decrease as peNDF increases (because much of the increase comes from eating time, not cud chewing time)
• Forage-to-concentrate ratio has a stronger effect than particle length alone in modifying rumen pH (Yang and Beauchemin, 2009)

4. Effect of Different Feed Types/Particle Categories

4.1 Long-Stem Dry Hay

• Maximum eating time and cud chewing time
• Strongest rumination stimulus
• Highest saliva output
• Best rumen buffering
• May reduce lying/resting time (competition between eating and resting behavior)

4.2 Silage (Corn/Grass)

• Particle length is highly variable depending on theoretical length of cut (TLC)
• A TLC of 12-22 mm is a general practical recommendation for dairy cows
• Corn silage processing (kernel processing score) also affects fermentation; fine processing improves starch availability but reduces peNDF
• Kononoff et al. (2003) showed altering corn silage particle length alone did not always affect rumination times, suggesting silage fermentation characteristics interact with particle effects
• Alfalfa particle length manipulation (Kononoff and Heinrichs, 2003) did affect ruminating time and rumen pH, with a quadratic response - intermediate particle length produced the highest rumination times

4.3 Total Mixed Rations (TMR)

• In TMR diets, sorting behavior is a major concern - animals may selectively consume fine particles (concentrates) and leave coarse forage
• This creates an effective particle size reduction for the individual animal, independent of diet formulation
• Uniform TMR particle distribution reduces sorting and maintains more consistent rumination stimulation

4.4 Concentrate/Grain-Based Diets

• Grain and concentrate are fine-particle, rapidly fermentable feeds
• They contribute minimal peNDF
• Increasing concentrate from 33% to 82% to 144% of diet resulted in a linear decrease in ruminating times (Robinson and McQueen, 1997)
• Forage:concentrate ratio of 35:65 vs. 60:40 - the higher forage diet increased rumination time, increased milk fat, raised mean rumen pH, and reduced time spent below pH 5.8 (Yang and Beauchemin, 2009)

4.5 Straw / Low-Quality Long Forage

• Very high peNDF, maximum rumination stimulus
• However, also low digestibility - very high uNDF240 (undigested NDF after 240 h in vitro fermentation)
• Animals spend excessive eating time without proportional nutritional return
• Mechanically reducing particle size of low-quality hay can actually improve DMI and milk production without sacrificing adequate rumination

5. Asymmetry: Decreasing vs. Increasing Particle Size

• Decreasing particle size has more impact on cud chewing than increasing particle size does
• Increasing particle size mainly increases eating time, not cud chewing time

• The cud bolus expelled from the reticulum has a surprisingly consistent particle size regardless of diet particle size - it's the reticulum that determines bolus composition, not the original feed
• Very fine particles may literally fail to trigger the reticulum's tactile contraction reflex, preventing regurgitation entirely
• Fine particles also pass rapidly out of the rumen, removing the substrate for rumination

6. Downstream Consequences for Rumen Function

7. Practical Recommendations

1. Minimum particle size targets: Forages should be harvested at TLC of 12-22 mm for dairy cows (adjustable for species, maturity, DM)
2. Minimum peNDF: Rations should meet minimum peNDF thresholds to sustain adequate chewing (~30-32% peNDF1.18 of DM as a starting guideline)
3. Monitor rumination as a health indicator: Healthy cows should ruminate >6-8 hours/day; <4 hours suggests dietary imbalance or illness
4. At least 2 kg of long forage equivalent per day (a rule of thumb) to maintain effective rumen buffering
5. Avoid excessive TMR sorting - uniform particle distribution in the bunk is as important as the formulated particle size
6. Forage quality interacts with particle size - finer chopping of low-quality, high-uNDF forage can actually improve intake and performance while still providing adequate rumination stimulus

Key References

• Grant RJ (2023), J Dairy Sci - Review of peNDF system development (PMID 36400612)
• Beauchemin KA (2018), J Dairy Sci 101 - Comprehensive review of eating and ruminating activity (cited 554 times)
• Yang and Beauchemin (2009) - Particle length vs. forage:concentrate ratio effects on rumen pH
• Kononoff and Heinrichs (2003) - Alfalfa particle length and quadratic rumination response
• Mertens DR (1997) - Original peNDF concept paper, J Dairy Sci
• Maekawa M et al. (2002) - Chewing activity, saliva production, and ruminal pH in dairy cows, J Dairy Sci 85:1176-1182
• Smith WA et al. (2026), J Dairy Sci - Undegraded vs. physically effective fiber (PMID 41679634)