Effect of Feed Particle Size on Cud Rumination in Ruminants

1. Overview of the Relationship

2. Mechanics of Cud Rumination

• Trigger for rumination: The rumen's fibrous mat (large floating particles) stimulates reticular/rumen wall mechanoreceptors, initiating the rumination reflex. Larger particles create a denser, more stimulatory mat.
• Goal of rumination: To reduce particle size below the "escapable threshold" (~1-2 mm) so particles can pass through the reticulo-omasal orifice.
• Salivary buffering: Each chewing cycle produces saliva rich in bicarbonate and phosphate buffers. Longer rumination = more saliva = better rumen pH buffering, reducing risk of subacute rumen acidosis (SARA).

3. Effect of Particle Size on Rumination Time

General Pattern

• 4.5 h/day eating (range: 2.4-8.5 h)
• 7 h/day ruminating (range: 2.5-10.5 h)
• Maximum total chewing time: ~16 h/day

• Long particles (≥19 mm) predominantly increase eating time
• Medium particles (4-19 mm) and forage NDF intake predominantly affect rumination time
• Finely ground or pelleted feeds drastically reduce both eating and rumination time

Physical Effectiveness Factors (Mertens, 1997)

4. Dietary Particle Size and Rumen pH

• Increasing peNDF content increased ruminating time (h/day)
• Chopped hay vs. ground hay: cows on chopped hay had more time at rumen pH >6.2 and less time at pH <5.8 (acidosis threshold)
• Feed particle size was moderately correlated with ruminating time but not with eating time
• peNDF was negatively correlated with the area-under-the-curve below pH 5.8, confirming its protective role against acidosis

5. Sorting Behavior and Particle Length

• Total rumination time was not significantly different across diets when peNDF was held constant
• Cows sorted against longer particles with the sorting behavior increasing linearly as particle length increased
• Greater forage particle length increased sorting behavior but had no effect on rumen fermentation or total chewing behavior (when peNDF was matched)
• This means it is peNDF, not absolute particle length alone, that drives rumination

6. Forage Quality × Particle Size Interaction

• High-quality forages (low uNDF240 - undigested NDF at 240 h) break down more quickly due to plant fragility, reducing eating time even if cut long
• Low-quality forages (high uNDF240, e.g., mature hay, straw) take more chewing to reduce particle size, increasing both eating and rumination time
• The bolus (cud) particle size at re-swallowing is remarkably consistent (~10-11 mm) regardless of incoming feed particle size - cows chew to a uniform endpoint before swallowing

7. Measuring Feed Particle Size in Practice

• Long (>19 mm): target 2-8% of DM in total mixed rations (TMR)
• Medium (8-19 mm): target 30-50%
• Small (1.18-8 mm) and fine (<1.18 mm): remainder

8. Species Differences

• Dairy cattle and beef cattle: Well-studied; the peNDF framework applies directly
• Sheep and goats: Smaller rumen, finer particle size threshold for passage; goats show more variable responses - a study on goats showed that reducing peNDF did not significantly affect chewing activity in some settings, contrasting with cattle (Animal Bioscience review)
• Buffalo: Similar to cattle; long-stem roughage strongly stimulates rumination
• Camelids (camels, llamas): Modified "pseudo-ruminants" with a 3-compartment stomach; particle size effects on rumination are qualitatively similar but less studied

9. Practical and Clinical Implications

Key References

• Beauchemin KA (2018) - Eating and rumination review - PMID 29627250 - Comprehensive review of dietary/animal/management factors affecting chewing behavior
• Grant RJ (2023) - Physical characterization and peNDF system - PMID 36400612 - Historical development and future directions of the peNDF framework
• Beauchemin KA et al. (2003) - Alfalfa particle size effects - PMID 12647969 - Direct measurement of rumination time, rumen pH, and peNDF
• Suarez-Mena FX et al. (2013) - Forage particle length and chewing - PMID 23031313 - Sorting behavior and its interaction with particle size
• Mertens DR (1997) - Creating a system for meeting the fiber requirements of dairy cows. J Dairy Sci 80:1463-1481 - the foundational peNDF paper

Research Papers: Effect of Feed Particle Size on Cud Rumination in Ruminants

REVIEWS & FOUNDATIONAL PAPERS

1. Beauchemin KA (1991)

2. Mertens DR (1997)

3. Heinrichs AJ, Buckmaster DR, Lammers BP (1999)

4. Grant RJ (1997)

5. Martz FA & Belyea RL (1986)

CONTROLLED EXPERIMENTS - DAIRY CATTLE

6. Grant RJ, Colenbrander VF, Albright JL (1990)

7. Krause KM, Combs DK, Beauchemin KA (2002)

• Fine chopping decreased rumination time from 485 to 320 min/day (a 34% reduction)
• Fine forage plus fermentable starch: rumen pH dropped to 5.81 vs. 6.02 with coarse forage
• Time at rumen pH <5.8 more than doubled with fine-cut forage (6.4 to 11.8 h/day)
• peNDF intake (particles on top PSPS screen) correlated strongly with rumination time (r = 0.61)

8. Beauchemin KA, Yang WZ, Rode LM (2003)

• Increasing peNDF significantly increased ruminating time (h/day)
• Chopped vs. ground hay: more time at rumen pH >6.2 and less time at pH <5.8
• peNDF negatively correlated with area-under-the-curve below pH 5.8 (acidosis index)
• Particle size was moderately correlated with ruminating time but NOT with eating time
• Milk production was not affected by particle size, but rumen health clearly was

9. Teimouri Yansari A et al. (2004)

• Decreasing particle size reduced rumination time, eating time, rumen pH, and milk fat
• Finer particles had higher bulk density, faster hydration, and shorter rumen mean retention time (RMRT)
• Faster passage rate with fine particles = less time for fiber digestion in rumen
• NDF digestibility decreased with finer particles
• Demonstrated that particle size affects both chewing behavior and rumen kinetics simultaneously

10. Zebeli Q, Tafaj M et al. (2007)

• In low-concentrate diets: increasing particle size (6→30 mm) significantly increased rumination time and rumen mat consistency
• In high-concentrate diets: coarser hay extended rumination by 100 min/day and increased rumen mat
• BUT in high-concentrate, coarser hay did NOT improve rumen pH or fibrolytic capacity - fiber digestion shifted to the hindgut
• Demonstrated that rumination time alone is insufficient to assess dietary physical effectiveness when concentrate levels are very high

11. Yang WZ, Beauchemin KA, Rode LM (2001)

12. Haselmann A et al. (2019)

• Reduced particle size increased DMI by +1.8 kg/day
• Reduced eating and ruminating time per kg DMI (4.8 and 1.9 min/kg, respectively)
• Total-tract digestibility improved with fine particles
• Energy-corrected milk yield was higher (+2.3 kg/day) on reduced particle size
• Important nuance: in very high-forage diets, reducing particle size can actually improve performance despite lower rumination efficiency per unit DM

SORTING BEHAVIOR AND PARTICLE LENGTH

13. Suarez-Mena FX, Zanton GI, Heinrichs AJ (2013)

• Total rumination time was NOT different when peNDF was matched across particle lengths
• Cows sorted against longer particles, with sorting increasing linearly as particle length increased
• Confirms that peNDF, not raw particle length, is the key driver of rumination behavior
• Longer particles actually create sorting problems that may reduce effective fiber intake

14. Suarez-Mena FX, Lascano GJ, Heinrichs AJ (2013)

SMALL RUMINANTS (SHEEP & GOATS)

15. Krone B, Hummel J, Riek A, Clauss M, Hünerberg M (2024)

• Sheep showed higher rumination:eating (R:E) ratio than goats - sheep ruminate proportionally more
• Sheep had a higher R:E ratio for short hay than long hay - suggesting sheep compensate for reduced particle stimulation by ruminating relatively more
• Goats consumed more long hay than short hay (preference)
• More daily boluses, higher chewing frequency, and more chews per day in sheep vs. goats
• No species difference in fecal particle size, suggesting similar end-point comminution
• Discusses evolutionary context: sheep may have adapted to abrasive diets requiring more ruminal "washing" before effective chewing

PARTICLE RETENTION & PASSAGE KINETICS

16. Zhang X, Li Y, Terranova M et al. (2023)

• Higher chewing intensity (not just frequency) was associated with shorter MRT in the reticulorumen - more effective particle size reduction allows earlier passage
• Chewing behavior could serve as a non-invasive proxy for MRT
• Highlights that individual cow variation in rumination behavior is substantial and clinically meaningful

17. Singer M, Codron D, Lechner I et al. (2024)

• Particles that escaped without being ruminated had shorter MRT than ruminated particles
• Higher intake led to a higher proportion of intact (non-ruminated) particles passing through - not faster rumination, but more particles bypassing the rumination cycle entirely (pressure-driven escape at high gut fill)
• The critical size threshold for forestomach escape is body-size dependent across ruminant species
• Reindeer retained 1 mm particles longer than cattle, muskoxen, and moose - reflecting body size effects on rumen morphology

peNDF SYSTEM DEVELOPMENT

18. Grant RJ (2023)

• Reviews how forage particle size influences chewing activity, ruminal pH, volatile fatty acid profiles, and milk fat
• Traces the peNDF system from its origins to incorporation into nutrition models (NRC, NASEM)
• Highlights the corn silage fragmentation index as an extension for starch availability assessment
• Identifies improved physicochemical evaluation of rumen fiber degradation as the top future research priority
• Concludes that starch and fermentation pH interact with peNDF to determine minimum fiber requirements

19. Beauchemin KA (2018)

• Cows spend ~4.5 h/day eating and ~7 h/day ruminating (max total chewing = 16 h/day)
• Long particles (≥19 mm) primarily affect eating time; medium particles (4-19 mm) primarily affect rumination time
• Particle size reduction during eating/rumination results in a bolus of relatively uniform ~10-11 mm regardless of original feed size
• Reviews use of electronic sensors (noseband sensors, accelerometers) for real-time monitoring of rumination on commercial farms
• Discusses how decreased rumination time can flag early illness, estrus, and parturition

SUMMARY TABLE