lecture 11 - factors affecting fiber digestibility

What factors affect fiber digestibility?

Intake, passage rate, particle size, forage:concentrate ratio, plant type, variety, and maturity.

How does passage rate affect fiber digestibility?

Slower passage increases digestibility; faster passage decreases it.

How does intake affect passage rate?

Lower intake → slower passage → greater fiber digestibility.

How does particle size affect fiber digestibility?

Longer particles increase rumen retention and digestibility.

How does a high-concentrate diet affect fiber digestion?

Faster passage, less fiber digestion, and increased intake.

What is the order of fiber component digestibility?

Pectin > hemicellulose > cellulose > lignin (0)

What makes a forage more digestible?

More pectin and hemicellulose; less cellulose and lignin.

What is the apparent total-tract NDF digestibility range?

Approximately 40–60%.

What is the apparent total-tract ADF digestibility range?

Approximately 30–50%.

Why is ADF less digestible than NDF?

ADF contains cellulose and lignin but excludes more digestible hemicellulose.

How do grasses and legumes differ in digestion?

Legumes digest faster but to a lower extent; grasses have a longer lag phase.

How does plant maturity affect fiber digestibility?

Maturity increases cellulose and lignin and decreases pectin and hemicellulose, lowering digestibility.

How does plant variety affect digestibility?

Some varieties are genetically more digestible than others.

What happens to larger fiber particles in the rumen?

They remain in the fiber mat until broken down, then leave the rumen.

What is the average particle size of feed leaving the rumen?

About 1.3 mm.

How does poor-quality fiber affect intake?

It fills the rumen and limits intake.

How does moderately digestible fiber affect intake?

It exits faster, allowing greater intake and energy consumption.

Where does most NDF and ADF digestion occur?

In the rumen; approximately 90%.

Can intestinal fermentation compensate for reduced rumen fiber digestion?

No

What nutrients do fibrolytic bacteria require?

Ammonia and branched-chain VFAs.

What rumen pH do fibrolytic bacteria need?

Relatively high pH; digestion decreases below 6.7 and is substantially impaired at 6.0 or lower.

Why is rumen buffering important?

It maintains a pH suitable for fiber-digesting microbes.

Why are fibrolytic bacteria sensitive to low pH?

VFAs enter cells, release H⁺, and force bacteria to spend energy removing acid.

What happens when acid accumulates inside fibrolytic bacteria?

Growth and fiber breakdown decrease; excessive accumulation causes cell death.

How do acid-tolerant organisms respond to low pH?

They allow intracellular pH to fall and tolerate more acidic conditions.

What are the 4 Rs of rumination?

Regurgitation, rechewing, reinsalivation, and reswallowing.

What does rechewing accomplish?

Reduces particle size and increases surface area.

How much do dairy cows ruminate per day?

About 8–9 hours across approximately 20 bouts.

How much saliva does a dairy cow produce daily?

About 50 gallons.

What is the main buffer in saliva?

Sodium bicarbonate (NaHCO₃).

How does rumination support rumen health?

Chewing stimulates saliva and bicarbonate, helping maintain rumen pH.

Which forage stimulates the most saliva production?

Hay

How does hay compare with grass and silage for saliva production?

Hay: 3.6; silage: 1.1; fresh grass: 0.9 g saliva/g feed.

What are the disadvantages of very long forage particles?

Slow passage, reduced intake, and increased sorting.

What are the disadvantages of very short forage particles?

Less rumination and saliva, faster passage, and potentially lower fiber digestion.

What particle sizes do cows tend to select?

Smaller particles, which are higher in starch and lower in fiber.

What happens when cows sort against long particles?

Some cows may consume an imbalanced diet.