lecture 13 - starch digestion and rumen acidosis

How does decreasing grain particle size affect starch digestion?

It increases ruminal and total-tract starch digestion, especially for corn

How does grain type affect ruminal starch digestion?

Corn (~75%) < barley (~82%) < wheat (~88%)

Which grain varieties are more digestible: floury or vitreous?

Floury varieties

How does grain processing improve starch digestion?

It breaks the pericarp and disrupts prolamins and starch structure

How does faster passage rate affect ruminal starch digestion?

It decreases ruminal starch digestion

How does greater dietary starch affect ruminal starch digestion?

It decreases the proportion of starch digested in the rumen

Where does starch digestion occur in ruminants?

Rumen, small intestine, and large intestine

What percentage of consumed starch is typically fermented in the rumen?

Approximately 70–100%

How efficiently is starch digested in the small intestine?

Approximately 60% of the starch entering it

Why is starch fermentation in the large intestine undesirable?

It is inefficient and can cause health problems

What is the overall goal of starch digestion?

Maximize ruminal starch fermentation

What is fecal starch used for?

As an inexpensive indicator of dietary starch digestibility and availability

What is the relationship between fecal starch and milk production?

A 1-percentage-point increase in fecal starch is associated with about 1 lb/day less milk production

What happens to rumen pH after an animal eats?

pH decreases as fermentation produces acids, then rises as acid production slows and acids are absorbed or used

What is considered a healthy rumen pH range in the lecture?

Above 5.6, with very little time below 6.0

What is subacute ruminal acidosis (SARA)?

Rumen pH remains between 5.0 and 5.6 for multiple hours per day

What happens to fiber digestion during SARA?

Fibrolytic bacteria cannot fully recover, so fiber digestion decreases

What causes the rumen pH to drop during SARA?

Faster VFA production and/or less salivary buffering

What dietary factors can cause SARA?

Too little chemical or effective fiber, too much starch, and sorting against long particles

What management factors can contribute to SARA?

Overmixing, large meals, feed-bunk competition, overcrowding, heat stress, and reduced rumination

Why does increased intake raise acidosis risk?

It can increase fermentation and acid production

Why can changing to a higher-energy diet increase acidosis risk?

Rumen papillae may be underdeveloped after a lower-energy diet, limiting acid absorption during the transition

What is acute ruminal acidosis also called?

Lactic acidosis

Why is acute ruminal acidosis a life-threatening emergency?

Rapid lactic acid accumulation can sharply lower rumen pH, damage tissues, and cause systemic illness or death

Which bacteria commonly produce lactic acid in the rumen?

Streptococcus bovis and Lactobacilli

What normally prevents lactic acid from accumulating in the rumen?

Lactic acid-utilizing bacteria rapidly convert lactate to propionate

Name the main lactic acid-utilizing bacteria.

Propionibacteria, Selenomonas ruminantium, and Megasphaera elsdenii

What can happen if an unadapted animal is suddenly switched to a very high-starch diet?

S. bovis grows rapidly and shifts toward lactate production, causing lactic acid to accumulate

Why is lactic acid more dangerous to rumen pH than VFAs?

It is about 10–12 times stronger as an acid

What happens when rumen pH drops below 5 during acute acidosis?

S. bovis can no longer thrive, but Lactobacilli can produce more lactic acid, worsening the acidosis