Ruminal acidosis. What changes does it entail?

Ruminal acidosis continues to be a common digestive disorder in beef cattle and can lead to marked reductions in cattle yield.

This disease reflects an imbalance between the microbial production of volatile fatty acids (VFA) and their ruminal absorption.

The severity of acidosis, usually related to the amount, frequency, and duration of grain feeding, ranges from acute acidosis due to lactic acid accumulation, to subacute acidosis due to accumulation of volatile fatty acids in the rumen.

Microbial changes in subacute acidosis resemble those observed during adaptation to grain feeding. A declining population of ciliated protozoa is a common feature of both forms of acidosis and can be a good microbial indicator of an acidotic rumen. Other microbial factors, such as the presence of endotoxins and histamine, are suspected of contributing to systemic effects of acidosis.

The reticculo-rumen is an anaerobic ecosystem in which microbial digestion of food converts fermentable substrates into organic acids mainly. These are then mostly absorbed.

As long as the substrate availability is not excessive and the absorption rate is maintained at the level of production, rumen fermentation remains stable and rumen pH ranges from 5.8 to 6.5 in grain-adapted cattle.

Rumen pH fluctuates considerably over a 24-hour period and is influenced by:

• Fermentable carbohydrate intake.
• The inherent ability of the animal to provide buffer substances
• And acid use and absorption rates

In beef cattle fed with highly concentrated diets, the animal’s ability to buffer the rumen is limited by inadequate salivary secretion. If the absorption capacity of the rumen wall is affected by abnormal rumen papillae or rumenitis, the animal’s ability to maintain a stable rumen pH is affected.

Rumen pH is a critical factor in the normal and stable function of the rumen due to its profound effect on microbial populations and fermentation products. It  is also crucial for mantaining physiological functions of the rumen like motility and absorption.

When the ruminal pH drops below 5.6 there is a change towards lactic acid producing microbial populations, which reduces ruminal pH even more.

• • • Lactic acid is about 10 times stronger than VFAs (pKa 3.9 vs. 4.9). Therefore, it is less protonated and accumulates in the rumen, contributing to the downward spiral of rumen pH.

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