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Acidosis is also known as "grain overload".  As the common name, grain overload suggests, the condition is a sudden intake of too much grain or concentrate.  This high level of grain causes a change in the rumen acidity and bacteria population.  The increase in acid causes an imflammation of the rumen wall (rumenitis) and a reduction of bacteria needed to digest fiber. 
Symptoms may include:
Animals off feed
Founder (stiffness, or tender feet)
Occasionally death.

Occasionally lambs develop a thiamine (B vitamin) deficiency (polio) due to the inability of rumen to produce adequate thiamine in the highly acidic environment.`

Bicarbonate of soda can be added to rations or self fed to help buffer the rumen.

Ewes require 1.5 to 2 pounds of forage to maintain healthy rumen function.

Lambs need 1/2 to 1 pound of forage.

From the article Sheep, Grain and Acidosis link is listed below
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