Last week I made a tongue-in-cheek comment about feeding SPINACH to the chickens as a source of calcium. I was not serious, and since no one else commented on it, I want to make sure no one took me seriously. DO NOT feed large amounts of Spinach to chickens. Like rhubarb leaves, it contains high amounts of oxalic acid which binds iron and calcium. Some report that it leaches calcium from their bones. You can get more info. by doing an internet search.
Here is a quote regarding oxaltes in rhubarb, taken from a document on plant poisoning of livestock from the University of Vermont Extension:
"Some oxalates are insoluble and cause local irritation but the oxalates in rhubarb (and other
species, such as sorrel or dock, Rumex) are soluble, and cause systemic problems, especially
in the kidneys, or they can affect the electrolytes in the body, such as the balance of calcium
and magnesium. Poisoning can be acute, when large amounts of oxalates are consumed
quickly, or may be chronic, where smaller amounts are eaten over a longer period of time.
Low blood levels of calcium and kidney failure are commonly reported findings in soluble
oxalate toxicity. Affected animals will appear depressed, and may stagger and tremble and be
weak. Often, they will drink and urinate more as kidney function declines. Diarrhea may be
noted, and affected animals may die if the electrolyte balance is extremely deranged or if the
kidneys fail."