Here's what Cornell says about comfrey: ". . . comfrey . . . contains at least eight pyrrolizidine alkaloids (PA). . . PAs are hepatoxins and can cause irreversible liver damage. One of the problems is that the effects of the alkaloids can be cumulative. Therefore, damage to the liver may not be associated to the alkaloids in comfrey. Sometimes toxicity signs will not be present until an animal is stressed by something that requires greater liver function (e.g., lactation). Also, the leaves and roots of comfrey have been shown to be carcinogenic."
Plants Poisonous to Livestock - Cornell University, Department of Animal Science
Here is a little more information about pyrrolizidine alkaloids. They put comfrey into the group including tansy. " . . . most toxic for pigs, then poultry, cattle, horses, goats and sheep, with sheep being the least susceptible. These alkaloids cause photosensitization, liver and kidney damage and can also cause cancer and heart failure."
Oregon State Unversity
So, what might we be losing by NOT feeding or eating comfrey? Looks like very little: "comfrey has lower amounts of eight amino acids that are essential for humans than turnip greens or spinach, but more than cabbage. Comfrey, like most green vegetables, is deficient in methionine and is also low in phenylalanine. Three ounces of dried turnip greens or spinach, in comparison to 20 oz of dried comfrey, supply adults with the total daily requirement of all essential amino acids, except for methionine."
University of Minnesota, University of Wisconsin, and Purdue University.
Steve
ETA: You may notice that the Mother Earth News article in the April post above is over 35 years old.
