This section is from the book "A Guide To The Poisonous Plants And Weed Seeds Of Canada And The Northern United States", by Robert Boyd Thomson, H. B. Sifton. Also available from Amazon: A guide to the poisonous plants and weed seeds of Canada and the northern United States.
Certain animals are poisoned by hay, straw, ensilage, etc., when they could have pastured with impunity on the fields where the feed was raised. There are several reasons why this is so. In the first place, animals in pasture have a better chance to pick their food. The plants grow up separately and distinct from each other, and there is usually enough wholesome feed to allow of a choice. It thus often happens that a weed, refused by an animal in pasture, may be eaten when cured and mixed with the hay. This is especially so where the amount or concentration of the ration is insufficient. Again, certain weeds which are eaten with impunity in pasture may cause trouble when eaten by an animal confined to a non-succulent ration. The laxative nature of the green grass and the freedom of exercise give the animal in pasture or on the range a much better chance to overcome the effects of toxic substances. It may also be pointed out that moulds and bacteria grow on forage under conditions that would rarely if ever obtain in the field.