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.
The plants in this division are more numerous than those of the preceding, in spite of the facts to which attention was directed. (See p. 17). Several circumstances are to be considered in this connection. Some poisonous plants thrive in early spring and are shrivelled and gone before the time for cutting hay arrives. Others lose much of their poisonous character with maturity, and some contain volatile poison which evaporates and is lost during the curing of the hay. In addition to these there are numerous poisonous plants which are never cut with hay but are often found in pasture and eaten by animals with fatal results.