Some things to keep in mind is that many of these plants can and will be eaten by sheep, some are even quite tasty to them. Depending on the amount of toxic plant included in their diet may result in no outward signs of toxicity or symptoms mistaken for something else.
A good example is Oak. In the Fall sheep love the crunchy "potato chip" like leaves. It's important to remember that when we reference toxicity, we're speaking of an abundance consumed. The takeaway is that if your sheep are starved for fresh grasses or hay, and they turn to toxic substances to make up over 50% of their diet over a protracted period of time you'll likely experience issues.
Another example is lantana - often found in Texas as a decorative flower that can attract sheep and cause significant damage or death.
Below the toxic plants list are some excellent sources of additional information.
In the list below you will see a few extra's labels listed on some plants, these labels are for:
(Alk) - Alkaloid Containing Plants. Class of organic, nitrogen-containing ring compounds that have a bitter taste, that are usually water-insoluble and alcohol-soluble, that form water-soluble salts, and usually exhibit pharmacological actions, as nicotine, morphine, or quinine.
(Cya) - Cyanogenetic Containing Plants. These types of plants are usually deadly when damaged or frozen.
(Gls) - Glucosides - Glycosides Containing Plants. Any plants in this class have compounds that yield a sugar and a noncarbohydrate upon hydrolisis.
(Int) - Plants That can cause mechanical injury. Self-explanatory really but some plants and shrubs have physical characteristics that can cause injuries to animals. Example, thorned plants. Thorned and spiked plants may be eaten in some cases but once they reach the palate, punctures and tears can occur internally which cause a great degree of injury. Also, certain plants are known to twine or bind causing great intestinal difficulties.
(Pht) - Photosensitizing Plants. These types of plant will cause a reaction where the ingredients interact with light. An animal ingesting such a plant is susceptible to sunburn, heat related illnesses and more. Not all photosensitizing are considered extremely harmful, dependent upon climatic conditions/light, this class of plants can do great damage if the animal is not monitored.
(Vol) - Volatile/Essential Oils Containing Plants. Plants, trees and shrubs that contain volatile chemicals that go beyond a general classification. This simply means that there may be one or more ingredients within the chemical structure of a plant that causes adverse reactions in the animals who ingest it. More of a warning.
Additional notes: Tannic Acid Containing Plants, such as Oaks. Oak trees contain Tannic Acid, as do their acorns and can be harmful. Resin Containing Plants, such as discarded Christmas trees. More of a warning to those considering feeding cuttings or trees. Resin containing plants may not produce immediate, noticeable results, it can be a cause of abortions months later.