Broccoli. The very word can send children screaming in terror and cause adults to tremble in fear. Despite the bad press broccoli gets, it is quite edible and nutritious. Most parts of the broccoli plant can be eaten, from the immature flowers to the stalks and even the leaves. The only parts of broccoli that have been shown to be poisonous are the seeds and roots. Unless your broccoli hangs around in the back of the fridge until it blossoms and sets seed, there is nothing to fear but broccoli itself.
Broccoli may be the bane of every child's existence, but it is in fact very safe for humans to eat. The part of the broccoli most commonly eaten by humans is the immature flower. Stalks and leaves can also be delicious when steamed until softened and served like kohlrabi or cabbage. Some members of the broccoli family can be dangerous when consumed; however, broccoli is only poisonous to humans when the flowers are mature and contain seed capsules.
Broccoli is in the genus Brassica. This group includes a wide variety of plants, including the common cauliflower and cabbage and the lesser known wild mustard. Both wild mustard and white mustard can cause serious reactions in humans. These plants are truly poisonous. Symptoms run the gamut from gastroenteritis to pain, diarrhea and even upper gastrointestinal disturbances. In addition, kale, cabbage and turnip have been known to cause goiters.
Brassica and Livestock
Often, livestock eat the most dangerous kinds of brassicas. Wild and tansy mustard, field pennycress, yellow rocket and Virginia pepperweed are among the brassicas found as pasture weeds. Fortunately, pasture brassicas -- and cultivated ones, as well -- must be eaten in large quantities over a period of time before livestock begin to demonstrate symptoms. Carefully check pastures for brassicas, and destroy them as soon as they are found. All kinds of livestock, including poultry, cattle, horses, goats and sheep, can be affected by brassica poisoning.
The main chemicals in brassicas that are poisonous are glycosinolates and nitrates. When a ruminant consumes plants containing glycosinolates, it breaks them down into isoallyl thiocyanates, which is where the trouble begins. If these chemicals are synthesized in the gut, they can interfere with thyroid function and reduce production of thyroxine. Nitrates, on the other hand, are constructed within plant material in periods of plant stress. Excess nitrate intake interferes with the ability of hemoglobin to carry oxygen, leading to abdominal pain, labored breathing and a blue color in mucus membranes. Nitrates in the rumen can also cause vitamin A deficiency.