...specific problem with brassicas is the prescence of GOITROGENIC compounds as well as some SAPONINs andThiaminase (which destroys thiamin) dependent upon which specific vegetable (and
part of the vegetable being eaten ) and the quantity... sometimes cooking may help, sometimes it is best just to avoid
Info below is from
www.finchinfo.com but is applicable to all birds incl poultry :
"....Protease inhibitors inhibit digestive enzymes and, when present in high levels in the diet, may decrease the avian body's ability to digest proteins and lead to pancreatic hypertrophy. Although these enzyme inhibitors are present to a degree in all plants, significant levels are found in all legumes, corn, lettuce, oats, peas, peanuts, barley, beets, buckwheat, wheat, rice, rye, turnips, sweet potatoes and potatoes (in VERY high amounts).5 Luckily protease inhibitors are readily inactivated by cooking,5 so if any of the above-listed food sources are going to constitute a major part of your birds' diet, you should cook them first.
Acorns, lettuce, carrots, rape seed, grape seeds, bananas, spinach, onions, milo, grapes, and raisins have high levels of tannins. (Rhubarb, tea, coffee, and chocolate also contain high levels of tannins but these foods should never be fed to birds anyway as they may be harmful or toxic). Tannins can inhibit digestive enzymes, bind protein, and reduce the bioavailability of vitamin B12 and iron.5 Additionally, at high levels, tannins can cause epithelium and liver damage.5 Tannins (tannic acids) are the chemicals that are responsible for the normal browning of fruits and vegetables that have been bruised or cut. If feeding any of the above-listed "safe" foods that contain tannic acid, only feed them in moderation.
Oxalate (oxalic acid) is an organic acid makes calcium and other trace minerals unavailable to birds because it binds them.5 Lower levels of oxalate can cause reduced growth, kidney stones, and poor bone mineralization.5 Peas, beets, beet greens, lettuce, turnips, carrots, and berries have lower levels of oxalates; high levels of oxalates can cause diarrhea, poor blood clotting, vomiting and convulsions.5 Very high levels are found in spinach (tea also contains very high levels of oxalate, but it should not be fed to birds due to its caffeine content).
The complex of phosphoric acid and sugar is called phytate or phytic acid. This complex effectively forms a heterocyclic ring with metal atoms (minerals such as iron, zinc, and calcium), making the minerals unavailable.5 Phytates are found in nuts, cereal grains (germ and bran), and legumes. Phytic acid is also present in green beans, berries, carrots,
broccoli, potatoes, and sweet potatoes, but at lower levels.
.When Safe Foods May Become Unsafe
Any of the foods listed in this section that are normally considered safe
may cause problems for a bird if fed in high quantities. Therefore, keep in mind that moderation is key. "
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