Your livestock can't really avoid poisonous food in some situations.

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You as a person will never know if a plant is poisonous unless you're told by an experienced person, or a professional botanist. So what about the livestock? How can they tell this mushroom is poisonous so I won't eat?

You may assume that farm animals possess some sort of instinctive recognition ability which would allow them to know if a particular plant is toxic or non toxic. If they're developing traits or features for survival, wouldn't this be super important?

You would probably assume that the cow can see a poisonous weed by its smell just as well as any expert can determine the quality of wine by tasting. But this assumption is not exactly the case. It is all about the learned behavior rather than some innate ability.

Farm animals apply their instincts to perceive the environment around them and rely on memory. Toxic plants taste bitter. It makes animals avoid eating such foods. But the learned part can only come after the first bite. Once a sheep has eaten something poisonous, it will associate a certain smell and flavor with it. So it will avoid such foods in the future. This is something called the conditioned food aversion. An animal never understands that this food contains poison. It just knows it tastes bad.

We just hope your first is not your last, dear Cow.

The thing though about this strategy is that it will not work once an animal is extremely hungry. Poor maintenance and neglect is the biggest threats to the welfare of livestock. When an animal doesn't have grass and food sources during droughts, its instincts kind of vanish. A starving sheep would prefer eating deadly hemlock and toxic cherry leaves to dying of starvation.

Cattle are used to searching for large edible mushrooms, but so many experiments showed that animals fail to distinguish edible mushrooms from poisonous ones most of the time. So you cannot rely on the common sense of your livestock to avoid eating poisonous plants. It is up to you to identify potential hazards and take necessary measures. If a starving horse finds itself in a meadow with poisonous plants like Datura, it will not see the risk. Only a proper fence or an abundance of food will save the animal from death.

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