All kinds and all parts of the oak can be poisonous, but typically the fall is when we see problems in this area. Acorns are especially a problem during dry summers, when forages are scarce, or in very wet falls, when heavy rains dislodge them. Calves are the most severely affected, then adult cows, then sheep and goats. Deer are resistant to oak poisoning, which may be due to certain chemicals in their saliva called proline.
Animals affected by oak poisoning usually stop eating and become constipated with dark, mucus covered stools. Later they may have a bloody or very dark, tarry diarrhea. During this time they get very thirsty, and are commonly found in or around water. Usually they separate themselves from the rest of the herd. They typically lose body condition rapidly, and may get edema, such as bottle jaw. Bloody noses have also been seen. This poisoning does not happen quickly; it may take one to two weeks before you notice changes in the animal.
This poisoning is much easier to prevent than to cure. Obviously avoiding acorn covered pastures or regular cleaning may be the simplest way. It is difficult to supply feed to prevent acorn ingestion since some animals seek them out. Some people have promoted the addition of calcium hydroxide or proline containing feeds such as corn gluten during the fall to prevent problems. This is obviously something to speak to your veterinarian about before attempting.
Fall is a beautiful time of year, but it can be dangerous for your livestock. Now is the time to check your pastures if you have not already, for prevention is always much easier, and cheaper, than trying to treat a plant poisoning.