Our local feed dealer has deer salt blocks on clearance, and I was wondering if there is anything in the block that could be harmful to my horse. The packaging of the blocks state they are made with real acorns. The other ingredients are 93-98% salt, 1% calcium, and 0.8% phosphorus.
Giving horses feeds or products designed for other species is usually not a good idea unless you have the ingredients verified by a nutritionist or veterinarian. In this case, the only ingredient that may be of concern in the salt block would be acorns (the nuts of oak trees). However, the percentage of acorns in the block is probably minimal (less than 5%). A horse would probably not consume more than a few ounces per day, and the resulting intake, equivalent to less than one acorn, would not be enough to cause any problems, especially if the horse has access to plenty of forage.
What may be a concern is whether the horse will lick the block at all. Acorns are typically bitter, and horses not accustomed to them might not lick the block. In light of these concerns, I would suggest purchasing a salt block intended for horses.
Acorn toxicosis occurs when horses consume large amounts of acorns, though it is highly unlikely in the case of the salt blocks you mentioned in your question. Acorns contain tannins, though the amount varies based on the maturity of the nut. Green acorns have much higher levels of tannins than ripe ones. Tannins are also found in other parts of oak trees, especially tender buds, young leaves, and bark. Because the tannins are associated with a bitter taste, horses do not usually consume these things unless there is a shortage of forage. Rarely, a horse will develop an unusual obsession for acorns.
Signs of acorn poisoningare initially depression, lack of appetite, and colic. Other signs may include emaciation, poor or rough hair coat, increased thirst, mouth ulcers, frequent urination, bloody urine, constipation with hard, dark feces or bloody diarrhea, edema of the abdomen and legs,excessive salivation, slow or irregular heart rate, elevated temperature, pale mucous membranes, watery eyes, and sometimes choke with saliva passing out of the nostrils.
The type and severity of clinical signs are dependent on how much tannin was ingested and the duration of the poisoning. Tannins are astringents, so they draw fluid out of tissues. Bloody diarrhea is caused by the destructive effect tannins have on the intestinal lining by dehydrating and damaging cells, thus drawing abnormal amounts of fluid into the intestinal tract. Not only do the tannins destroy the intestinal tract, but they can be absorbed and cause excessive damage in the liver and kidneys. This damage may not be immediately apparent but will be obvious within a week of ingestion. Sometimes the horse will die within a day of eating large quantities of acorns, or it may live for five to seven days after the onset of clinical signs. If the liver and kidneys are severely affected, death occurs about 85% of the time unless treatment is administered.
There is no cure for acorn poisoning, and it is usually handled by giving supportive care. Once poisoning is suspected, it is important to remove the horse from the acorn source. Intravenous fluid therapy will help counteract the dehydrating effect of the tannins on cells, flush the toxins from the organs, and prevent kidney failure. Administration of paraffin, mineral oil, calcium hydroxide, or activated charcoal by stomach tube has been recommended to stop further damage from occurring in the intestinal tract and to bind the toxins. Encouraging the horse to eat hay and drink water will also dilute the toxins in the digestive tract. If the horse is colicky or painful, analgesics can be administered.