A:   There is not enough evidence for a diagnosis of a poisoning, but we can't rule it out either.

The first poison that comes to my mind when calves die is lead. Anything odd visible at necropsy in the rumen content or downstream from the rumen in the GI system? We have diagnosed cases of lead poisoning, even though the animal owners swear that there is no way the animals could have been exposed to it. Calves can find things as they explore their environment that are not obvious to us humans. A liver and kidney specimen from the dead animal could be analyzed for lead to explore that possibility.

Regarding the possibility of death due to poisonous plant ingestion, what was found in the rumen at necropsy? Was any forage present, and if so, did any of it look like something other than the grass on which they were run? A plant-containing rumen content specimen could be examined for evidence of poisonous plant exposure.

Any evidence of a recent algae bloom in the pond? The time of the year is right for blue-green algae blooms, which could poison animals exposed to the bloom. There is one blue-green algae poison that damages the liver, and from what was mentioned in the message, it does not sound like that poison was involved. There are two other blue-green algae poisons that would not produce visible lesions at necropsy. However, if there is no evidence present in or around the lake of an algae bloom, then the possibility of a blue-green algae poisoning is slim, at best.

Was there any evidence of dehydration in the calves? Did they know where their water source was, especially if they had been recently separated from their dams? Did they have access to a salt block or mineral supplement? Water deprivation/salt toxicity may be a long shot, but it might be worth considering.

Did the calves have access to anything that has an ionophore in it, especially if the product was offered free choice?