Veterinarians and researchers from New Zealand Food Safety Authority (NZFSA) found the proprietary feed was contaminated, but not to a fatal extent for horses. Hence, the actual cause of death remains unknown.

They found no evidence of exotic disease, infectious agent, or poisoning from 1080  (sodium monofluoroacetate) or selenium. The chaff and copra was cleared of any risk.

Allergy in horses
Horses are known to be allergic to monensin  and lasalocid,  known as ionophores,  commonly used in feeds manufactured for poultry and ruminants such as cattle, pigs and sheep, but are not registered for use in horse feed.

The manufacturer, whose name has not been released, has been ordered to improve manufacturing procedures, and its products are being monitored by the NZFSA.

Too low levels
An NZFSA official says the levels of monensin were not judged sufficient to cause sudden death in the two horses, but the presence of any ionophore in horse feed was considered an indicator of potential contamination. No horse feed should contain monensin.

Compliance action was taken against the manufacturer, which changed the manufacturing process for new batches of the feed. This is being monitored by the NZFSA.