Random drug tests will begin next year for horses in polo matches in the United States after the deaths of 21 elite horses in Florida that were injected with an incorrectly mixed supplement shortly before a championship match, the United States Polo Association said Wednesday.

The decision was made last month to begin a pilot program for testing horses starting in January, said association spokeswoman Amber Owen.

The Lexington, Ky.-based organization put together a committee to study the possibility of testing shortly after the South Florida deaths in April. The random testing will be mandatory.

Horses from the Venezuelan-owned Lechuza Caracas team began collapsing April

19 at the International Polo Club Palm Beach in Wellington as they were unloaded from trailers before the match. Some died at the scene, others hours later.

Florida's top veterinarian later ruled the deaths were caused by an overdose of a common mineral that helps muscles recover from fatigue. State Veterinarian Thomas J. Holt, DVM, said at the time that toxicology tests on the dead horses showed significantly increased selenium levels. (Read more: "State Vet: Selenium Overdose Likely Cause of Polo Pony Deaths.")

A Florida pharmacy that mixed the brew of vitamins and minerals for the team has acknowledged that the strength of selenium was incorrect in the product it delivered. The compound also contained vitamin B, potassium, and magnesium, and is similar to a name-brand supplement known as Biodyl, which is used around the world in the sport to help horses recover from fatigue. It hasn