WinStar Farm, a leading Thoroughbred racing, stallion and breeding operation in Versailles, Kentucky, has announced the death of their breeding stallion Pioneerof the Nile.

In statement on their website, they said:

Pioneerof the Nile died suddenly today. The 13-year-old son of Empire Maker—Star of Goshen, by Lord at War (ARG), bred a mare this morning and started acting uncomfortable once he was back in his stall. He was en route to the clinic when he died.

“We are all extremely saddened by the loss of Pioneerof the Nile,” said WinStar Farm’s Elliott Walden. “He was a superior physical specimen, a Triple Crown Sire, and a unique personality. All of us at WinStar are heartbroken.”

A multiple Grade 1-winning millionaire on the racetrack, Pioneerof the Nile is the sire of two 2-year-old male Eclipse Award winners and two Breeders’ Cup winners from his first four crops to race—Horse of the Year and Triple Crown winner American Pharoah and Classic Empire.

Pioneerof the Nile was a leading sire of his generation for the past three years, including No. 1 sixth-crop cumulative sire in every category: Grade 1 winners, G1 horses, graded stakes winners, black type winners, earnings, etc.