An unusual throat problem has forced the premature retirement of talented filly Assail who is being offered for sale as a brood mare.

Her disappointed trainer Clarry Conners said the three-year-old was no longer a racing proposition after efforts to fix the condition failed.

"She has had two operations but they can't fix it so she won't race again. The vets tell me they only see one of these a year," Conners said.

"They say it could be caused by hay or something else getting caught in the epiglottis and infection setting in.

"She can run around the paddock just fine but under the pressure of a race she just can't get enough air down.

"She is just not competitive as a racing proposition so I am trying to find a buyer to get the best for the owners who were very patient with her.

"She is also in the brood mare sale at the end of the month.

"It's a shame because she just has so much talent."

Assail was the raging favourite for the $2 million Magic Millions in January only to be withdrawn on race morning with a deep seated stone bruise.

Conners' attempts to get her to the Golden Slipper failed but he had high hopes for a spring campaign until the final diagnosis.