There is an adage in racing that punters should back grey horses on wet tracks.

But what happens when the whole field are greys?

Those who supported the highest-rated horse in Thursday's Subzero Challenge at Flemington were rewarded when bush gelding Slate On Edge ($9) raced to a convincing three length win on a soft track.

The Subzero Challenge (1400m) is an annual race during the Melbourne Cup carnival restricted to grey horses and is the culmination of a three-race series during the spring.

Slate On Edge, trained by Brett Thompson in the NSW country town of Gulgong, was having his second attempt at winning the Oaks day race after finishing 10th last year.

He was runner-up in the series heats at Caulfield and Moonee Valley but went one better on Thursday with an all-the-way win under Dwayne Dunn.

"I rode him in this race last year and he bombed the start on me," Dunn said.

"We didn't have a fair go that year, so Brett has brought him back again and he's in great form.

"He's got a high rating for consistency and today he really loved the ground and extended nicely."

Thompson's son Jay represented the stable at Flemington and said Slate On Edge had been set for the Oaks day race.

"It's a big thrill bringing him all the way down here," he said.

Slate On Edge turned the tables on Secret Toy Bizness who won the second heat at Caulfield last month, while Harbour Grey was another length away third.