updated 5:22 p.m. ET April 17, 2008

About 40,000 thoroughbreds are born in the United States each year. About 23,000 make it to the race track. Only 20 run in the Kentucky Derby. A new documentary, “The First Saturday in May,” follows six hopefuls on their path to Churchill Downs, including the late Barbaro.

Filmmaker brothers Brad and John Hennegan focused their handheld cameras and wireless mics on trainers Frank Amonte, Dan Hendricks, Dale Romans, Michael Matz, Bob Holthus and Kiaran McLaughlin along the 2006 Derby trail.

The movie opens Friday in 20 cities, including Los Angeles, New York, Chicago and the racing capitals of Louisville and Lexington, Ky. It earned awards at film festivals in Savannah, Ga., and Austin, Texas.

“This film is almost like a 101 for horse racing,” Brad Hennegan said in a phone interview Thursday. “If you’re a fan you can bring someone there and say, ‘This is why I like it.’ It’s a film about people that just happens to be set at the racetrack.”

The brothers shot about 500 hours of film during a 150,000-mile journey that included stops in Southern California, New York, Kentucky, Florida, Arkansas and Dubai. Their travels culminated at Churchill Downs on the first Saturday in May, when undefeated Barbaro won by an amazing 6½ lengths — the largest winning margin in 60 years.

The colt broke down in the Preakness two weeks later and was euthanized in January 2007 from complications of his injuries.

“This film could have been made 131 times in a row and we happened to pick the most dramatic year for it to get made,” Hennegan said.

Five of the six horses in the 90-minute film actually made it to the Derby. Matz trained victorious Barbaro; Hendricks trained Brother Derek and McLaughin saddled Jazil, who tied for fourth; Holthus guided Lawyer Ron to 12th; and Romans oversaw Sharp Humor to 19th. A leg injury knocked Amonte’s horse, Achilles of Troy, out before the hard-scrabble trainer could get to Louisville.

In a poignant moment, Amonte tells his horse, “I’m 48, I want to go to the Derby before I die.”

In another scene, Matz coaches his young son Alex on how to pet Barbaro, urging the boy to stand closer and not be afraid of the colt. Near the end of the film, Barbaro’s little brother appears, not having made it to the racetrack yet.

Cameras poke into the barns, cars and homes of the trainers and their families, their horses and their stable help from pre-dawn workouts to race days, when emotions run high in victory and defeat.

“When you get to be my age you don’t have to go to the plastic surgeon,” Holthus’ wife Bonnie says as Lawyer Ron wins a Derby prep in the film, “it’s an instant face lift.”

By contrast, the camera captures Amonte’s profane reaction when Achilles of Troy is vanned off in an ambulance and his stable help realizes they won’t be part of the Derby fanfare.

“We put a mic on these guys and stayed back. After about 10 minutes they would forget we were there,” Hennegan said. “We were able to get such intimate stuff because there was no crew. It was just us and these guys knew we knew our way around horses a little bit.”

The Hennegan brothers — Venice, Calif.-based Brad is 36 and Brooklyn-based John is 39 — spent their formative years at racetracks in Maryland and New York, where their grandfather and father held various positions.

“What stuck with my brother and I were the amazing characters you can find at the racetrack,” said Hennegan, who recalled meeting tycoon Alfred Vanderbilt and musician Cab Calloway as a child. “You’d meet all these guys and you don’t know the social stratas, some are going to Newport, Rhode Island, and some are smoking Newports.”

The brothers held jobs doing everything from scooping manure to ice cream (“Not at the same time,” Hennegan noted) at the track.

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After college, they started working in television and film, always wanting to make their own film that involved racing.

“We’re not the Coen brothers or the Warner brothers or the Weinstein brothers,” Hennegan said. “We decided our best route was to make a documentary.”

So they quit their jobs and followed the Derby trail from July 2005 to the Breeders’ Cup in October 2006. They shopped around a sample tape of their project to potential investors.

“A lot of people weren’t interested, but we found a couple people who believed in us and believed in the story and had a little disposable income,” Hennegan said.

The film is being distributed through Truly Indie, a company co-owned by Dallas Mavericks owner Mark Cuban which helps place films lacking major studio backing into theaters.

Truly Indie plans to donate 25 percent of the opening weekend box office receipts to equine research.

“I think the whole Barbaro thing helped shed light on the sport,” Hennegan said. “So much was learned while treating him. Maybe that’s the good that comes out of all this.”

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