After crossing the rugged Farewell Burn between the Rohn and Nikolai checkpoints, Mackey rested for less than five hours before leaving for McGrath, the next checkpoint 48 miles down the trail.

His team was down to 15 after Mackey dropped Hobo, the 5-year-old male who was one the handful of dogs that helped Mackey to last year's Iditarod victory as well as Mackey's last two Yukon Quest championships. Hobo won the Quest's Golden Harness Award twice.

In the hours before the race began in Willow on Sunday, Mackey had to break up a scuffle between two of his most trusted dogs, Larry and Hobo.

"Larry looks like he's aged in the last week because of it," Mackey said at the time. "He's got battle scars all over his nose. Hobo's got a few little puncture wounds up and down his forearm, which was unfortunate and something of concern."

Earlier Tuesday, five former champions chased Norway's Kjetil Backen through wet falling into Nikolai.

Backen was the first musher to reach the checkpoint, checking in at 10:39 a.m. with 15 dogs.

Backen coasted into the checkpoint with 15 dogs. Told he was in the lead, he said, "I can see that."

Backen told onlookers the going was rough on the frozen Kuskokwim River.

"I fall over but it's all right," he said. "No problem."

Veteran Gerry Willomitzer of Whitehorse, Yukon, was second in to Nikolai, population 109. Willomitzer, a veteran of several Yukon Quests, clocked in at 11:17 a.m.

Last year's runner-up, Paul Gebhardt of Kasilof, was next, arriving at 11:33 a.m. with all 16 dogs. He was followed three minutes later by Mackey.

The next checkpoint of McGrath is a Kuskokwim River community of 400. The 54-mile run from Nikolai is mostly flat, passing through stands of spruce and alders, and running on the river itself.

Race organizers said unusually high temperatures along the early stretch of the trail are a concern.

Race spokesman Chas St. George said some areas were reporting temperatures into the 40s, which "is too hot for the dogs to run in."

Last year at this time, temperatures were below zero throughout most of this portion of the trail.

This year, teams are dealing with snow instead.

"But there are no blizzard conditions," St. George said, "Everybody's moving."

About 20 dogs have been dropped so far, but there are no serious casualties, according to St. George. He said the numbers were tapering off.

"Usually in the first third of the race, mushers expect to drop a lot of dogs. This is an area with a lot of opportunities for sprains or other injuries."

Two mushers scratched Tuesday. Tom Roig of Shreve, Ohio, dropped out, citing concern for his health and the health of his team. Joe Garnie of Teller also quit, citing health issues with his dog team. A record field of 93 mushers remains.

The Associated Press contributed to this report.