Every spring, assistant trainer Cat McGee is amazed by how young thoroughbreds change during the winter. The 2-year-old rookies she schooled in summer and fall have grown up at 3.

“They’re like babies when they first come to the barn,’’ the Amityville native said from trainer Graham Motion’s base in Fair Hill, Maryland. “They develop so much in a few months. It’s like going from being a teenager to 21. I’ll look at a 3-year-old colt and say, ‘Wow, now you’re a man.’ ‘’

Motion, a 52-year-old Englishman, won the 2011 Kentucky Derby and the 2013 Dubai World Cup with Animal Kingdom, one of the century’s best horses. He’s been around thoroughbreds all his life, yet he still shares McGee’s sense of wonder about how dramatically they mature from 2 to 3.

“I think 3-year-olds at this time of year change a lot,” Motion said. “It amazes me that after I take them down to Florida in December how different they are when we bring them back in March and April.”

A select few are potential stars, and Team Motion’s Alpha male is Irish War Cry, dominant winner of Aqueduct’s Wood Memorial and among the leading contenders for the May 6 Kentucky Derby.

Photo Credit: Coglianese Photography / Joe Labozzetta

“The Wood was pretty unbelievable,” McGee said. “I hadn’t laid a hand on him since before Thanksgiving, but there was still a sense of pride. My horses are like my children.”

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When the rangy chestnut son of two-time Horse of the Year Curlin arrived at Fair Hill in September, McGee became his kindergarten teacher.

“Last year was the first time I really handled the babies on my own,” said McGee, who joined Motion as a hot walker after graduating Purdue with a degree in animal science in May 2012. “I had him in the barn from when he came in to pretty much his first start.

“At first, he was a bit of a handful — memorably a handful — but only when he was walking to the track, not when he was galloping. After a week or two, he figured out the program. He was really chill. He’s very athletic and always loved his job.”

So does McGee, even though she has to get up at 4:30, start work by 5:15 and has few days off. Unlike most people, her mantra is “I never don’t want to go to work.” She checks feed tubs, temperatures and legs before training begins at 6:30, and tells her foreman or Motion of any problems. She’ll do “live gate work with the babies,” teaching them how to break. After training, the staff consults the veterinarian. McGee leaves about 1:30 and usually returns at 3 for feeding time, often with her Australian Shepherd mutts, Alex, 10, and Amelia, 3.

Irish War Cry looked as if he might be special in his debut Nov. 11, when he closed powerfully to score by 4½ lengths at Laurel. Six weeks later, he won a minor stakes there by a nose. Motion shipped him to Florida and tested him for class in an early Derby prep, Gulfstream Park’s Holy Bull Stakes.

McGee spent the winter at Tampa Bay Downs supervising 15 runners, and her mother, Janet, was visiting Feb. 4, the day of the Holy Bull. When Cat (given name Carolann) saw Irish War Cry in the post parade, her reaction was “Wow, he’s really filled out.’’

He ran to his looks, leading throughout. As he remained unbeaten with a 3¾-length runaway under Joel Rosario, the McGee women shared a peak experience.

“I was with my mom at my apartment,’’ Cat said. “I said, ‘I can’t believe one of my 2-year-olds is running in this race. I can’t believe one of my babies is winning this race.’ ”

Racing’s highs are glorious, its lows abysmal. Irish War Cry’s next race couldn’t have been much worse. He finished seventh, beaten 21 lengths, as the even-money favorite March 4 in the Fountain of Youth. Motion called it “a really appalling race” and couldn’t figure out why it happened. He decided to draw a line through it and hope it was too bad to be true.

The Wood was pressure time, when Motion would find out whether he had a Derby horse. He switched from Rosario to Rajiv Maragh, who let Irish War Cry settle before accelerating clear in midstretch for a 3 1⁄2-length victory. On to Louisville, but McGee won’t be there. “I’ll probably be watching the Derby from Fair Hill,” she said.

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Irish War Cry was conceived in Kentucky but was foaled in New Jersey, a breeding backwater that has produced only two Derby winners — the filly Regret (1915) and Cavalcade (1934). Frank McGee would be pulling hard for his daughter’s horse regardless, but Irish War Cry’s birthplace provides an extra rooting interest.

“My dad is a Jersey-bred, too, so he’s pretty excited,” McGee said. “In Florida people kept saying this horse ‘is only a Jersey-bred,’ so it was nice to see him prove them wrong in the Wood.”