Mud-clad up to the top of his rubber boots, large-animal veterinarian Jim Holt, arms gloved to the shoulder, gently examined the uterus of a stoic Angus cow, assessing whether she was pregnant.

On this gray, rainy day in late fall, about 35 brood cows were lined up for their exams at Jay Heim's Glen Oak Farm. The rest of the year, the herd lives out in the fields of the 282-acre Wallace Township property that Heim has farmed for 20 years.

It was suggestive of a relatively recent past in which northern Chester County was predominantly farm country and farmers relied on local vets who made house calls.

But here, as elsewhere, the animal doctor with a solely owned practice is becoming increasingly scarce, as developments have gobbled up land and many vet-school graduates choose to work in teams or to go into lucrative and less stressful small-animal practices.

Holt, who until recently ran his practice out of his Wallace Township farm, represents a breed in transition: a circuit rider called out to doctor every type of domesticated hoofed species.

"It's a lifestyle, not a profession," says the 16-year veteran. "You have to have a passion for it."

Holt's day begins around sunrise, as he travels from one dairy farm to another to inspect cows after farmers have completed their early morning milking. Often he is out well after dark.

Most of his bovine clients at the approximately 60 Chester County farms he visits are dairy cattle. There are not many beef cattle left in Chester County, Holt said.

Although Holt also cares for white-tailed deer, sheep, goats, llamas, alpacas and other animals kept as pets, they occupy a small slot on his schedule, he said. Horses are another matter. Holt estimates he has 1,500 East Coat equine clients, most of them on farms in the five-county area.

After administering a shot of Adequaf, used to treat equine arthritis, to a well-mannered Arabian Paint horse named Jack on a chilly December afternoon, Holt and caretaker Lee Williams watched carefully to see how Jack would react.

Both remembered the frightening time when Holt was a half-hour down the road and learned that the horse had gone into anaphylactic shock. But this visit to Jack's North Coventry barn was uneventful.

Although much of his practice is devoted to farm visits, Holt also reflects the changing face of large-animal veterinary medicine, with its emphasis on food-animal production and health.

Mondays, Tuesdays and Wednesdays he can be found at Lancaster County's New Holland Sales Stables, a large-animal auction house, where he does health certificate inspections and pregnancy tests on 200 to 400 horses and 300 to 600 head of cattle.

Bucking the tide in large-animal veterinary care, with its move toward group practice, Holt moved last year to downsize his practice.

But even his smaller client list and less hectic schedule still leave him and his back-up emergency vet, Keith Olin, with more work than they can handle.

Veterinarians face some of the same challenges as do others in the medical field, said David Wolfgang, Pennsylvania State University's director of field investigation and Extension veterinarian.

As concerns about global animal-borne diseases, such as avian influenza, and about food contamination are rising, there is a call for more animal doctors expert in preventative medicine and epidemiology. In other words, specialists.

And many newer vet school graduates are seeking more control of their time, job-sharing, and more specialization, said Wolfgang.

Many of those graduates gravitate to small-animal practice, unlike Holt's large-animal practice.

"We are at a tipping point," Wolfgang said.

The University of Pennsylvania's School of Veterinary Medicine is aware of the dimensions of the problem, said Gary Althouse, chair of Penn's New Bolton Department of Clinical Studies in East Marlborough.

With a teaching hospital and other facilities, Penn's New Bolton campus employs more veterinarians than any other business or practice in Chester County, said Althouse.

There are 319 veterinarians who live or work in Chester County, according to Charlene Wandzilak of the Pennsylvania Veterinary Medical Association. This includes the vets who work at New Bolton or for area pharmaceutical firms.

But only 20 of those 319 are food-animal vets like Holt who treat cows, sheep and pigs, according to the Web site of a national group, the American Veterinary Medical Association.

And no one currently keeps track of how many solo practitioner vets like Holt there are in the county.

Judging by the national statistics, "it is clear that the United States is not producing enough veterinarians to fill the needs of the veterinary profession, including demands for more large-animal veterinary practitioners," Althouse said in an e-mail. The pressure on vets to fill the void created by the shortage "can only be sustainable for so long."

Holt doesn't have time to be distracted by the larger forces influencing the future of large-animal veterinary medicine.

Running lab tests at his office while he talks on his cell phone, he says he is fulfilling a lifelong dream. "I love what I do," he says. "I can't imagine doing anything else."

The next morning he will again rise at 5:30, hop into his truck and make his rounds - a day that will end, he says, only when the work is done.