PANYU, China — Over the past decade, China has become an industrial colossus by mastering a variety of commercial skills, from the manufacture of high-speed computer chips to the production of top-quality automotive steel.

Learning to stimulate the sex drive of crocodiles has proved more difficult.

China's forestry department eliminated steep duties on imported breeder crocodiles nearly a decade ago. The hope was that low wages, highly skilled farmers and well-developed road and port networks would turn China into a highly competitive producer of crocodile meat, hides, shoes, purses and other goods.

But impotence, obesity, runny noses and finicky palates among the crocodiles have made this dream difficult to realize. Imported by the tens of thousands from tropical Thailand, the crocodiles have had trouble adapting to slightly cooler southeastern China and have been slow to breed, prone to infections and reluctant to eat anything but expensive chicken breasts.

The biggest problem has been that male crocodiles eat more in the late autumn and early winter here than they do in Thailand. They become so plump that they show little interest in sex during the spring mating season, said Li Mingjian, the deputy general manager of Crocopark Guangzhou in Panyu, now one of the world's largest crocodile farms, with 60,000 to 70,000 animals.

"They don't chase the females," he said. "They're very fat guys. They just eat, eat, eat."

Nighttime temperatures that sometimes dip to 50 degrees Fahrenheit also make the crocodiles vulnerable to illness, although the farm tries to protect them by covering their pools with canvas. So farmers are forced to spend heavily on medicines, sometimes sneaking into concrete-lined pools with several thousand crocodiles to jab the ailing ones with syringes filled with antibiotics.

Even with the medicine, "they get runny noses and sneeze a lot," said He Zhanzhao, Crocopark's chief breeder, who for emphasis does his best imitation of a crocodile sneeze.

With too few crocodiles being born to make the farm profitable by itself, the management has tried since the summer to raise extra money by opening the site to tourists, even letting children feed the crocodiles by paying $1.25 for a bamboo pole, a length of string and two chicken torsos.

China's spectacular economic success over the past quarter of a century has given its economic planners the same kind of reputation for invincibility that Japan's top bureaucrats enjoyed in the 1980s. Yet the difficulties afflicting crocodile production here show that even in China, industrial policy has its limits.

The crocodile farmers are now trying to strike just the right dietary balance, one that will keep male crocodiles from sickening and dying in cool weather, without making them overweight. The tricky part is throwing the chickens so the larger, more aggressive crocodiles do not eat too many and the smaller ones get enough to eat to stay healthy.

"When we control the food," He said, "the slim males chase the females longer than the fat ones."