Hsing and Ling were tops in their time too. Well, there's always artificial insemination. At the Wolong reserve, which has produced more panda cubs than anywhere else, they use natural mating followed by a dose of AI, just to make sure. I do not know length. Wolong is doing a great job at producing babies, and they have learned some things about how to do it that the zoo hopes to emulate.
Conditions there range from excellent to somewhat depressing.
They have new natural enclosures on a hillside that are really neat, but also some old-style rusty cages and plain yards. The Chinese are supposed to use the loan fees to preserve the giant panda in the wild. There is a national panda plan, and the money is supposed to be used for items on the plan list. The U.S. Fish and Wildlife Service approves each project as well. As I mentioned in an earlier response, the Wolong reserve uses both natural and artificial insemination, so I'm not clear on which one worked to produce Mei and Tian.