Xanadu Weyr is a self sufficient Weyr - thus all food stuffs are traded for rather than tithed. Xanadu Weyr does have its own fishing fleet, which provides a great deal of seafood for the area, and the free-roaming herds of the southern plains are often rounded up for both the dragon pens as well as consumption by the Weyr itself. However, they have minimal fields, so most crops and produce is imported from nearby holds, or other locations if necessary.
Xanadu Weyr provides many services to the holds in return for a share of their goods - be they leather, wool, wood, or metal - which are then used by resident crafters to augment the various shipments from the craftholds themselves.
Xanadu Weyr's main export is technology support, rather than a physical good, as their riders often do work for other areas in return for goods, rather than trading away their goods.