As Iowa Gov. Terry Branstad completes the confirmation process to become the new U.S. ambassador to China, demand for agricultural products from China is playing a growing role in the health of the U.S. farm economy.

With domestic supplies of crops and meats rising and the government farm safety net waning, American farmers are relying heavily on export demand to buttress commodity prices, economists say. And China, with its huge population and growing middle class, has emerged over the past decade as a key factor in the health of the U.S. farm economy, according to a new analysis by Iowa-based Decision Innovation Solutions (DIS).

The rapid expansion in the Chinese...