CHICAGO, Oct 12 (Reuters) - U.S. corn and soybean harvests will be smaller than previously forecast, with hot and dry weather during critical parts of the growing season dragging on yields, the government said on Thursday.

The corn harvest was pegged at 15.064 billion bushels and soybean harvest at 4.104 billion bushels, according to the U.S. Agriculture Department's monthly World Agriculture Supply and Demand Estimates report. Average yields were seen at 173.0 bushels per acre for corn and 49.6 bushels per acre for soybeans.

In September, USDA estimated that corn harvest would be 15.134 billion bushels, based on an average yield of 173.8 bushels per acre, and soybean harvest would be 4.146 billion bushels, based on an average yield of 50.1 bushels per acre.

Analysts had been expecting the report to show a corn harvest of 15.101 billion bushels with an average yield of 173.5 bushels per acre and a soybean harvest of 4.134 billion bushels based on an average yield of 49.9 bushels per acre. (Reporting by Mark Weinraub)