Heat stress is not just a problem for the southern dairyman anymore.  High-producing cows can start to suffer from heat stress at air temperatures as low as 80^oF.  Paying attention to cow comfort during the summer is something that will pay off.

Coping with Summer Weather - From Kansas State University.  Written for dairy producers.

Embryo transfer as a tool for improving fertility during heat stress  - from the 2004 University of Florida Reproduction Road Show

List of links about heat stress from Animal Environmental Specialists, Inc.