According to Bill Krohn's Hitchcock at Work, the scene was filmed  "...at Lost Hills in Northern California's San Joaquin Valley."

 

Krohn goes on to say that "renting the location and planting a cornfield there came to $5,200."...and  "required only one matte." As an aside, I've been told that on another part of that road is where James Dean had his fatal accident.

 

Sarah Nichols