Cows in heat move more than those that aren't, and that activity can be recorded by electronic boxes on their collars.
At Saxon Homestead Farm in Cleveland, Wis., data from the boxes is scanned and sent to a computer when cows go through a gate in the milking parlour. The gate sends cows in heat in one direction to be bred and the rest another way.
Activity monitoring systems aren't new, but farmers say interest is growing as the technology becomes more accurate and easier to use.
Farmers say the collars don't eliminate hormone use because some cows still need drugs to get pregnant, but they do reduce it.