on wheat, plant growth regulator is applied about a 7 -10 days before the wheat heads out.  So, the size of the head is already determined.   It will shorten up the stalk about 8" or so, and stiffen it up enough to make it stand better.

I had a customer use it on winter barley.  He liked what he saw.  The difference in barley stalk heighth was not extremely obvious - you could see it was shorter - but he liked how the straw was stiffer and baled nicer.  The product he used was Ethephon. 

You do need to be careful.   I believe you can negatively impact grain yields under certain weather conditions.  I believe hot weather at application time is a concern.