I asked the same question to the owner about 3 weeks ago and received the following response:
calving on correctly managed heifers has not been a problem any where. birth weigts have stayed down, he will power them up but you will probly not like them for about 30 days, then they explode and come on to the point that you may not reconize them, had a lady send a message the other day about GM calves on her angus donor. the calves on recips were in with her clubby bred cattle and she was not really liking the GM bull calves at the time. she messaged back a couple of weeks later and apoligized because they were stouter and thicker than her club calf cattle and loved them. as far as GM cattle in IA,I dont really know we sold about 17,000 units on the bull last year all across the country and im sure their has to be some out there.
Johnnie
Hope this helps, I tried to breed some last year but didn't get any stuck. Will try again this year. Good luck.