According to a University of Wisconsin study, using a presynch estrus-synchronization protocol does not increase heifer pregnancies. In an on-farm trial with 166 heifers, researchers treated half of the animals with GnRH seven days before beginning a synchronization and timed-A.I. protocol. The other half followed the same synchronization and timed-A.I. protocol, but did not receive the extra shot of GnRH.
At 30 days after breeding, the proportion of heifers diagnosed pregnant did not differ between treatments. The researchers theorize this is due to the variation in follicular dynamics among individual heifers. The research was published in the October 2006 Journal of Dairy Science.