I only found a few things in a quick Google Search:

A question about it, answered by a vet:
http://www.horseadvice.co...e/messages/3/289777.html

"The results demonstrate that two immunizations with G6k-GnRH-tandem-dimer-OVA conjugate in a suitable adjuvant such as CoVaccine HT caused a rapid and complete reduction of serum testosterone levels in sexually mature stallions..." (from a study in the Netherlands, the vet cited)

And a journal article on a study on something similar:
http://www.animalreproduc...4320(04)00063-6/abstract