The Kaimanawa wild horses near Waiouru may soon be on the pill again, as the Department of Conservation tries different types of equine contraception as an alternative to culling the herd.
Bill Fleury, a conservation analyst with the Kaimanawa Wild Horse Advisory Group, is liaising with colleagues in the United States to come up with a vaccine that works.
Mr Fleury says initial trials using an extract from pigs' ovaries resulted in all the mares getting pregnant after a year.
He says a second vaccine developed by Massey University shows more promise, but DoC is waiting for more conclusive research being conducted on a wild herd in the American state of Colorado.
The Kaimanawa horses are counted every year. DoC says if the herd exceeds 300, it becomes unsustainable and threatens fragile plants in the area.
The pill would be administered either from a pellet fired by an air gun or by an injection during the annual muster.
