Shipping the bison to the mainland was costly, and it raised concerns about the stress on the animals during shipment and the expansion of the herd beyond ecologically sustainable numbers between shipments. Beginning in 2009, the Conservancy's scientists injected the female bison with porcine zona pellucida (PZP), a contraceptive that had been used for fertility control in zoos, wild horses and white tail deer. In addition to substantially reducing the number of new calves, the PZP had no apparent effect on pregnant females or their offspring. A peer-reviewed study published in 2013 reported that the contraceptive program was effective in controlling the herd. Previously, more than two-thirds of the cows delivered calves every year. After receiving the contraceptive, the calving rate dropped to 10.4% in the first year and 3.3% the following year.