Horses are being mustered from Theodore Roosevelt National Park this week as part of its herd management programme.
Park management intend removing about 90 horses, to be offered at public auction on October 23, and reducing the herd to about 70 animals.
This year the park is starting a research project to evaluate a new, multi-year contraceptive vaccine as a potential tool in wild horse management.
The contraception study is a four-year project to assess vaccine feasibility and will involve 50 of the park mares, about half of which will receive a contraceptive vaccine.
Park staff will monitor the effectiveness of the vaccine, its durability over time and any changes to horse behaviour.
"Keeping the population within our objective requires periodic roundups that are time consuming and have inherent risk for horses and park staff," says park superintendent Valerie Naylor.
"We want to see how well the contraceptive vaccine works on horses as we may want to consider using it as a tool in the future to reduce the need for frequent horse roundups in the park."
The mares were being tested by ultrasound to determine if they were pregant, and dung samples were being taken to compare estrogen levels with those in blood samples.
Scientists are exploring whether it might ultimately be possible to determine pregnancy by monitoring the wild mares and conducting a test after they are seen to drop dung.
Theodore Roosevelt National Park maintains wild horses as a "historic demonstration" herd so park visitors can see horses in a natural setting.
Since 1954, the park service has conducted more than 25 roundups to maintain the herd at desired levels.
The Theodore Roosevelt National Park horses are scheduled to be sold at public auction at Stockman's Livestock Exchange in Dickinson on Friday, October 23 at 2pm. About 90 are expected to be offered. Estimates indicate about 30 foals, 20 yearlings, 13 2-year olds and some older stallions and mares will be auctioned.