GREEN RIVER -- Federal land managers plan to use roundups, and possibly fertility control, to significantly reduce the number of animals in wild horse herds north of Lander.

The Bureau of Land Management is proposing to remove more than 250 excess wild horses from public rangelands within the Muskrat Basin, Conant Creek, Rock Creek, Mountain and Dishpan Butte Herd Management Areas, known as the complex, beginning in October.

The proposed action, and three other alternatives involving fertility control efforts and research, are examined in a recently released Environmental Assessment of the North Lander Herd Management Area Complex Capture/Removal and Fertility Control Project.

BLM Lander Field Office wild horse specialist Roy Packer said wild horses were last gathered in the North Lander Herd Management Area complex in 2001.

He said at the completion of the roundup, the population was estimated to be approximately 325 wild horses.

Since that time, BLM wranglers estimate the population has grown to about 590 wild horses as of this spring. That number exceeds the area's Appropriate Management Level by 270 animals.

Packer said in a phone interview that herd populations increase about 15 percent a year. At that rate, the wild horse population would double within five years.

The agency wants to reduce the herd by 270 horses to achieve a population objective of about 320 animals, he said.

Another proposal (alternative 1) examined in the Environmental Assessment calls for gathering approximately 470 wild horses, or about 80 percent of the herd complex population.

Some 270 horses would be removed from the herd under the alternative. About 100 mares would be subject to an immunocontraceptive research project.

The project involves inoculating mares with a contraceptive vaccine known as PZP using a dart-injection delivery method. The vaccine provides about three or four years of contraception to treated mares.

Other alternatives call for capturing 470 wild horses, removing 160 from the herd complex, and vaccinating approximately 155 mares (alternative 2); and gathering 470 horses and removing 160 horses from the herd complex, with no vaccination. There is also a no-action alternative examined in the document.

The contraceptive vaccine has been used in past pilot projects in the Cloud Peak wild horse herd near Cody, Packer said.

"Strictly in the past ... it's been used for research, and as it gets more effective, we're using it more," he said. "Because they increase at 15 percent per year now, (using fertility control methods) will delay our roundups and cut our costs as far as trying to control numbers."

In recent years, Wyoming's wild horse population has reached as high as 7,000 animals, more than double the appropriate management level of about 3,263 animals statewide.

Last year, the state and BLM avoided a possible costly and long-term litigation by agreeing to a consent decree after the state said it intended to sue the BLM. The state claimed the BLM had mismanaged wild horses to the point that grasslands were overgrazed, hurting elk and deer herds.

Under the consent decree, the BLM promised to reduce the number of horses to the appropriate management level in all 16 of the state's Herd Management Areas by December.

Southwest Wyoming bureau reporter Jeff Gearino can be reached at 307-875-5359 or at gearino@trib.com.