The Sand Wash Basin horse roundup is now the largest in the history of Colorado, as the Bureau of Land Management has gathered more than 500 horses from in and around the basin, about 55% of the total herd.

After operations finished Tuesday with 79 horses gathered, the BLM had rounded up 501 total horses since operations started Sept. 1. The plan is to gather 783 horses total, before releasing 50 of them back into the herd management area.

The total surpassed the 457 horses gathered from the West Douglas Herd Area in August, which had been the largest roundup in Colorado history.

One horse has died in Sand Wash since the start of the roundup. BLM officials said the horse was euthanized because of a preexisting condition, and it was not related to roundup activities.

The gather intends to remove 733 of the estimated 893 horses that live in and around the Sand Wash Basin, and the effort has gotten contentious, said Chris Maestas, public information officer with the BLM’s Little Snake Field Office in Craig.

“It seems like every day it has gotten a little bit more contentious,” Maestas said. “As for the actual gather itself (on Tuesday), it has gone extremely well. I know some people won’t appreciated that I say it went well, but we didn’t have any deaths. It just was a very smooth day.”

According to the BLM, horses need to be removed because overpopulation and drought are combining to produce “unacceptable negative impacts” on the land, agriculture producers and wildlife.

Wild horse advocates disagree, saying other livestock grazing on the range is leading to the limited amount of forage available for the horses. Colorado Gov. Jared Polis, Rep. Joe Neguse, of Boulder, and others have called for the gather to stop, and they would like to see a moratorium on further roundups put in place, but operations have continued.

The gather started last week with horses grazing on the public and private lands beyond the boundaries of the Sand Wash Basin Herd Management Area in Moffat County. Maestas said it is estimated there are about 120 of these horses, and to date, about 80 of the horses collected are from outside the management area.

Over the weekend, efforts moved inside Sand Wash Basin, and horses that have been named and followed by advocates for years were rounded up. About 50 of the horses are on a list curated by advocates to release if they are caught in the roundup.

The Sand Wash Advocacy Team put together this list of 25 stallions and 25 mares, and members are working with the BLM to identify these horses. The mares will be treated with birth control before being released.

Advocates with the group are going to holding corrals after gather operations each night to try to identify horses. They return at 6:45 a.m. the next day to pull horses out before they are loaded to be shipped away from the herd management area.

“The contractor has trailers backed up and ready to load around 8 a.m.,” the group wrote on Facebook. “Loading goes fast, and we have to identify them before they are loaded, so they don’t get shipped.”

Of the horses gathered so far, 259 of them have been moved to the BLM’s facility in Cañon City. Horses waiting to be shipped and those slated to be released are held in temporary corrals within the management area.

Advocates have identified one horse that was euthanized as 6-year-old Brennen, according to the advocacy team on Facebook. The horse had been seen with a shoulder injury about two weeks ago, the post said, and the BLM said it was put down because of a preexisting leg injury.

After just seven days — one where there was no gather operations — the gather has been moving quickly with large numbers of horses gathered in a single day. Maestas said this is because the terrain in the Sand Wash Basin is flatter and more open than in the West Douglas Roundup last month, which saw between 20 and 30 horses gathered on most days.

“It has gone pretty quick, but there are a lot of horses in the area,” Maestas said. “I participated in West Douglas, and that terrain was completely different within a very mountainous area with a lot of trees so that went extremely slow, whereas here it is very flat.”