The muster, expected to take eight weeks, is now over half way, with 1447 gathered from the area and 1322 so far taken through to the handling facility at Fallon.
The Bureau of Land Management wants 2500-2700 horses removed from the five herd management areas in the complex, saying it is necessary to maintain an ecological balance. The targeted numbers represents about 80 per cent of the horses in the complex.
Animals welfare groups have branded the operation as cruel and unnecessary.
The bureau reported that four mares died on Friday. Two were found dead and two were euthanized because of poor condition.
"Facility staff are aware of 20 miscarriages as a result of poor body condition among the 659 mares at the facility," the bureau said.
"Many of the mares are in thin to very thin condition, which is directly attributable to the availability of forage and water on the rangeland, and is exacerbated if the mare is providing nourishment for a foetus, herself and a weanling-aged foal that continues to nurse."
However, In Defense of Animals says the spontaneous abortions "occurred after the heavily pregnant horses were stampeded by helicopter over treacherous terrain for up to 10 miles or more at full-gallop speeds".
It says an additional 20-25 horses are being treated for injuries and lameness sustained in the roundup.
The group says access for observers is being limied. "Even so, during limited viewing of the helicopter roundup, public observers have witnessed horses being run at high speeds in frigid temperatures and sometimes over rugged terrain."
The group has continued its attack on the bureau over the muster.
"The bureau's brutal roundup methods clearly violate Congress' intent that our nation's wild horses be managed humanely and in a minimally intrusive manner," said William Spriggs, who is lead counsel on a lawsuit filed pro bono on behalf of the group, ecologist Craig Downer and noted children's author Terri Farley to stop the roundup.
While the judge rejected their bid to stop the muster, he said he did see merit in their challenge to the bureau practice of moving captive horses further east to holding facilities off their natural rangeland.
Spriggs said the bureau was in the process of "stockpiling" horses in Midwestern holding facilities. "The entire programme must be fundamentally reformed."
The roundup began on December 28 and is scheduled to last through February.