Water being delivered to Nevada's Owyhee, Rock Creek, and Little Humboldt herd management areas.
The bureau said it had trucked 12,000 gallons of water to the horses in the northern Owyhee area but tracks indicated only 15 or so horses had taken water.
Bureau staff had hoped that the dehydrated horses would take water before being captured in a helicopter-driven muster.
The Tuscarora roundup, covering the Owyhee, Rock Creek and Little Humboldt herd management areas, has been surrounded by controversy and has been the subject of legal action.
Contractors gathered 228 horses last Saturday in a muster lasting 150 minutes. However, 12 of the horses died in the days following from complications arising from dehydration. A 13th suffered a broken leg in a pen accident.
The deaths angered wild horse advocates, but the bureau responded that the horses' deaths related to the dangerously dry conditions on the range and not the gather operation.
On Wednesday, US District Court Judge Larry Hicks, sitting in Nevada, issued a temporary restraining order to stop the muster.
However, in a hearing on Thursday, he lifted the order amid growing fears for the safety of the Owyhee horses.
The judge made several other determinations, deciding that the other pending roundups in the Rock Creek and Little Humbolt herd areas were separate events.
He also ruled that the blanket closure of the area imposed by the bureau, preventing public access, was unconstitutional and ordered they be relaxed, while still satisfying health and safety responsibilities.
Author and wild horse advocate Laura Leigh, who sought the order to halt the muster, said: "This ruling was made by a judge that is obviously willing to listen.
"He does not want the deaths of 75 per cent of the horses in Owyhee on his heart and mind. As an advocate I completely understand why he ruled the way he did."
The bureau said it now considered the Owyhee muster to be an emergency rescue operation.
"The bureau is pleased that the decision of the court that will allow us to move forward with the Tuscarora emergency rescue gather, which is necessary to prevent the mortality of the wild horses that are currently suffering from water starvation and dehydration." said the bureau's Nevada state director, Ron Wenker.
After the ruling, the bureau contractor immediately brought in 54 wild horses suffering from water starvation/dehydration.
The bureau submitted an interim report to Judge Hicks during the hearing, prepared by a review team composed of bureau and independent experts.
It proposed beginning the emergency gather but, for those horses on the range showing obvious signs of distress from water dehydration and in too weakened a condition to gather, to be left alone.
For those already down, unable to be trailed and not responding to the helicopter, the bureau would attempt to humanely euthanise them.
The water danger became apparently last Tuesday in a flyover of the area by the bureau contractor. Two large groups of wild horses in the Owyhee area were identified. The Dry Creek group consists of about 125 wild horses and the Star Ridge group has about 400 animals.
The Star Ridge group is located around a dry reservoir and has made no attempt to move to the nearest water source about 15km away at the South Fork Owyhee River.
The bureau installed six water troughs with a combined capacity of 3000 gallons near and around a reservoir located within 400 to 800 metres of the horses within the Star Ridge pasture, and used a water tanker to fill the troughs and the reservoir.
The bureau said it had hauled 12,000 gallons of water to the North Owyhee horses, yet only 15 or so had drunk, based on tracks in the mud.
The bureau said it would continue to take in water in the hope the animals would drink.