The roundup, scheduled to last six days, has been delayed until early July.
The bureau said in a statement that the postponement of the operation in the Confusion Mountain Wild Horse Herd Management Area would allow it to update environmental documentation for the gather to reflect 2010 population and vegetation data.
"Because horses naturally concentrate in the early summer, the July date will also make it easier to gather the bands. The postponement also allows more co-ordination with gather contractors and/or bureau gather crews in planning," it said in a statement.
However, the group In Defense of Animals believes the delay stemmed from what it said was thousands of protest emails from supporters of the group.
The organisation accused the bureau of planning to carry out the muster without public comment and without conducting a current environmental assessment on the impact of its plan to leave only 60-100 horses behind in the 225,000-acre area.
In Defense of Animals said it had alerted its supports to the bureau plans. "Less than 24 hours later, the bureau has completely reversed course, postponing the roundup until at least July," it said.
"Clearly, the bureau's complete about-face on the Utah horse roundup is directly related to the massive public outcry," said Eric Kleiman, the organisation's director of research.
"The public is sending a strong message to the Obama Administration that it is time to stop the assault on our nation's wild horses and burros. The bureau's mismanaged, wasteful and inhumane wild horse and burro program must be reformed, and the first step is a moratorium on unnecessary and inhumane roundups."
The bureau has plans to remove nearly 12,000 wild horses in the 2010 financial year from western rangelands and place them in holding facilities.
An estimated 35,000 are already held in captivity.
Wild horses comprise 0.5 percent of grazing animals on public lands, where they are outnumbered by cattle at least 200 to 1, In Defense of Animals says.
The bureau, it said, manages more than 256 million acres of public lands of which cattle grazing is allowed on 160 million acres; wild horses are only allowed on 26.6 million acres of this land, which must be shared with cattle.