US president Donald Trump's budget proposal seeks to save 10 million dollars (£7.1 million) next year by selling wild horses captured throughout the West without a guarantee from buyers that the animals will not be resold for slaughter.
Wild horse advocates said the change would remove nearly a half-century of protection for wild horses and could send thousands of free-roaming mustangs to foreign slaughterhouses for processing as food.
Campaigners said the Trump administration is kow-towing to livestock interests who do not want the region's estimated 59,000 mustangs competing for precious forage across more than 40,000 square miles of rangeland in 10 states managed by the US Bureau of Land Management.
Mr Trump's budget anticipates the savings would come through a reduction in the cost of containing and feeding the animals.
The savings would also include cutbacks involving round-ups and contraception programmes.