The letter follows an ongoing legal challenge to the Bureau of Land Management's wild-horse gather operations in the Tuscarora complex, comprising the Owyhee, Rock Creek and Little Humboldt horse management areas. The challenge failed last week to halt the bureau's gather plans.
Wild horse advocates were angered by the first of the musters, in Owyhee, which went ahead as an emergency operation after the bureau told a judge around 500 horses were in danger of dying through lack of water.
The operation had initially been suspended following the deaths of half a dozen of the first horses gathered from complications arising from dehydration.
In total, around two dozen horses have died following the operation, most from dehydration.
The Cloud Foundation had warned the bureau of the dangers of mustering horses in the height of a Nevada summer.
The letter to Salazar has been signed by Representatives Raul Grijalva and Nick Rahall, who have both campaigned hard for better management of wild horses, and 52 other Representatives.
The signatories seek a halt to muster opations "until the agency demonstrates that it has addressed the failings of the current programme and can ensure the safety and well-being of the animals [the bureau is] charged with protecting".
The letter said the bureau "must account for temperature extremes and the impacts of stampeding young, elderly or injured animals across long distances when planning roundups".
It said the bureau needed to ensure transparency by allowing members of the public to observe agency activities.
The letter argues that current roundup policies are unsustainable.
"The bureau's aggressive use of roundups has resulted in unsustainable increases in the number of horses in holding facilities (now at 38,000) and continues to undermine the bureau's overall budget.
"Unfortunately, the frequency of roundups has only increased under this administration."
It calls for a review of bureau policies by the National Academy of Sciences to provide "a clear determination of the most accurate, science-based methodologies to estimate wild horse and burro populations, provide an assessment of appropriate management levels based on the goal of maintaining sustainable herds and provide an assessment of practical, effective, non-lethal and publicly acceptable management alternatives to current BLM policies".