The alliance has continued its attack on the bureau over the eight-week Calico muster in northern Nevada, in which up to 2700 horses will be removed from five herd management areas.
Wild horse advocates argue the muster is cruel and unnecessary, while the bureau says it is necessary to protect the delicate rangelands.
Protests were held last week against the muster and protesters will gather in front of the Capital Building in Albany, New York, on Thursday in another show of opposition. Protests are also planned for Denver and Lexington next week.
The alliance has highlighted discrepancies in bureau accounts of how many horses will be removed, suggesting the plan may not be to return 600 to 800 horses to the area, but as few as 380.
It pointed also to the May 13 evidence of a bureau staff member who undertook a flyover of the area.
She expressed surprise at the number of the horses - being over the bureau's "established levels" by 500 per cent, yet monitoring objectives were being met. The significantly larger population was not evident in their forage consumption.
"The test for appropriate wild horse population levels is whether they achieve and maintain a thriving ecological balance on the public lands," the alliance said. "The law does not require the bureau to maintain specific numbers of animals but does require specific research and analysis for determining removals."
Based on the testimony, there was no justification to reduce the wild horse population at Calico, it argued.
The alliance also pointed to concerns raised by academic veterinarian Nena Winand of Cornell University regarding a condition known as metabolic syndrome when horses are removed from their native habitat and placed in more nutrient rich pastures.
She said: "Once, I addressed this with the BLM wranglers that auction horses here every year or two when I suggested that they mention this syndrome in their presentation to potential adopters so that they would be better prepared to manage their horse's needs.
"They looked at me like I was an alien - they had no clue."