The Bureau of Land Management (BLM) should rely more on contraception than on roundups to effectively manage wild horse herds, according to study results released this week.

The National Academy of Sciences/National Research Council (NAS/NRC) is an independent nonprofit group that advises government agencies on scientific issues. In 2010 the BLM asked NAS to review technical aspects of their wild horse and burro program, including science-based population estimation methods, annual herd growth rates, and population control measures. The BLM also asked the group to make recommendations for future wild horse and burro management techniques. The $1.5 million study began in 2011 and the results were released June 5.

In its report, the NAS said the population of wild horses under BLM care on public rangelands in Western states increases at an unsustainable rate of 15% to 20% annually. In managing its herds, the BLM has estimated the ideal number of horses each range can support, then reduced herd populations to meet that estimate by gathering and removing horses from the range. But according to the NAS report, the BLM has not used scientifically rigorous methods to estimate the horse and burro populations on each range or to model the effects of management actions on the animals under BLM care. The report said that the current methodology the agency uses also fails to assess the availability and use of forage on rangelands the animals occupy.

In addition, the report said the BLM fails to effectively use contraception tools—specifically porcine zona pellucida vaccines for mares and a chemical vasectomy vaccine in stallions—to achieve appropriate wild horse and burro population control. The report said that conclusion was based on delivery method, availability, efficacy, duration of effect, and the potential for side effects