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Computer Simulation Studies Although not part of the contract between the Bureau of Land Manage- ment (BLM) and the University of Minnesota, the research group engaged in computer modeling to compare the population control effectiveness and costs of various roundup and fertility control options. The group presented its results to the committee at the February 1990 review in Denver. The researchers modeled the following three, herd management scenarios over a 20-year period: 1. The roundup and adoption procedures used to date. This scenario assumed that a hypothetical herd would need to be rounded up every 4 years. Initial herd size was 600, and at each roundup it would be reduced to 300. The captured horses would then be placed for adoption. Costs were assigned to roundup transportation, holding, and adoption based on BLM's records. 2. Mare contraception. An initial herd of 600 would be reduced to 300 through roundup and adoption. All of the mares that were older than 3 years would be implanted with estrogen. Every 3 years thereafter, enough animals would be rounded up to implant 85 percent of the mares more than 3 years of age. 3. Contraception and selective removal. An initial herd of 600 would be reduced to 300, and the removals placed for adoption. All of the mares older than 3 years would be implanted with estrogen. Every 3 years there- after, enough animals would be rounded up to implant 85 percent of the mares older than 3 years, and all of the animals 1 to 3 years old would be removed and placed for adoption. The model predicted that scenarios 2 and 3 would cost 30 to 50 percent less than scenario 1. Furthermore, they would significantly reduce the 21
22 WILD HORSES: FIELD STUDIES IN GENETICS AND FERTILITY number of animals needing to be adopted and ameliorate the chronic prob- lem of holding unadoptable animals. The major drawback is that the horses would be captured and handled more frequently in order to implant the mares every 3 years. Hence, the modeling shows that there would be trade- offs, but nonetheless a distinct gain in costs and a reduction in the problems associated with the roundup and adoption program. As in any modeling exercise, the output is contingent on assumptions made. The Minnesota investigators caution that anyone substituting alternate assumptions, or modifying the simulated protocols, must not expect the same results as those encapsulated above.