sensitivity-analysis and economic-optimization capabilities, both of which could help managers of equid populations to set priorities for management actions.

A model that captures the population-level effects of contracepting males and females could help in designing efficacious, herd-specific contraceptive treatment plans to meet management goals. According to BLM’s presentation to the committee, the agency treated an average of 500 mares a year with the porcine zona pellucida (PZP) vaccine from 2004 to 2010; just over 1,000 were treated in 2011 (Bolstad, 2011). Contracepting 500-1,000 mares a year with a 2-year vaccine will not substantially lower the rate of growth of a population of over 30,000 horses. To reduce the population growth rate with contraception, a much higher proportion of the population would need to be treated in a comprehensive, strategic fashion, making use of PZP (in the PZP-22 or SpayVac® formulation) and GonaCon™ for females and chemical vasectomy for males. Recording information on the date and type of treatment applied would allow BLM to measure the success of its contraception management actions and adapt its strategy accordingly. It would also contribute to knowledge about the effects of contraception on individual reproductive success if the contraceptive is administered multiple times, on the longevity of treated mares, and on behavior in individuals, harems, or the larger population. Tracking responses to a large-scale fertility-control strategy would be particularly important for BLM to be able to respond quickly and appropriately to known and unknown side effects that may affect population or genetic health. Any information learned from analysis of management actions could be used to modify the model or models to continually improve their predictive ability and hence their utility going forward.

Another way to reduce the growth rate is to allow horses and burros to self-limit. As reviewed in Chapter 3, few scientific studies have been conducted on equid self-limitation. However, there is substantial evidence in wild ungulate populations that self-limitation will involve shortages of forage and water for the population, which will increase the number of animals that are in poor body condition and dying, either directly from lack of food and water or indirectly from increased vulnerability to disease. Although increased mortality would reduce population growth rates, it is unclear how much the growth rate would be lowered and what effect this strategy would have on the health of the rangeland and on the welfare of other animals on the range. Without further research, experimentation, and modeling exercises, it is difficult to predict mortality, body conditions of all animals, and rangeland ecological conditions at the point of horse and burro self-limitation.

An issue that is vitally important for improving the operation and the image of the Wild Horse and Burro Program is the connection of the establishment of AMLs to results of scientific research. AMLs involve policies that allocate rangeland resources among many uses of the land, but information regarding the interaction of horses and burros with the environment and other species, which informs these policies, should be robust and of the best quality possible. The committee suggests that a science-based assessment of the range and the interaction of animals with the range, consistently applied over time and among districts, could inform the establishment of AMLs more accurately. The committee could not identify a science-based rationale used by BLM to allocate forage and habitat resources to various uses within the constraints of protecting rangeland health and listed species and given the multiple-use mandate.

The committee also finds that, if AMLs remain set at their 2012 levels (Appendix E, Table E-1), contraception or self-limitation strategies may not reduce horse and burro populations to target levels. To manage horses at 2012 AMLs, horses may first have to be removed. Large-scale removal would require the public to accept gathers on a number of HMAs over a short period, probably within less than 5 years, which would be expensive.

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