As stated in the Wild Horse Fertility Control Field Trial Plan, the Bureau of Land Management (BLM) has an immediate need for a safe, effective contraceptive agent to assist in the management of the large number of wild horses on western rangelands. The BLM and the U.S. Geological Survey-Biological Resources Discipline (USGS/BRD) are testing the immunocontraceptive agent Porcine Zonae Pellucida (PZP) in field trials with three free-roaming herds of western wild horses. Extensive research has already been conducted on the safety, efficacy, and duration of PZP applications in both domestic and feral horses on eastern barrier islands and in some select trials I with wild horses in Nevada managed by the BLM. However, significant questions remain concerning the effects of I PZP application at the population level in the wild, as well as effects at the individual level on behavior, social structure, and harem dynamics of free-ranging animals. These questions are best answered with field trials on wild horse herds under a tight research protocol. The ultimate goal is to provide the BLM with the protocols and information necessary to begin using fertility control to regulate population growth rates in wild horse herds on a broader scale. Fertility control is intended to assist the conventional capture, removal, and adoption process as a I means of controlling excess numbers of wild horses and burros, and to greatly reduce the adoption costs and numbers of animals handled. Fertility control is not intended to totally replace the removal and adoption process.

The USGSIBRD began assisting the BLM with field trials of immunocontraceptive fertility control of wild horses in early 2001. The first PZP treatments were applied during gathers at the Pryor Mountain Wild Horse Range in September 2001, and the Little Book Cliffs Wild Horse Range, Colorado, in July 2002. At those gathers, 5 horses were treated in the Pryor Mountain WHR, and 23 were treated in the Little Book Cliffs WHR with PZP. These initial treatments were followed by booster injections in 2002. The second injection is required in order to raise, and I maintain, the titer levels of mares high enough to be considered contracepted. By the end of 2002, 13 horses on the Pryor Mountain WHR had received both injections, as had 11 horses in the Little Book Cliffs WHR. In 2003, intensive research efforts were carried out by the USGSIBRD at three field locations; Pryor Mountain WHR, Little I Book Cliffs WHR, and McCullough Peaks Wild Horse Management Area. The work at these sites during this I calendar year included treatment of wild horse mares with PZP in the Pryor herd and Little Book Cliffs herd, development and implementation of behavioral research to investigate potential affects of PZP treatment, continued tracking of demography and foal production in all three herds, and early phases of investigating aerial population estimation survey techniques. Detailed descriptions of these research topics can be found in the Wild Horse and Burro Management Strategic Research Plan and the Wild Horse Fertility Control Field Trial Plan. Field work in 2003 was conducted by USGSIBRD and BLM staff with the assistance of many dedicated individuals. See I Acknowledgments for more details.

This report is meant to highlight the activities of the 2003 field season, as well as to provide a general overview of the data collected. More in-depth data analysis will be conducted following the conclusion of each I phase of the research project, and in many cases will not be possible until several seasons of data are collected.