Pryor Mountain Wild Horse Range: Joint Herd Management Plan Revision Wild Horse Gather Plan, and Proposed RMP Plan Amendment

On March 15, the BLM-Billings Field Office released the EA for the plans listed above. This opened up a 30-day comment period which lasts until April 14, 2023. Your comments are very important. The BLM is looking for good, substantive comments as they prepare the final decision for the plans.

First off, the link below to the BLM ePlanning site will take you to the documents. Look for the section: 30-day Comment Period on Preliminary EA. The main document is clear at the bottom. It is the 2022 Pryor Mountain HMAP. Be forewarned…these documents are not easy to read. The entire plan strictly adheres to wild horse law and BLM Policy/Guidance. In addition, it uses technical concepts of genetics and range management. This makes for a pretty challenging read.

The Mustang Center has been working to read and understand the various parts of the plan and the appendices. We will provide main topics for you to review along with recommendations we have in regards to those topics. We will also add pages to guide you to the sections within the HMAP or the related appendix.

Big Picture Observation…Movement from a Specialized Plan: This Herd Management Plan has its roots way back to 2020 when the scoping notice was released. Perhaps the strength of that plan was this sentence from the introduction: Due to monitoring and documentation of the Pryor Mountain wild horses, management within the Pryor Mountain Wild Horse Range JMA can be more specialized than that of most BLM managed herd management areas.

Unfortunately, this statement was removed from the current document. The strict adherence to BLM guidance has moved from a specialized plan to a very standardized plan that is common in wild horse ranges across the country. These plans often rely on statistical measures rather than onsite observation and real-time documentation of the horses. This concept will be discussed within other topics as we proceed.

RMP Amendment (page 5-7): BLM lands are managed in accordance with Resource Management Plans. The plans establish how the lands will be used for different purposes. The BLM Billings Field Office RMP has three main management goals and nine management decisions (MD) that specify how the BLM should manage the Pryor horses. Due to a 2018 court order, it was determined that MD 7 needed to be amended. The intent of this management decision was for genetic diversity.

So the BLM made a radical departure and are proposing an amendment that changes genetic management to the use of a statistical measure, Observed Heterozygosity (HO). This is one example of how a specialized plan using bloodlines was changed to the use of a statistical measure. All you really need to know is there is a certain level for HO and if the level drops lower than that level, one of three actions would happen:

  • Maximize the number of fertile, breeding age wild horses (6-15).
  • Adjust the sex ratio in favor of males.
  • Introduce mares or stallions from other wild horse HMAs.

Concerns: This is a reactive measure and remedies are taken after the damage is done when the levels dip too low.


  • Engage in ongoing monitoring of mare lineage charts (aka bloodlines), to ensure that no lines are being eliminated or over represented. The mare lineages are unique for each founding mare. The Mustang Center has a full set of lineage charts for the current horses on the range.
  • Use the mare lineage charts as a proactive way to manage genetics. The use of observed heterozygosity can then be monitored periodically to ensure that it is meeting the desired level.
This is an example of a mare lineage chart that shows successive generations from left to right.

AML – Appropriate Management Level (Appendix C Page 1): This one is a tough one as it has the potential for major impacts to the population. Appropriate Management Level (AML) defines carrying capacity of the horse range. It is the number of horses that the land can support. The AML has been recalculated a number of times throughout the years. It is a range of numbers from high to low.

The AML has changed through the past years as a result of recalculations. The upper limit is the maximum number of wild horses. The lower number is the number that allows the population to grow to the upper limit (over a 4-5 year period) without any interim gathers. So essentially, you manage for the low end. When it gets to the high end…you have a gather.

  • 2009 Herd Management Area Plan: 90 – 120 horses
  • 2016 Recalculation: 98 – 121 horses
  • Current EA Recalculation: 108 – 121 horses


  • The current population of the Pryors is just over 200.
  • What will the ten-year plan look like as the population is moved to the low end of AML?
  • The BLM has set a target goal of 150 with an initial gather of 56. This is a high number and could have a negative impact on genetic diversity and social structure.


  • Don’t rush to AML. Define a cautious approach to meet the low-end objective.
  • Determine the need for multiple small gathers to reach the target number of 150 using population numbers at the time of the gather.

Gather Plan Alternatives: The BLM has presented four gather plans which are presented starting on page 13. Each plan has an objective to balance color, sex ratios, and age structures.  Each plan would remove horses with any genetic defects or closely inbred individuals.

  • Alternative 1: Continue Existing Management with RMP Plan Amendment. Selections would be based on a Colonial Spanish Matrix which was last used in 2009.
  • Alternative 2 (BLM Proposed Action): After making the initial removal determinations (i.e. genetic considerations), horses would be randomly removed.  
  • Alternative 3: Lineage Based Decisions – Selective removal criteria would be used with actual lineage data.  
  • Alternative 4: Continues Existing Management, No RMP Plan Amendment (No Action Alternative).

The appendices show the lists of horses that would be slated for removal under the various alternatives. These lists show the impact of each plan:

  • Appendix E: Alternative 1: Selective Removal Criteria (p. 1-3)
  • Appendix F: Alternative 2: Horse Gather/Removal List (p. 1-5)
  • Appendix G:
    • Alternative 3: 2023/2024 Horse Gather/Removal (p. 1-9)
    • Table 2: Alternative 3 Horses (p. 10-24)


  • Alternative 1 is not recommended as it uses scoring based on horses in 2009. This plan would remove all descendants of any horse not scoring high in the rating matrix at that time and could drastically reduce the genetic diversity in the herd.
  • Alternative 2 is not recommended due to a more rapid potential decline of genetic diversity (p. 53). It is the BLM proposed action with this statement, “Alternative 2 most closely aligns with management guidance in the BLM handbook.” This is another example where standardized practice could replace a specialized plan.
  • Alternative 3 is our preferred alternative, and with revisions could serve as a valuable specialized plan for the Pryor Horses. These are the main concerns of this alternative as written:
    • The ambiguous criteria for selection decisions.
    • The information in Appendix G shows an inconsistent use of the selection criteria. For example, Greta is considered a non-responder and would be slated for removal, as would five of her seven offspring. On the other hand, Feldspar, who has the highest current number of offspring on the range, only has three of eight slated for removal. Feldspar has not been identified as a non-responder, and she, herself will remain on the range.
    • In addition, the information in this section has many errors which brings into question to the validity of the section.
  • The Mustang Center is willing to work with the BLM officials to further develop a clearly defined selection procedure for Alternative 3 using lineage charts and other parent/offspring relationships.

5/28/2023: It has been a month since the end of the comment period for this Environmental Assessment. At this point, the Bureau of Land Management has not issued any updates regarding the EA.

Published in: on March 23, 2023 at 7:41 pm  Comments (6)