November 6, 2009 – Appropriate Management Level

In the first blog post of my management series, I discussed the geography of the Pryor Mountain Wild Horse Range so that it is easier to visualize that the wild horses do live in areas that are a certain size that can thus produce a certain volume of resources that can support a certain number of wild horses. (Please click here to read this blog post.) With that, let’s go through what should be the most math-heavy post of this blog series.

Today I’d like to focus on actual numbers; how does one define what the aforementioned “certain number of wild horses” is for a given area? This number is referred to as the Appropriate Management Level, and it is commonly referred to as AML. In the glossary of the BLM’s Pryor Mountain Wild Horse Range Evaluation (February 2008), there is this definition: Appropriate Management Level means the maximum number of wild horses or burros excluding the current year’s foal crop that can be maintained within an area without causing deterioration of rangeland resources. So the previous blog post walked though the concept of carrying capacity as related to the PMWHR, and all of this is basically the reasoning behind the definition of AML.

All herd’s have their own AML. To get a feeling for what typical AMLs are for different herds, I used the BLM’s 2008 wild horse census information; and I made a histogram showing the range of AMLs that exist. I only used data from herds that have wild horse AMLs. Thus, this isn’t about burro AMLs, though some herds with horse AMLs may also have burro AMLs; these aren’t the focus here though. This chart includes data from 171 Herd Management Areas, which is the number I counted with an AML of at least 1 wild horse. In the histogram, the X-axis shows different values of AML, and the Y-axis shows how many herds have that particular AML. So, for example, the first column shows us that there are 53 HMAs with AMLs of 50 or less wild horses and the second column shows us that there are 42 HMAs with AMLs between 51 and 100 wild horses. Here is the histogram (Please click on it and all following images for full-sized versions.):

Histogram of AML ranges for HMAs
So what is AML for the PMWHR? The first AML set for the Range occurred in 1984; it was determined through the 1984 Herd Management Area Plan and the 1984 Billings Resource Area Resource Management Plan. This AML was 121 adult wild horses, so from 1984 to 1992, the AML for the PMWHR was 121 adult wild horses.

AML was recalculated in 1992 with the revision of the 1984 HMAP. Recall from the previous post that there was a change in the size of the PMWHR that occurred due to the loss of National Park Service lands; the result was a smaller area. Because of this, AML went from 121 adult wild horses to 95 plus or minus 10% horses. Thus, starting in 1992, the AML was 95 to 105 wild horses. Due to concerns over the herd’s genetics, recommendations were made in 1994 to not bring the wild horse population down below 100 individuals one year of age and older plus that year’s foal crop. This is basically why the population was always floating around 140 individuals from 1992 onward into the early 2000’s. However, AML was still legally 95 to 105 wild horses. I learned of this information from an interesting discussion of Appropriate Management Level that was included in the BLM’s Environmental Assessment for the 2001 roundup.

The AML of 95 to 105 wild horses was still in existence until recently, when the BLM developed a new Herd Management Area Plan. This HMAP will reset AML at 90 to 120 individuals one year of age and older plus that year’s foal crop. The target population size is the high end of the range, 120 individuals plus that year’s foal crop.

So now we know what AML means and what it is for the PMWHR, but how is it determined? In the BLM’s Pryor Mountain Wild Horse Range Evaluation (February 2008), Staff Technical Recommendations section B as well as Appendices VII and VIII go through the actual calculations that led to the determination of the current AML. The formulae used in the following calculations come from BLM manuals on rangeland evaluation.

The low end of AML (90) was calculated with one particular formula that estimates carrying capacity. The equation works with ratios. It says that if you have a population of a certain size that are utilizing a certain percentage of resources, then how big of population size would you need to have a set amount of resources utilized? Put another way:


So the actual population size is measured each year, of course. The utilization, which is expressed as a percentage, is found through field observations on range conditions. There is an actual, objective process used by the BLM to quantify utilization. Basically, the utilization describes the percentage of forage consumed by the entire population each year. The goal in this calculation is to see 45% of forage consumed each year. Where did 45% come from? It is arbitrary, but it within the commonly used range of values for desired utilization. It basically is saying that the goal is for the population to be using just under half of the available forage. So reworking the formula with this information, we can represent the formula like this:

In the actual calculations that occurred, the BLM determined the “desired use,” which we can think of as the estimated carrying capacity, from 1995 to 2006. Let’s calculate the desired use for 1995 as an example. That year, there were 146 wild horses, and the measured utilization was 81%. Here’s how these numbers are used to calculate that the desired use for that year was 81 wild horses:

The average of the values from all those years was then taken, and this average is 92 horses. Thus, according to this calculation, from 1995 to 2006 there should have been an average of 92 horses on the PMWHR each year. This was rounded down to 90, and this is where the AML’s low end of 90 horses comes from. Remember, this was determined only from observations on the wild horse population and the observed forage utilization for the years 1995 to 2006.

The high end of AML (120) was calculated with another particular formula that also estimates carrying capacity. However, instead of only relying on wild horse population size and observed forage utilization, this formula brings in another factor: Precipitation. This formula modifies the observed utilization with precipitation data; it assumes that utilization is influenced by both wild horse grazing patterns and by precipitation. The first step in performing this calculation is to determine the modifying factor for each year. This factor is called the Yield Index. The Yield Index is based on the annual precipitation from October to September (Crop Year Precipitation) and the 30 year average of this aforementioned precipitation (Average Crop Year Precipitation). A constant factor ((1.23)-.23) is also in the formula. The Yield Index is determined from this formula:

Precipitation data from Lovell, Wyoming, was mostly used in calculating the Yield Indices for the years 1995 to 2006. This is because the Lovell, Wyoming, data are the most complete collection of precipitation data in the vicinity of the PMWHR. These Yield Indices were put into the formula described earlier to make this new formula:

This equation was used to calculate another value of “desired use”; but this value has been modified by the Yield Index to give a new set of values for desired use from 1995 to 2006. Let’s calculate the desired use for 1995 using this new equation. Remember from above, in 1995 there were 146 wild horses on the range, and the measured utilization was 81%. Also in 1995, the Crop Year Precipitation was 6.67 inches. The Average Crop Year Precipitation used in all calculations was 6.79 inches. Here is what the calculation for 1995 looks:


There isn’t a big difference in the results of the two calculations for 1995. This is because 1995 experienced just below average precipitation. Lower precipitation yields higher values for desired use. This is because lower precipitation leads to lower forage production. If there is lower forage production, then actual utilization is more heavily influenced by a combination of precipitation and wild horse grazing than wild horse grazing by itself. The year 2000’s low end desired use result was 95 wild horses. However, 2000 was also a year with below-average precipitation. Because of this, the high end desired use was calculated to be 151 wild horses.

When the average for these values was taken, it was determined to be 117. This was rounded up to 120, and this is where the AML’s high end of 120 horses comes from.

So that is what Appropriate Management Level is and how it relates to the PMWHR. With that, I think there some relevant questions:

When and how can AML be raised? After all, a larger herd size is a definite goal here and in many other herds.

How will AML be reached? We’ve seen how the 2009 roundup was used to move the herd size closer to 120, but roundups aren’t the only way to manage a wild horse population.

These are the topics of upcoming posts. This post and the previous one have worked toward establishing the scientific and legal reasoning behind management. Future posts will focus on actual methods of management. Thank you for taking the time to better understand AML!

Published in: on November 6, 2009 at 5:12 pm  Comments (14)  

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  1. I can see a lot of work went into that explanation of the AML and what it means. Thank you for that. The ability of the land to support the number of horses is of course number one. I have another concern, that being the numbers of foals surviving their first year due to predators and other factors, winter,accidents, etc.,therefore not needing birth control. I’m concerned about the birth control and the effect on when some foals are born (too close to winter) Isn’t the natural predator control enough for the Pryor Mountain Herd? I’m afraid the herd numbers may get too low and be irreversible. I can see the birth control maybe being an option in some other areas that have wild horse numbers that are way to high for the land to support. What’s your thoughts on that?

    • Hello,
      Thanks for taking the time to read the post! Your concerns are definitely valid. I will soon be putting up posts on the pros and cons of predation, roundups, and fertility control in managing wild horse populations. What I’d like to have happen is for the same questions be asked of these three strategies, and I think that you’re onto this already. I’ll be focusing most of the attention here on predation and fertility control as I think roundups are simple enough and common enough for less attention to be focused on. The other two, though, are topics I feel are worthy of some good discussion.
      Thanks again,

  2. Dear Matt,

    Will you share a little of your c.v. with us? That is to say, your “schoolin’.”
    Obviously you are knowlegeable in your field, committed to the welfare of the horses, probably permanently, from what I can tell. Are you a wildlife biologist? When did you first become involved with the mustang? Was it through your family?


    • P.S. Where is the First Blog of the Management Series? I do not want to miss any of these. Thanks

    • Sorry, I’d missed this post earlier. I have a BS in environmental science from UNM. I am from Lovell and have pretty much always lived here, and so I’ve always known about the Pryor horses. This, however, consisted mostly of trips to Bighorn Canyon. Growing up here I spent a lot of time all over the lands of the northern Bighorn Basin, and natural science is everywhere to be seen here. It wasn’t until 2004 when I started to pay attention to the horses at an individual level. This came about with my family getting more involved in the horses and the Center.

  3. Thank you for this additional resource to study (The Management Series) and the improved links. Reading about your background you shared was so interesting. So glad your family are all enjoying the horses together.

  4. Matt, Thanks for sharing this information. Very informative.

  5. I appreciate the time you have spent explaining how the AML’s are decided. I do have one question, though. On a recent Ken Mcnabb show they talked about the grazing on the Pryors and they were only counting certain species of grasses as forage. In Nevada the horses seem to thrive on mostly small forbs and sagebrush. And obviously certain parts of the range have better forage than others. As you pointed out in one post good water sources lead to heavier grazing in certain areas.

    Do you feel the calculations to determine AML are accurate in taking all those variables into account? I know its best for my pasture to err onthe side of caution when it comes to figuring carrying capacity. I am assuming the calculators also do that.

    • When doing the range evaluations, only forage species are counted. I am pretty certain that forbs and brushes aren’t counted. Definitely in the area Jared and Ken were talking at in the snow, there is a predominance of forbs that have little or no forage value compared to grasses. When I watched an evaluation being done it was definitely grass that was important, but this was in one of the desert areas of the range. I am hoping I can do a more thorough blog post on these evaluations in the future. Definitely there are areas under heavy grazing pressure due to water availability. This is going to be someone I talk about a little more in detail in an upcoming post. I think that the AML calculations are made to do as good of job as possible in determining carrying capacity given observed grazing patterns. It is tricky due to all of those other factors, though.

  6. Matt, It does seem that there is an assumption that 146 horses use 81% of the available resources. There needs to be a better measure than a calculation. That can only come from actual range study. Wild horses are such efficient grazers that they may pass over an area many times and it will regrow each time, like making hay. Mar

    • And I understand that, unlike some other grazers, wild horses “replant” a great many of the seeds of the plants they consume, since these seeds pass thru their systems intact, and are presented back to the earth with their own fertilizer! This provides for not only more food for the horses, but for many other species, including birds, and helps the entire ecological balance, unlike other grazers.

      • I very much agree with you that wild horses can benefit rangeland health.

    • There probably is a lot of room for improvement in these determinations. Unfortunately, this seems to be the best process that is currently available for the BLM to calculate AML.

      In 2004 the Natural Resources Conservation Service did a range study on the PMWHR, and they concluded the range could support 45 to 142 horses. This study also recommended that midslope water sources be developed to allow for better grazing distribution. This is something that I am a big supporter of as are many others, and it’s something I’m going to be touching on in an upcoming blog post.

      I’m referring to different reports a lot, so I am thinking about creating an area on our main website where these can all be read.

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