Published May, 2014

As many of us know, horse ownership is like eating potato chips; you can’t have just one! The definition of too many horses depends on who you are asking and what parameters you are considering.

Before trying to decide if your checkbook can stand to take on one more horse, ask yourself, “Is my land capable?”

More specifically, can the land you have support the horses you own in an economical and environmentally friendly way? There are tools to answer this question, one of which is the National Cooperative Soil Survey (NCSS) program.

National Cooperative Soil Survey

The NCSS program began in 1896 as an attempt to survey and map soils in the United States. The program started small, surveying only 2.8 million acres in Maryland, Connecticut, Utah, and New Mexico. Today, soil survey data is available online for the entire country as the Web Soil Survey (WSS) and is maintained by the USDA-NRCS (United States Department of Agriculture, Natural Resources Conservation Service). The NCSS has a wealth of information and uses, both in and out of agriculture.

Using Web Soil Survey

One of the many features of WSS is the ability to calculate your farm’s carrying capacity, answering the question of “How many horses can my farm hold?”

Below is a step-by-step guide to viewing soil characteristics for any piece of land in the United States. A PowerPoint presentation has also been created to graphically walk you through each step and can be found on the UK Horse Pasture Evaluation Facebook page.

For this example, we will select roughly 80 acres of prime horse pasture located on the University of Kentucky Research Farm near Lexington, Kentucky.

1. Navigate to the Web Soil Survey athttp://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm or search for “web soil survey.” Click the green “START WSS” button.
2. Enter Address. Click the dropdown arrows, enter the address, and click view.
3. Find Your Area. Entering an address in WSS is like using a GPS--it isn’t always completely accurate. Use the “+” magnifying glass to zoom in, the “-“ magnifying glass to zoom out, and the “hand tool” to move the map left, right, up, or down.
4. Select Your Farm. Use the “AOI” button on the right to select your farm by clicking on points outlining the farm. The program will connect the points with a straight line. Double click on the last point to complete the area. If you make a mistake and need to try again, simply click the “AOI” button again to start over. When you are finished, the total acres selected will appear on the left.
5. View Your Soil Map. Click the “Soil Map” tab at the very top (above the map). This will show a map of the selected area with each soil type outlined. A table containing all soil types in the selected area will be displayed on the left. To view this as a PDF or to print, click “Printable Version” on the far right. In the table, you can click on the soil types to learn more about that series.
6. View Soil Ratings. Click “Soil Data Explorer,” then the dropdown arrows for “Vegetative productivity.” Click “Yields of Non-Irrigated Crops (Map Unit)” and select “Pasture” from the dropdown menu. Finally, click “View Rating.” You will now see a map of the selected area with soil types colored in. Click “Printable Version” in the right corner to view as a PDF or to print. Scrolling down will show a table of the soil types and the ratings for each type in Animal Unit Months (AUM). This unit tells us how many months one acre of this soil type can carry a 1,000-pound animal with average precipitation and recommended fertilization.

Interpreting Soil Ratings

As stated, AUM soil ratings indicate how many months one acre of land can carry one animal unit. This is useful for other livestock species whose numbers will fluctuate throughout the year, such as farms where calves are bought in the spring and sold in the fall. Generally, horse numbers are more stable, especially on nonbreeding farms. Therefore, those ratings are converted to acres per horse per year (AHY).

1. Adjusting for Horses. An animal unit is defined as 1,000 pounds. The average horse's weight is closer to 1,200 pounds, so divide the AUM rating by 1.2 to get the adjusted AUM rating. Obviously, the horse's breed and age has a significant impact on the animal's average weight, so you might need to use a different adjustment factor. Light breeds (ponies) that average only 800 pounds would adjust by 0.8, while draft breeds that average 1,600 pounds might adjust by 1.6.
2. Converting Months to Year. Divide 12 by the adjusted AUM rating to convert years. This gives acres per horse per year (AHY).
3. Carrying Capacity by Soil Type. Dividing the number of acres you have of the soil type by the AHY will tell you the number of horses you can carry for a year on that soil type.
4. Total Farm Carrying Capacity. You can repeat the calculations for each soil type and add the number of horses each soil type can carry to determine your farm's total carrying capacity.

For example, 80 acres on the UK Research farm in Lexington, which included 12 acres of Huntington silt loam, would be calculated as follows:

9.5 (AUM rating) / 1.2 (adjustment factor) = 7.92 (adjusted rating)
12 (months) / 7.92 (adjusted AUM rating) = 1.5 (acres per horse per year)

Now we know that we need 1.5 acres of this Huntington silt loam to carry one horse for one year. Now we can calculate how many horses we can have on 12 acres:

12 (acres of Huntington silt loam) / 1.5 (AHY) = 8 horses

This tells us that we can carry eight horses on 12 acres of Huntington silt loam. We can repeat this process for each soil series present to calculate how many horses the entire farm can carry.

Uses and Limitations

Understanding a property's soil types is valuable in many ways. Consider the production potential of a piece of land before a rent or purchase decision. Use carrying capacity to estimate the land's profitability based on the number of horses you could house for boarding, training, or breeding purposes. Plan buildings, roads, and fencing to utilize the best soils on your farm for pasture and understand what challenges you will face managing the land.

Proper soil fertility is key in obtaining maximum production. Permanent pastures benefit from recycled nutrients in manure and urine. Pastures should be soil sampled every three years and fertilized according to laboratory recommendations. Cutting hay from pastures removes many more nutrients; therefore hay fields should be soil sampled every year and fertilized accordingly to maintain production. For more information on soil sampling, see the UK publication “Taking a Soil Sample” or contact your local county extension agent.

Just like making a budget and sticking to a budget are two different things, so is determining the capacity of your farm and implementing it. When rating soil types, the WSS makes a few key assumptions. The rating assumes that you will practice good pasture management, including maintaining good grass cover, managing weeds, using rotational grazing, and maintaining soil fertility. WSS also assumes average weather conditions. Events such as a late spring, hard winter, or dry summer will all impact the carrying capacity. Keep in mind that these are yearlong averages; in most years there will be excessive pasture growth in the spring that will require clipping and hay feeding that will be needed in the winter. Year-round grazing is possible in some areas but requires intense management. Finally, most horse farms will also have roads, barns, and common areas that are not included in pasture; remember to account for these nonproductive areas when determining your farm's carrying capacity.

Knowing the production potential and limitations of the soil under your farm is key to reducing feeding costs when managing horses and being a good land steward.

Krista Lea, MS, UK’s Department of Plant and Soil Sciences provided this information.