Using Web Soil Survey to Estimate Cool-Season Pastures’ Carrying Capacity

The stocking rate of livestock on a pasture has a significant impact on both forage productivity and financial profitability of an operation. Too many animals in a field results in overgrazing and the need to provide costly supplemental feed while too few animals results in underutilized forages and, therefore, lost income. While many factors influence how many animals a farm can carry, soil type has a major influence and should be considered when purchasing, leasing, planning or managing livestock on pastures.

“Carrying Capacity” is the number of animals the environment can sustain indefinitely given its food, habitat, water, and other availabilities necessities. For livestock, this is how many animals a farm or pasture can carry throughout the year without negative environmental impacts. Carrying capacity is based on the soil type and slope of the land, characteristics that take thousands of years or more to change. The productive capacity will dictate what that land is most useful for.

National Cooperative Soil Survey

The National Cooperative Soil Survey (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 Connecticut, Maryland, New Mexico, and Utah. Today, soil survey data is available online for the entire country as the Web Soil Survey (WSS) and is maintained by the United States Department of Agriculture Natural Resource Conservation Service (USDA-NRCS).

Using Web Soil Survey

Web Soil Survey has a tremendous wealth of information both in and out of agriculture. This publication will focus on using Web Soil Survey to calculate the livestock carrying capacity on cool season pastures.

• Navigate to the Web Soil Survey homepage, and click the green “START WSS” button.
• Enter the address of the property in question and click “view” to display the land area. The area of interest can also be located by other means such as using latitude and longitude.
• Find the property using the tools at the top of the satellite photo. 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.
• Outline the “Area of Interest” (AOI) using the AOI rectangle or polygon buttons on the right. Double click on the last point to complete the area. When finished, the total acres selected will appear on the left. This step can take a few seconds to a couple minutes depending on connection speeds, so be patient.
• To view the soil map for the selected area, click the “Soil Map” tab at the very top (above the map). Click “Printable Version” on the right to view these documents in a PDF form (be sure you’ve disabled your browser pop-up blockers).
• To retrieve the pasture 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 under “Basic Options.” Finally, click “View Rating.” Click “Printable Version” in the right corner to view as a PDF.

Interpreting Soil Ratings

Soil ratings range from 1 to 9.5 Animal Unit Months (AUM). AUM is how many months one acre of land can carry one 1,000-pound animal (1,000 pounds = 1 animal unit). Acres per Animal (A/A) is an easier unit to work with and easy to calculate. To convert the AUM to animals per acre, you must first calculate an animal adjustment factor by dividing the average weight of your animals by 1,000 (because not all livestock are 1,000 pounds), which provides the number of animal units per animal. Next, divide the AUM rating provided by Web Soil Survey by the animal adjustment factor. Finally, divide 12 (for months in a year) by the adjusted AUM rating to convert to acres per animal.

Calculations from table 1 indicate you’d need 0.4 acres to house sheep on well-rated soils (9.5) and slightly more for poorer soils. Two acres would be needed for a 1,000 lb. cow on low rated soil and almost 2.5 acres for a 1,200 lb. horse on similar land.

Comparing Productive Capacities

For the purposes of this publication, we’ve compared two tracts of land. Farm A contains just over 1,000 acres in Fayette County currently used for horse pasture, while Farm B includes around 950 acres in Owen County currently used for beef production.

Farm A is dominated by Bluegrass-Maury silt loam with a 2 to 6% slope (uBlmB) and has a rating of 9.5. This is one of the highest ratings in Kentucky for pasture and could sustain a 1,000 lb. cow on 1.3 acres or a 1,200 lb. horse on 1.5 acres. Another soil type, Maury-Bluegrass silt loam with a 6 to 12% slope is similar but has a slightly lower rating of 9, mainly due to the increased slope. Water will run off sloped land more quickly, increasing erosion and decreasing water available for pasture plants.

Farm B is dominated by Eden flaggy silty clay with 20 to 30% slope and is severely eroded (EfE3) with a rating of 4.8. Farm B will need to allocate 2.5 acres for a 1,000-pounds cow or 3 acres for a 1,200-pound horse. While this soil may not have the highest rating, it can still be productive and profitable when managed correctly and not overgrazed.

Uses and Limitations

Understanding the soil types available on a piece of property is valuable in many ways, especially when considering a piece of land’s production potential before renting or purchasing. You can use carrying capacity to estimate the profitability of the land and, ultimately, its value, based on the number of animals it might carry. In addition, understanding soil properties can help you develop a strategy to locate buildings, roads, and fencing on marginal soils and use the best soils for pasture. Fundamentally, a clear understanding of your soils and how they are located will help you to better manage your land base.

To maximize your soils’ production potential and maintain optimum soil fertility, collect soil samples routinely and follow laboratory fertilizer recommendations. Because permanent pastures benefit from recycled nutrients in manure and urine, you only need to soil sample pastures every three years. Cutting hay from pastures removes many more nutrients; therefore, you should soil sample hay fields every year. When doing so, take multiple soil cores (10-20) with a soil probe and mix them together to form one sample for lab analysis. Soil samples should represent the top 3-4 inches of soil in untilled fields such as pastures. Generally, each sample submitted should represent no more than 20 acres. Submit separate samples for areas within a pasture that have major differences in soil properties or historic management (for example, if there was previously a tobacco patch in a pasture). Finally, apply fertilizer and lime according to laboratory recommendations. You can find more information on fertilizer and lime recommendations in AGR-1 “Lime and Nutrient Recommendations.”

When rating soil types, Web Soil Survey makes a few key assumptions. The rating assumes good pasture management. This includes maintaining good grass cover, managing weeds, rotational grazing, and maintaining soil fertility. Web Soil Survey 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 may not be utilized and hay feeding will be needed in the winter. Year-round grazing is possible in some areas but requires intensive management.  Finally, most farms will also have roads, barns, and common areas that are not included in pasture. Remember to account for these nonproductive areas when determining a farm’s total carrying capacity.

Summary

Soil type has a significant impact on a pasture’s productivity and limitations. Web Soil Survey provides farm managers and owners with valuable information they can use to calculate their farm’s carrying capacity. Proper pasture management is essential to reaching maximum utilization without overgrazing and damaging pastures. Find more information and soil sampling resources by contacting your local county Cooperative Extension agent; visit www.ca.uky.edu/county to locate your extension office.

Ray Smith, PhD, professor and forage extension specialist within UK’s Department of Plant and Soil Sciences, and Krista Lea, MS, research analyst and coordinator of UK’s Horse Pasture Evaluation Program within the Department of Plant and Soil Sciences provided this information.

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