As one moves from east to west across the United States, precipitation declines, especially after you cross the 100th meridian. With this decline in average annual precipitation, plant species change–there are less trees and different species of grasses and broadleaved plants. This change can easily be seen on a satellite image of the earth; west of the line colors tend to be in the brown family, while on the east side of the line, colors are more in the green family. Because of the difference in plant species, time and timing of precipitation, and amount of precipitation the number of animals that you can sustainably graze on a piece of land throughout the year differ greatly. To determine how many animals you can graze on a piece of land, you will need to know two things, 1) how much forage the particular animal will consume, and 2) how much forage is available.
When comparing different livestock grazing systems it is necessary to convert all units to a common language. To do this we use what is known as an animal unit month (AUM). An AUM is the measure of forage required by an animal for 1 month, which in most cases is defined as 760-800 pounds depending on what land management agency you are working with. An animal is defined as one, 1000 pound cow or her equivalent. See the chart below for the equivalent definition. This allows us to compare different types of animals as well as different sizes of animals in the same grazing system. Let us look at two different systems–one found on the east side of the 100th Meridian and one found on the west.
Jasper County, Missouri is located in the Southwest corner of the state. It is characterized as a loamy upland prairie. Its elevation is 1000-1700 feet above sea level. It has an average annual precipitation of 41-45 inches and its growing season is 195-225 days long. The soils in the county are characterized as deep to very deep, moist but well-drained, and medium to fine textured. The annual vegetation production is 5,000-10,000 pounds per acre depending on soil type and precipitation. Native grasses include big bluestem, little bluestem, Indian grass, and switch-grass.
On the west side of the 100 Meridian is Converse County, Wyoming. It is located in the East Central part of the state, in the Northern Prairie, and has an elevation of 2950-5900 feet above sea level. Its average annual precipitation is 9-14 inches, with a growing period of 145-170 days. Soils are characterized as shallow to very deep, dry, well drained, and medium to fine textured. Annual forage production is 700-2,000 pounds per acre depending on soil type and precipitation. Native grasses include rhizomatous wheatgrasses, green needle, needleandthread, and blue grama.
So let us just say we had 10 acres of land in each of these counties. How many 1000 lb cows could we graze for one month?
Jasper County, Missouri
Forage production = 7,000 lbs/acre/year
Forage for 1 month = 5,833lbs
We will use 60% of the forage (40% is left for the wildlife and to ensure plant health) = 3,500lbs
Animal Units (or 1000 lb cows) = 3500/800 = 4 cows
We could graze 4 cows for 1 month on 10 acres
Converse County, Wyoming
Forage production = 1,000lbs/acre/year
Forage for 1 month = 833lbs
We will use 40% of the forage (60% is left for wildlife and to ensure plant health) = 333lbs
Animal Units (or 1000 lbs cows) = 333/800 = 0.5 cows
We could only graze 1/2 a cow for 1 month on 10 acres; or we would need 20 acres to graze 1 cow for 1 month; or we could graze 1 cow for 15 days on 10 acres.
Stocking rates differ greatly depending on forage production which is determined by the amount and timing of precipitation, plant species present, and soil type. Figuring stocking rates is fairly easy once you get the hang of it and is one of the more important tasks in a successful livestock production system. Forage production and stocking rate records are critical in making timely management decisions. If you need assistance in calculating your stocking rate be sure to contact your local University Extension Office.