How to Calculate Animal Unit Months (AUMS)

Calculating Animal Unit Months (AUMs) is a fundamental practice in sustainable livestock and range management, representing the amount of forage required to sustain one animal unit (AU) for one month. This calculation helps determine appropriate stocking rates for pastures, ensuring the long-term health and productivity of both the land and the livestock.

To determine the number of AUMs needed to support your livestock, follow these two primary steps:

1. Calculate Total Animal Units (AU): Multiply the number of animals planned for grazing by their respective Animal Unit Equivalent (AUE).
2. Calculate Total Animal Unit Months (AUMS): Multiply the calculated total AU by the number of months the animals are planned to graze.

Understanding Key Concepts

Before diving into the calculation, it's crucial to understand the terms involved:

• Animal Unit (AU): A standard measure representing the forage consumption of a 1,000-pound cow (with or without a calf). All other livestock types are converted to this standard based on their relative forage intake.
• Animal Unit Equivalent (AUE): A factor used to convert different types or sizes of livestock into standard Animal Units (AU). It reflects how much forage an animal consumes relative to a 1,000-pound cow.
• Animal Unit Month (AUM): The quantity of forage required to support one Animal Unit (1,000-pound cow) for a period of one month.

The AUM Calculation Formula

The process can be broken down into these two distinct formulas:

1. Total Animal Units (AU) = Number of Animals × Animal Unit Equivalent (AUE)
2. Total Animal Unit Months (AUMS) = Total Animal Units (AU) × Number of Grazing Months

Step-by-Step Guide to Calculating AUMS

Follow these steps to accurately calculate your AUM requirements:

1. Identify Your Livestock and Their Numbers:

   • Begin by listing all the types of animals you plan to graze on a specific pasture.
   • Record the exact number of each animal type.

2. Determine the Animal Unit Equivalent (AUE) for Each Animal Type:

   • Use a reliable table of AUE values to find the appropriate factor for each animal. These values account for differences in body weight, metabolic rate, and whether the animal is lactating or growing.

   • Example AUE Values:
     Most AUE values are based on a 1,000-pound animal consuming approximately 26 pounds of dry matter forage per day (or about 780 pounds per month).

     Animal Type	Average Weight (lbs)	Typical AUE Factor
     Cow (with calf, 1000 lbs)	1000	1.00
     Heifer (800 lbs)	800	0.80
     Bull (1500 lbs)	1500	1.50
     Horse (1000 lbs)	1000	1.25
     Sheep (ewe with lamb, 150 lbs)	150	0.20
     Goat (nanny with kid, 100 lbs)	100	0.17
     Deer (adult)	150	0.20

     It's important to note that specific AUE values can vary slightly based on animal breed, size, and physiological state. Consult local agricultural extension services or ranching guides for the most accurate values relevant to your region and livestock. A detailed guide on stocking rates and AUE can often be found through resources like the USDA Natural Resources Conservation Service (NRCS) or university extension programs.

3. Calculate the Total Animal Units (AU):

   • For each animal type, multiply the number of animals by their AUE factor.
   • Sum these values to get the total AU for all livestock on the pasture.
   • Formula: Total AU = (Number of Cows × AUE_Cow) + (Number of Horses × AUE_Horse) + ...

4. Determine the Planned Grazing Duration:

   • Specify the exact number of months (or fractions of months) your livestock will graze on the particular pasture.

5. Calculate the Total Animal Unit Months (AUMS):

   • Multiply the total AU calculated in step 3 by the total number of grazing months determined in step 4. This final figure represents the total AUMs required.
   • Formula: Total AUMs = Total AU × Number of Grazing Months

Practical Example

Let's say you plan to graze 25 cows (with calves) and 5 horses on a pasture for 4 months.

1. Number of Animals:

   • Cows: 25
   • Horses: 5

2. Animal Unit Equivalents (AUE):

   • Cow (with calf): 1.00
   • Horse: 1.25

3. Calculate Total Animal Units (AU):

   • For Cows: 25 animals × 1.00 AUE = 25 AU
   • For Horses: 5 animals × 1.25 AUE = 6.25 AU
   • Total AU = 25 AU + 6.25 AU = 31.25 AU

4. Planned Grazing Duration:

   • 4 months

5. Calculate Total Animal Unit Months (AUMS):

   • Total AUMS = 31.25 AU × 4 months = 125 AUMS

Therefore, you would need 125 AUMs of forage to support this livestock combination for a 4-month grazing period.

Importance of AUMS

Calculating AUMs is crucial for:

• Sustainable Stocking Rates: It helps ranchers and land managers prevent overgrazing, which can degrade pasture health, reduce forage production, and lead to soil erosion.
• Forage Inventory and Planning: By knowing the forage production capacity of a pasture (often expressed in AUMs per acre) and the livestock's AUM demand, managers can balance supply and demand.
• Lease Agreements: AUMs are often used in grazing lease agreements to specify the amount of forage allowed.
• Wildlife Management: The concept can also be adapted to assess forage competition between livestock and wildlife.

By accurately calculating AUMs, you can ensure your grazing practices are both economically viable and environmentally responsible, promoting healthy rangelands and productive livestock.