How Much Pasture do I Need and What are AUMs?

How Much Pasture do I Need and What are AUMs?
Jan 30, 2023

By Ryan Benjamin

Determining how much pasture is needed for summer grazing can be difficult. There are many factors that affect pasture productivity including current year precipitation amounts and timing and previous year growing season’s conditions. The drought conditions that much of the U.S. experienced last year limited the amount of energy plants were able to store, especially if they were heavily grazed. It is important to consider this when developing a grazing plan for this year. This will aide in understanding how to set stocking rates and prevent further damage.

Many ranchers and government agencies discuss grazing in AUMs (Animal Unit Months). However, this measurement of forage can be confusing for many. AUMs provide a standard unit of measurement for forage needs and availability. One AUM is the amount of forage a 1,000-pound animal will eat in a month or about 780 pounds of air-dry forage (90% dry matter). AUMs are based on body weight to account for variation between classes, ages, and species, The equation to convert any weight of animal to a standard animal unit is:

weight of animal/1,000 = animal unit equivalent.

For example, a yearling weighing 700 lbs. is 0.7 AU (Animal Unit), and a 1,500 lb. animal is 1.5 AU.

To help demonstrate how to utilize this math to calculate pasture needs and carrying capacity, below are a couple example ranches. The first is the Rocking PC Ranch consisting of 400 cows, 14 bulls, 10 horses and a small sheep enterprise. They need to determine how many acres of pasture they will need this year. The Rocking PC has historically had about 6 months of growing season grazing and 2 months of winter grazing, while the remaining 4 months hay is fed.

The Rocking PC Ranch Forage Demand

Summer Pasture Forage Demand

400 pairs x (1300lbs/1000) = 520 AU                520 AU x 6 Months = 3,120 AUM

14 bulls x (2000lbs/1000) = 28 AU                    28 AU x 6 Months = 168 AUM

10 horses x (1500 lbs/1000) = 15 AU                 15 AU x6 Months = 90 AUM

100 sheep x (200lbs/1000) = 20 AU                   20 AU x 6 Months = 120 AUM

Total requirement                     3,498 AUM of Summer Pasture


Winter Pasture Forage Demand

400 pairs x (1300lbs/1000) = 520 AU                520 AU x 2 Months = 1,040 AUM

14 bulls x (2000lbs/1000) = 28 AU                    28 AU x 2 Months = 56 AUM

10 horses x (1500 lbs/1000) = 15 AU                 15 AU x 2 Months = 30 AUM

100 sheep x (200lbs/1000) = 20 AU                   20 AU x 2 Months = 40 AUM

Total requirement                     1,166 AUM of Winter Pasture

In the area, forage production averages 1,600 lbs. of forage per acre. Using the "take half– leave half” rule of thumb, this leaves about 800 lbs. of forage for grazing, however, some of this will be lost due to trampling and soiling, plant material consumed by wildlife, or plant material that is just not consumed. For most limited management systems, the amount of forage actually consumed by the grazers is around 25-30%, meaning about 480 lbs. per acre is used to feed the animals. This is referred to as harvest efficiency, and refers to the amount of forage consumed, the amount of forage lost to trampling, and the forage left for plant health and vigor. Harvest efficiency can range from 20% in season long grazing (20% consumed) to 35% in some rotational grazing systems. Winter grazed pastures are often grazed more heavily without harming the pasture health. For this example, 50% of winter forage is consumed (ec127.pdf (

Acres of pasture or forage supply

3,498 AUM x 780 lbs. = 2,728,440 lbs. of forage required. If only 30% of forage is consumed by cattle (30% harvest efficiency, 20% trampled/loss, 50% for plant vigor = 100% of plant production), divide by 0.3 to determine total pounds of forage required and then divide by the lbs. per acre produced to determine the number of acres. For example,

2,728,440 lbs./0.3 = 9,094,800 lbs. of total forage required

9,094,800 lbs. of total forage required /1,600 lbs./ac (supply) = 5,684.25 acres of pasture needed for summer grazing.

This same process can be repeated to determine winter grazing acres:

1,166 AUM x 780 lbs.=909,480 lbs./0.5 = 1,818,960 lbs./1,600 lbs./ac = 1,136.85 acres of winter pasture.

The total number of pasture acres required by the Rocking PC Ranch is 6,821.1 acres. This does not include hay ground or feed required for the other 4 months of the winter. This estimation also assumes normal or average conditions. During drought more acres will be required, or forage demand may have to be reduced.

Carrying Capacity

This process can also be done in reverse, with a set number of acres to determine the number of animals and amount of time they can be on pasture. A ranch consists of 4,000 acres averaging 1,400 lbs. of forage per acre and wants to run some yearlings for 5 months starting at 600 lbs. and selling at 900 lbs. (averaging 750 lbs. while on pasture).

4,000 acres x 1,400 lbs./ac.=5,600,000 lbs. x 0.3 (harvest efficiency) = 1,680,000 lbs./780 lbs./AUM= 2,153.85 AUM/5 Months= 430 AU

750 lbs. (average weight)/1,000= 0.75 AU

430 AU/0.75 AU= 574 Yearlings

Drought Management

There are many factors that play into forage production on pasture, especially native rangelands. During periods of drought, forage production can be greatly reduced and can have effects that last into following years. This is compounded by the length of the drought, drought severity, and management decisions. Knowing and understanding the previous year’s grazing pressure and drought severity aides in assessing how plants will recover after a period of drought. In general, keeping stocking rates lower than normal and increasing rest periods can help pastures recover more quickly. Drought severity and grazing pressure should be the driving forces of post-drought management decisions. Having a plan for drought ahead of time is helpful in successfully managing through a drought. If drought conditions persist for multiple years, stocking rates must be reduced to match forage production. There are tools such as GrassCast, the rangeland analysis platform, and NOAAs season long forecast that can help livestock managers forecast possible drought conditions coming and help with developing a plan for lowering stocking rates ahead of drought. Having set decisions points allows you to take some of the emotion out of tough management decisions. Giving your pasture time to recover is imperative after a multi-year drought. Continual pasture condition monitoring, such as photo monitoring, and carefully following weather forecasts should help you make informed stocking rate decisions to keep your pasture as productive as possible.

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