Stocking Rates and AUM - Frequently Asked Questions

 
 
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 What is the difference between carrying capacity and stocking rate?
Stocking rate is the number of specific types and classes of animals that can graze a unit of land for a specific time period. It is usually classified as the number of animal units an acre of land can hold for one month (AUM/acre). Carrying capacity is the maximum stocking rate possible year after year without causing damage to vegetation of related resources. Though actual stocking rates may vary considerably between years due to fluctuating forage conditions, carrying capacity is generally considered the average number of animals a particular pasture or range will be able to sustain over time. It is largely determined by a combination of both environment and management.

If a greater difference exists between current and potential forage production of a pasture or range, then a greater difference will also be found between carrying capacity and recommended stocking rate. When current forage production is well below potential forage production, pastures will need to be stocked well below their carrying capacity to allow them to recover. Carrying capacity considers both livestock's forage needs and adequate carryover. Carryover, or litter, is the forage material left behind after grazing. It is important to leave enough carryover to help plants regrow, restore plant food reserves, help conserve moisture, and protect the soil from wind and water erosion.

What is an Animal Unit Month?
An Animal Unit Month (AUM) is the amount of forage required by an "animal unit" in one month. The standard animal unit is defined as one mature cow-calf pair that weigh a total of 1000 pounds (912 lb cow with an 88 lb calf at side), and is based upon the average daily intake of 25 pounds of dry matter (DM) forage per day. On a per month basis, that cow-calf pair is expected to consume approximately 800 pounds of forage dry matter, though more realistically the monthly consumption ranges between 600 and 900 pounds.
If you included waste and trampling at 25%, then total forage usage would be close to around 1000 to 1200 pounds per month. However, the amount of waste and trampling is highly variable depending on location, so it's less reliable to include waste and trampling with monthly consumption for a cow-calf pair than just monthly consumption on a dry matter basis.

How do I adjust for other types of livestock, and for larger cow size?
Animal unit equivalents are used to adjust for any animal type based on their forage requirements. Smaller animals need less forage, and larger animals need more forage.

To adjust for animal size, divide the actual average body weight of your animals by the body weight of the standard cow-calf pair. For example, if you had ewes with an average body weight of 200 pounds, then 200 lb 1000 lb = 0.20 AUE. The same formula can be used for larger animals, such as with the modern beef cow that weighs closer to 1400 pounds. The table below shows the average animal unit equivalencies for each animal type.


Class of Animal
Animal Unit Equivalents
Small cow
0.90
Cow-calf pair, standard
1.00
Large cow
1.40
Small bull, 2 years or older
1.50
Large bull, 2 years or older
2.00
Yearling heifer or steer
0.75
Weaned calf
0.50
Horse, 2 year old
1.00
Horse, 3 years old and over
1.50
Horse, yearling
0.75
5 Ewes or does, with or without lambs or kids
1.00
5 Rams or bucks
1.30
5 Weaned lambs or kids, up to 12 months
0.50
Bison cow (average)
1.00
Bison bull (average)
1.50
Bison yearling
0.65
5 Deer
1.00

How do I figure out my stocking rate?
You already know that approximately 800 pounds of forage is needed per standard animal unit per month. What you next need to know is the forage yield in your area, in terms of pounds per acre. Remember that precipitation directly affects forage yield, so if you are in a dry location with no irrigation (as in south-eastern Alberta), expect lower forage yields than if you were further north with higher precipitation.

Determining forage yield is best done by collecting some forage samples in your pastures using a 1 ft by 1 ft square at several locations and then drying them in an oven to get average dry matter weight per square foot. These results can be calculated to obtain your average forage yield on a per acre basis.

A much less reliable alternative is using hay production data for your area, or the amount of hay you or your neighbours get in the season, to estimate forage yield. Your pastures may not have the same species nor the same plant counts as in your hay fields, and you may have much less forage in your pastures than your hay fields, potentially giving you a higher forage yield value for your pastures than what may be more realistic. Please use this option with caution.

Once you have your forage yield, determine how much forage is going to be removed, based on pasture quality. This is called the utilization rate, and is a percentage of how much forage livestock will consume, versus how much is left behind. The average utilization rate for a fair to good pasture is 50 percent. Poor pastures will have a lower utilization rate (around 25 to 40 percent), and excellent pastures (with maximum forage yield for the area), will have a little higher utilization rate (55 to 60 percent). Utilization rates may also be lower if you are continuously grazing a pasture compared with using managed rotational grazing methods.

To calculate stocking rate for your area, multiply your average forage yield (lb/acre) by utilization rate, then divide that by the amount an animal unit is expected to consume per month. The formula would look like this:

Stocking rate = (Forage yield [lb/acre] x (Utilization rate [%] 100)) 800 lb/AU/month

Your stocking rate will not stay the same year after year, so you will need to adjust the number of animals you intend to graze according to the current stocking rate for your area.

References cited:
Wroe, et al. 1988. Guide to Range Condition and Stocking Rates for Alberta Grasslands. Alberta Forestry, Lands and Wildlife. Public Lands. Edmonton, Alberta.

Arychuk, C.E. 2000. Setting Stocking Rates for Pastures. Bison Producers of Alberta

For more information:
Using a Grazing Stick to Determine Stocking Rates on Winter Pasture
Custom Grazing Cattle
Agdex 130/53-1 Grazing Tame Pastures Effectively
How Much Feed Will My Cow Eat? FAQ
Foragebeef.ca
Animal Unit Months, Stocking Rate and Carrying Capacity


Prepared by Karin Lindquist, Ag-Info Centre, Alberta Agriculture and Forestry
 
 
 
 
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For more information about the content of this document, contact the Ag-Info Centre.
This information published to the web on June 4, 2003.
Last Reviewed/Revised on February 9, 2017.