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- Proper use of either native rangeland or tame pasture, depends on the amount of forage the area can produce and the amount of forage needed over the grazing season by the animals.
- The animal unit (AU) is a standard unit used in calculating the relative grazing impact of different kinds and classes of livestock. One animal unit is defined as a 1000 lb (450 kg) beef cow with or without a nursing calf, with a daily dry matter forage requirement of 26 lb (11.8 kg). An animal unit month (AUM) is the amount of forage to fulfill metabolic requirements by one animal unit for one month. One AUM is equal to 780 lbs (355 kg) of dry matter forage.
- Forage requirements change with the size and type of animal. Animal Unit Equivalents (AUE) have been calculated for various species and sizes of animals. For example, a 1500 lb cow is 1.5 AUE, a 700 lb heifer is 0.7 AUE, while a sheep is 0.2 AUE.
- Stocking rate is the number of animals on a pasture for a specified time period and is usually expressed in Animal Unit Months (AUM) per unit area. For example, an area that supports 30 (1,000 pound) cows for a four-month grazing season has a stocking rate of 120 AUMs for the area. If the pasture is 100 acres in size, the stocking rate would be expressed as 1.2 AUM/ac.
- Stock density is the number of animals in a particular area at any moment in time and increases as the number of animals in a paddock increase or as paddock size decreases and is based on level of grazing management. For example, a herd of 30 (1,000 pound) cows on a 2 acre paddock on the 100 acres has a stock density of 15000 lbs/acre or 15 Animal Units/acre, even though the stocking rate for the 100 acre pasture is 1.2 AUMs/acre.
- Carrying capacity is the average number of animals that a pasture can support for a season. It is a measure of a pasture’s ability to produce enough forage to meet the animal requirements over the long term and is expressed in AUMs.
- Along with proper grazing management, adequate rest after grazing and grazing during the correct time of year, appropriate stocking rates are critical for sustained pasture productivity.
- Condition of the pasture impacts stocking rate. Factors such as previous grazing management, species of forage, age of stand, soil type, texture, fertility level and moisture conditions all impact on forage yield and consequently stocking rate.
- Stocking rate histories on similar fields in the same area can be very useful in setting initial stocking rates.
- The optimum number of animals on a pasture makes efficient use of the forage and still leaves enough forage behind to allow a quick and complete recovery.
- Estimate the amount of forage available per acre by taking yield clippings from at least 10 random locations within a paddock. Dry down the forage and use the actual weights to calculate yield. Estimating the area of forage in the stand needed to feed on cow for one day is another technique. Once determined, measure the area and extrapolate to the whole field.
- Calculation of stocking rates or grazing acreage needed is done by the following three steps:
- Estimate animal consumption (per day) - Nursing cows (with calves) and growing steers or replacement heifers consume approximately 2.5% of body weight as forage dry matter.
- Estimate the amount of trampling and consumption to determine a level of forage disappearance.
- Estimate the production of the each paddock as it is about to be grazed each time to acquire a total production estimate.
- Calculate stocking rates (animals/acre) by dividing the amount of forage needed by an animal unit into the total estimate of production of the land.
Animal Unit Months, Stocking Rate, and Carrying Capacity
Budgeting Feed Requirements of Beef Cattle on Pasture - available in PDF format
Determining Your Stocking Rate - available in PDF format only
Estimating Livestock Forage Demand: Defining the Animal Unit
Guidelines for Setting a Proper Stocking Rate - available in PDF format only
How Many Animals Can I Graze On My Pasture? - available in PDF format only
Initial Stocking Rate Recommendations for Seeded Pastures in Saskatchewan
Managing Saskatchewan Rangeland - available in PDF format only
Methodology for Calculating Carrying and Grazing Capacity on Public Rangelands - available in PDF format only
Montana Grazing Animal Unit Month (AUM) Estimator - available in PDF format only
Montana Guide to Range Site, Condition and Initial Stocking Rates - available in PDF format only
Stocking Rates and AUM - Frequently Asked Questions
Stocking Rate and Grazing Management - available in PDF format only
Stocking Rate vs. Stock Density
What is an AUM? - available in PDF format only
Animal Unit Equivalent for Beef Cattle Based on Metabolic Weight
Cattle Weight Gains in Relation to Stocking Rate on Rough Fescue Grassland - available in PDF format only
Effects of Stocking Rate on a Rough Fescue Grassland Vegetation - available in PDF format only