Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Stocking rate is the number of specific kinds and classes of animals grazing a unit of land
for a specified time period. Carrying capacity or grazing capacity is
the maximum stocking rate possible while maintaining or improving
vegetation or related resources.
Animal Units and Animal Unit Months
Stocking rate and carrying capacity
are often expressed as animal unit
months (AUM).
Cattle
Mature cows without a calf
Cow with a calf
Weaned calf to yearling
Steers and heifers (1-2 years)
Mature bulls
Sheep
5 weaned lambs to yearlings
5 mature ewes with or without
lambs
5 mature rams
Goats
6 weaned kids to yearlings
6 does with or without kids
6 mature bucks
Horses and Mules
Mature horse (1200 lbs)
Mature mule
Wildlife
6 deer
Antelope, mature
Bison, mature
Animal
Unit
1.0
1.2
0.6
1.0
1.3
The original definition of an AUM
was the amount of forage a cow and
her calf would consume in 1 month.
This definition worked reasonably
0.6
well for several years until cows
1.0
started getting bigger and calf
weaning weights increased. To
1.3
accommodate bigger cows and
calves the definition of an AUM was
0.6
put on a weight basis. Today an
1.0
animal unit (AU) is commonly
defined as 1000 lbs of body weight
1.3
and an AUM is the amount of forage
that an animal unit will consume in 1
1 to 1.25
month. If the cow and her calf weigh
1 to 1.25
1000 lb then they are still 1 animal
unit. More likely the cow weighs
1.0
1200 lbs and her calf grows to 400
0.20
or 500 lbs by weaning. So the cow
1.00
without a calf is 1.2 animal units.
Table 1. Animal Unit Equivalents for domestic and wild
However, by weaning time the cow
herbivores.
and her calf are around 1.6 or 1.7
animal units. The 1000 lb animal unit can be applied to most large herbivores to get a
rough estimate of stocking rate. However, tables of animal unit equivalents are often
used to provide a more precise estimate that recognizes interspecies differences in
metabolic and intake rate. For example, a mature sheep has an animal unit equivalent
of 0.20. This means a sheep eats about 20% of the forage a cow will eat in one month.
Table 1 contains animal unit equivalents for several domestic and wild herbivores.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Forage and Feed Equivalents
The daily dry matter intake of a cow ranges from 1.5 to 3 percent of her body weight each
day. If she eats 2 percent of her body weight per day and she weighs 1000 lbs then she
will eat 20 lbs of forage per day on a dry matter basis. If she eats 3 percent of her body
weight she will consume 30 lbs per day on a dry matter basis. Multiplying 20 or 30 by 30
days in one month results in 600 to 900 lbs of dry forage consumed each month. Some
textbooks use 600 lbs for the amount of monthly consumption (Holechek 2004). Others
use 800 to 1000 lbs per month to be more conservative in their carrying capacity
calculations and to account for wasted forage. One AUM is often considered to be
equivalent to 800 lb of hay or 400 lbs of TDN.
Stocking Rate and Productivity
There is a fundamental trade-off between gain per animal and gain per unit of area
(Figure 1). At very low stocking rates animals can selectively forage with little competition
from each other. This promotes high gain or high body condition of individual animals but
does not result in maximum productivity per acre. As stocking rate increases competition
between animals for forage increases resulting in a decrease in individual animal
performance. At heavy stocking rates individual animal performance also decrease
because lower quality plants make up a larger portion of the diet and total intake can be
reduced. Between the extremes of light and heavy grazing there is an optimum stocking
rate that maximizes productivity per acre.
Figure 1. Influence of stocking rate on individual animal performance and
production per acre.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Potential Effects of High Stocking Rates






Animal performance reduced
Intake and forage quality reduced
Desirable forage plants replaced by less desirable species
Overall forage productivity reduced
Increase in bare soil and preferred grazing areas become degraded
Increased replacement feed costs
Potential for water quality impacts due to increased bacteria, sediment, and
nutrient loading
Potential Effects of Low Stocking Rates


Economic potential not fully realized, enterprise sustainability at risk
Mature animals maintain over-fat body condition which can reduce reproductive
capacity
On perennial dominated rangelands patchy grazing results in development of
“wolfy” plants that are used little or not at all. This reduces over all productivity.
This occurs less in annual dominated rangeland types but under used patches of
less desirable vegetation may occur
Some desirable forage species can be crowded out by taller growing species
Reduced biodiversity of species that thrive under moderate grazing
Stocking Rate
Many livestock operations base their stocking rate on carrying capacity estimates handed
down from generation to generation, on the advice of their neighbors or local experts and
on trial and error. Stocking rate is usually documented in private and public land leases.
Often carrying capacity is estimated from average annual productivity which is available
from soil surveys or ecological site descriptions (formerly range site descriptions). To
calculate carrying capacity you need to determine the total available forage in the
pasture and you need to determine animal demand for forage. There are two ways to
calculate total available forage. The first is the residual dry matter method used on
California’s annual rangelands. The second is the allowable use method used on
perennial rangelands throughout the western U.S. Finally you may need to adjust your
carrying capacity estimate for steep slopes and distance to water.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Figure 2. Sierra foothill and coast range oak-woodland carrying capacity is commonly in the
range of 10 to 30 acres per animal unit per year.
Figure 3. Mountain meadow carrying capacity commonly ranges from 1 ½ to 2 acres per
animal unit per year.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Calculating Total Available Forage (Allowable Use Method)
Total Available Forage (lbs/a) = Production (lb/a) X Allowable Use (%) X Pasture Size (A)


Production (lb/a) estimates based on averages for several years are often
available for ecological sites from the USDA Ecological Site Information System
(ESIS) website or from range production tables in Soil Data Mart. Current year’s
production can be determined by weighing dry forage clipped from small plots of a
known area.
Allowable Use guidelines are available in Table 2 or from textbooks (Valentine
2001 – pg 388-391, Holechek 2004 – pg 233-247) or from USDA Natural
Resources Conservation Service (NRCS) or other agencies.
If production is determined to be 1375
lbs/a, allowable use is 40 % and pasture
size is 1000 acres then:
Total Available Forage = 1375 lb/a X
0.4 X 1000 = 550,000
Suggested
Rangeland
proper use factor
Ecosystem or Type
(%)
35
Semi desert grassland
35
Sagebrush - grasslands
40
50
Short grass prairie
Oak-woodland
50
California annual
grassland
Table 2. Proper use factors for California
rangelands
Calculating Total Available Forage (Residual Dry Matter
Method)
On California’s annual grasslands and oak-woodlands stocking rate is calculated by
another method that insures that adequate residual dry matter (RDM) remains at the end
of the grazing season (UC Leaflet 8092)
Total Available Forage (lbs) = (Production (lb/a) – RDM (lbs/a)) X harvest efficiency (%) X
pasture area

Production can be determined in the same way as for the allowable use method.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile

The amount of RDM that should be left behind varies with rainfall, slope and
canopy cover and can be determined from UC Leaflet 8092.

UC Leaflet 8092: http://anrcatalog.ucdavis.edu/FreePublications

Harvest efficiency or grazing allocation is a term that has been used for the forage
that is available for grazing by cows or other livestock. Too maintain a
conservative stocking rate the grazing allocation or harvest efficiency should be
about 50 %.
If production is determined to be 1600 lbs/a, RDM to be 500 lb/a, harvest efficiency is 50 %
and pasture size is 1000 acres then:
(1600-500) X .5 X 1000 = 1100 X .5 X 1000 = 550,000 lbs of forage
Animal Demand for Forage
If we assume that one animal unit month is 800 lbs of forage on a dry matter basis and
that the pasture will be used for 12 months then:
Animal demand for forage = 800 lb/AUM
550,000 lb of forage ÷ 800 lb/AUM = 687.5 AUMs
687.5 AUMs ÷12 AUM/yr = 57.3 AU for 12 months = 57.3 animal unit years (AUY)
There for it takes about 17.5 acres (1000 acres/57.3) to support 1 animal unit for 1 year
(12 AUM).
If the pasture is to be used for only 6 months then:
550,000 lb of forage ÷ 800 lb/AUM ÷ 6 months on pasture = 114.6 AU for 6 months.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Figure 4. Annual grassland carrying capacity often is in the range of 6 to 12 acres per animal unit per
year.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Carrying Capacity Adjustments
While these calculations are based on long-term average productivity, range forage
productivity varies from year to year depending on prevailing weather conditions.
Therefore stocking rate must be adjusted
Slope
Reduction in
annually in response to these conditions. In
(%)
grazing capacity (%)
dry a year that means that fewer AUs are put
0-10
0
in the pasture or that the length of the grazing
11-30
30
period is reduced. When forage is in short
31-60
60
supply ranches purchase additional hay, rent
>60
100
additional pasture or reduced herd size.
Table 3. Approximate reductions in cattle
Carrying capacity is often adjusted for slope,
grazing capacity for different slope percentages.
distance to water and canopy cover.
Approximate adjustments for slope and distance to water are presented in Tables 3 and
4. Carrying capacity must also be adjusted when productivity is reduced by weeds,
brush, or trees that invade or encroach into pastures and range allotments.
Changes in grazing
management (season,
frequency, duration and
intensity of use) generally
0-1.6
will not change carrying
1.6-3.2
capacity. Grazing capacity
>3.2
of some range allotments
Table 4. Approximate reductions in carrying capacity as distance
can be increased by
from water increases.
improving livestock
distribution with such practices as water development, supplement placement, herding
and fencing (UC Leaflet 8217). Rangeland seeding and fertilization may also be options
for increasing carrying capacity. On irrigated pasture carrying capacity can be improved
with better fertility management and improved irrigation management.
Distance
from Water
(mi)
0-1
1-2
>2
Distance
from water (km)
Reduction in
grazing capacity
(%)
0
50
100
Livestock grazing has been shown to influence surface water quality and riparian area
health. While reducing heavy stocking rates may help protect water quality and riparian
areas, reducing residence time in streams and associated riparian areas using traditional
livestock distribution practices (George et al. 2007) is often more effective. Studies have
shown that riparian health is related to time invested in management by the land
owner/manager (Ehrhart and Hansen 1996, Ward 2002).
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Using Soil Data Mart to Determine Range Productivity
Step 1: Go to: http://soildatamart.nrcs.usda.gov/
Step 2: Select State and then county
Step 3 : Select Survey Areas
Step 4 : Select Generate Reports
Step 5 : Select soil series or soil mapping unit from the long list. Use Shift key to select
several consecutive soils. Use Control key to select several non-consecutive
soils.
Step 6: Select Range Productivity and Plant Composition from drop down menu.
Step 7 : Generate Reports: You should receive a file listing the unfavorable, normal
and favorable productivity for the range site that cover the selected soil(s).
Monitoring Stocking Rate
When estimating stocking rate the values used in the calculations for daily or monthly
intake or consumption rates, allowable use rates, residual dry matter (RDM), animal unit
equivalents, methods of estimating total available forage, and adjustments made for
distance to water and slope can result in different stocking rate estimates for the same
pasture. These potential differences support that these calculations are just estimates
that should be fine tuned based on end of season monitoring and experience. Selection
of conservative values for stocking rate calculations leaves room for adjustment upward if
a few years of experience show that the pasture is under stocked.
There are several indicators that stocking rate is too high or too low. High body condition
scores (greater than 5) may indicate potential to increase stocking rate and low body
conditions scores may be an indicator of a stocking rate that is too high. If desired forage
species are declining in vigor or decreasing in number stocking rate may be too high.
Utilization that exceeds the allowable use (see above) or recommended RDM is a good
indicator of overstocking. Monitoring of stubble height (Lile et al. 2003) or residual dry
matter (Bartolome et al. 2002) is sometimes used to assess utilization at the end of the
grazing season. Increasing amounts of bare ground or prevalence of soil disturbances
could also be an indication of over stocking.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Figure 4. Carrying capacity for Intermountain rangelands dominated by sagebrush may range from
24 to 36 acres per animal unit per year but carrying capacity decreases with invasion of western
juniper. In very cold or dry areas the carrying capacity will be less. Sagebrush rangeland
improved by seeding can have carrying capacities high as 6 acres per animal unit per year. The
normal season of use for these intermountain rangelands is April through October.
Figure 6. Well managed coastal and valley irrigated pasture carrying capacity is commonly 1 to 1.5
acres per animal unit per year.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
Literature Cited
Bartolome J.W., W.E. Frost, N.K. McDougald and J.M. Connor. 2002. California
Guidelines for Residual Dry Matter (RDM) Management on Coastal and Foothill Annual
Rangelands. Rangeland Monitoring Series, University of California Ag and Nat Res Pub
8092. Oakland, CA. 8 p.
Ehrhart, B. and P.L. Hansen. 1998. Riparian grazing management: Strategies and
techniques. In: Potts, D.F. (ed). 1998. Rangeland Management and Water Resources.
American Water Resources Association. May 27-29, 1998. Reno, NV. Pg 191-201.
George, Melvin, Derek Bailey, Michael Borman, David Ganskopp, Gene Surber, Norm
Harris. 2007. Factors and Practices that Influence Livestock Distribution. ANR Publ.
8217, Div. of Agric. And Nat. Res., Univ. of Calif., Oakland, Calif. 20 pgs.
Holechek, Jerry, Rex D. Pieper and Carlton H. Herbel. 2004. Range Management,
Principles and Practices. 5th Ed. Prentice Hall. 607 pgs.
Lile, David F., Kenneth W. Tate, Donald L. Lancaster and Betsy Karle. 2003. Stubble
height standards for Sierra Nevada meadows can be difficult to meet. Calif. Agric. 57:6064.
Valentine, John F. 2001. Grazing Management (2nd ed). Academic Press, San
Francisco, CA. 659 pgs.
Ward, T.A. 2002. A Cross-Sectional Survey of California’s Grazed Rangeland Riparian
Areas. MS Thesis. University of California, Davis. 75 pp.
Ecology and Management of Grazing: An Online Course
Module 4: Ranch Operations and Grazing Management
STOCKING RATE AND CARRYING CAPACITY
Melvin George and David Lile
FOR MORE INFORMATION
Forage Production References
Oak-Woodland Annual Grassland Ecological Sites:
http://californiarangeland.ucdavis.edu/Ecological%20Sites/ESD%20Web/esd.soil.conversi
on.htm
Soil Data Mart: Online soils information from USDA Natural Resources Conservation
Service. Follow directs in Soil Data Mart Tab to access this online source of rangeland
productivity information. http://soildatamart.nrcs.usda.gov/
USDA Ecological Site Information System: http://esis.sc.egov.usda.gov/
RDM Guidelines
Residual Dry Matter Guidelines: UC ANR Publication No. 8092 describes how to assess
and monitor the level of grazing use on annual rangelands.
http://californiarangeland.ucdavis.edu/Publications%20pdf/8092.pdf
http://anrcatalog.ucdavis.edu/FreePublications
Livestock Distribution
Factors and Practices that Influence Livestock Distribution: UC ANR Publication No.
8217
http://californiarangeland.ucdavis.edu/Publications%20pdf/8217.pdf
http://anrcatalog.ucdavis.edu/FreePublications
Drought Management
Livestock Management During Drought: UC ANR Publ. 8034
http://californiarangeland.ucdavis.edu/Publications%20pdf/8034%20drought.pdf
http://anrcatalog.ucdavis.edu/FreePublications