THE IMPORTANCE AND USE OF THE DETERMINED CARRYING CAPACITY OF RANGELANDS
C. Wayne Cook
Dept. of Rangeland Ecosystem Science
Colorado State University
The carrying capacity of a range is the number of animals that can be grazed without undue harm to the soils and vegetation. It is always assumed that when soils and plants are unharmed, animals on the range are likewise unharmed, but this is not always correct. Nutrition may be limiting for animal welfare on otherwise properly grazed ranges.
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Some rangeland managers say
that there can be no grazing capacity assigned to any particular parcel
of land such as a fenced pasture or a range site. Production does indeed
vary from year to year on any area of rangeland. As a result, the carrying
capacity must be flexible, generally not using more than 50% of the
total forage available from one year to the next. Over time there can
be more use from year to year. For cattle, it is suggested that the
breeding herd be at 65 to 70% of average capacity among years. Thus,
during good years steers can be added; and during dry years less than
the full breeding herd with no replacements and no cull animals. No
steers are carried over during dry years. They are sold as weaner calves
or no older than yearlings.
Estimated carrying capacities for tracts of rangeland must be recognized
as necessary and useful for management. Rangelands are sold on the basis
of animal units/acre capacity or animal units capacity/ranch such as
a 1500-cow ranch. IRS and estate transfer tax is based upon AU capacity.
Damages wrought on rangelands by pipelines, surface mining, road construction,
etc. are evaluated on the basis of animal unit months of capacity lost
to normal operations of the ranch. Eminent domain purchases and misrepresentation
of sales are settled in lawsuits based on carrying capacity of the land
involved. Calculated carrying capacity of range sites is a means of
comparing the value of rangelands of different types; for example, meadows,
rolling plains, breaks, brush types, etc.
Grazing capacity may depend upon frequency of drought years, species
of animals used, size of animals (breeds), physiological function of
animals, season of use or grazing system used, and distribution of animals.
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Examples of the above noted complications are:
•
Sheep are more adaptable to rough terrain and browse range than cattle;
however, common use by both with appropriate numbers of each would
have higher capacity than either class alone.
• Animal breeds averaging 1200 lbs/cow consume about 31% more forage
than breeds weighing only 915 lbs/cow.
• Animals in lactation or growing consume about 19% more than animals
in
gestation or maintenance.
• Seasonal grazing, such as on the northern desert shrub types,
if grazed
during the spring, has only half the carrying capacity as when grazed
in
the winter because only one growing period (April 1 to June 15) is available
for
recovery after grazing.
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• Management plans or grazing systems that provide flexibility
in grazing
intensity with relation to fluctuating forage production from year to
year is
less wasteful of the range forage than established numbers that leave
unused herbage during good years, and slightly overgrazed during drought
years (this practice requires conservative stocking and less overall
capacity
than flexible management systems).
• Hazards, such as down timber and thick brush may reduce grazing
capacity by
as much as 60%.
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• Distribution of animals by placement of water and salt, along
with
some trail construction, can increase capacity as much as 30 to 50%.
In case a ranch or an allotment on government land is divided into pastures
by fences the management becomes a matter of the suitability of each
pasture. It is important that each pasture is rather uniform or similar
throughout. Any variation in vegetation cover should be slight or if
different vegetation types are present the areas of such should be small
and generally few because management practices are assumed to affect
all of the pasture in the same way.
The capacity for any single species of animal on a specific area of
range depends upon the total aggregate of plant species and of all other
animal species using the area. Thus, common use of mountain range by
sheep and cattle in numbers representative of suitability will have
20 to 25% greater capacity than single use by either species. The reasons
for this are the differences in degree of use of plant species by different
animal species.
The practical application of carrying capacities are many and varied.
As mentioned before, when rangelands or ranches are bought, sold, or
traded, the value is calculated by means of a carrying capacity figure
for them and the going grazing price/animal unit on a year-long basis.
Thus, all rangeland brokers must somehow establish a carrying capacity.
Allotments on federal lands are based upon the calculated carrying capacity
of the land. Grazing permits on all Indian lands are based upon calculated
carrying capacities of the land. Charges for grazing in both of the
above cases are based on an animal unit month of grazing stated in the
permit.
Rangeland brokers generally arrive at capacity of a ranch by comparison
of neighboring ranches previously sold in the area. Grazing capacity
for a particular ranch may be arrived at primarily by talking to the
manager about how many cows, bulls, replacements, steers, and horses
are run on the ranch. For instance, a manager may answer that he tries
to keep about 600 cows, 15 bulls, 50-60 replacements, and 10 horses.
Thus, about 675 Au's are run on the ranch, but this number is not constant
from year to year by any means. This commitment by the manager generally
disregards the drought years when it was necessary to cut back animal
numbers rather substantially. It also does not include range supplements
or the amount of harvested forage fed during bad weather. Thus, the
carrying capacity used by a real estate brokerage will be higher than
the actual capability of the range forage, and as a result, should be
considered a more general figure for equating the value of rangeland
in a particular area rather than the actual carrying capacity of the
range involved.
How then does a scientific approach for calculating grazing capacity
of a ranch or a parcel of rangeland occur? The procedure is generally
recognized as an "ocular reconnaissance range survey". First,
it consists of obtaining production of vegetation by species composition,
and multiplying this by the degree of utilization expected; thereby
obtaining the actual pounds/acre of useable herbage or "forage"
(Table 1). Second, we need the expected daily consumption of the animals
that will be grazing the range in question. Table 2 describes some calculations
of air dry matter consumed per day under normal range conditions without
supplements.
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TABLE 1- CALCULATING CARRYING CAPACITY FOR CATTLE ON A YEARLONG BASIS |
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SPECIES
|
PRODUCTION
|
UTILIZATION
|
AVAILABLE |
|
(#/ACRE)
|
(%)
|
(#/ACRE)
|
| Western Wheatgrass |
600
|
55
|
330.0
|
| Galleta grass |
200
|
30
|
60.0
|
| Big Sagebrush |
250
|
15
|
37.5
|
| Winterfat |
100
|
35
|
35.0
|
| Bigalow Sagebrush |
100
|
35
|
35.0
|
|
TOTAL
|
1250
|
39.8(avg)
|
497.5
|
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TABLE 2 - AVERAGE WEIGHT, DAILY GAIN, CONSUMPTION OF AIR-DRY MATTER AND ANIMAL UNIT EQUIVALENT FOR VARIOUS KINDS OF LIVESTOCK DURING VARIOUS SEASONS |
|
KIND
OF ANIMAL
|
SEASON
|
AVERAGE WEIGHT (lbs) |
GAIN PER DAY (lbs) |
DAILY
CONSUMPTION (lbs/day)
|
ANIMAL
UNIT EQUIVALENT
|
INTAKE
PER ANIMAL (lbs)
|
|
Ewe
& Lamb
|
Spring
|
134
|
0.5
|
6.1
|
0.19
|
4.55
|
|
Summer
|
142
|
0.5
|
7.3
|
0.23
|
5.14
|
|
|
Average
|
139
|
0.5
|
6.9
|
0.22
|
4.96
|
|
|
Winter
|
139
|
---
|
5.1
|
0.16
|
3.67
|
|
|
Average
|
139
|
---
|
5.8
|
0.18
|
4.17
|
|
|
Cow
& Calf
|
Spring
|
892
|
2.14
|
32.1
|
1.02
|
3.60
|
|
Summer
|
956
|
1.42
|
35.3
|
1.12
|
3.69
|
|
|
Average
|
919
|
1.84
|
33.4
|
1.06
|
3.63
|
|
|
Winter
|
910
|
---
|
29.8
|
0.94
|
3.27
|
|
|
Average
|
914
|
---
|
31.6
|
1.00
|
3.46
|
|
|
Steer
|
Spring
|
590
|
2.25
|
20.4
|
0.65
|
3.46
|
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Summer
|
775
|
1.75
|
24.9
|
0.79
|
3.21
|
|
|
Average
|
667
|
2.04
|
22.3
|
0.71
|
3.34
|
|
|
Winter
|
435
|
0.39
|
15.4
|
0.49
|
3.54
|
|
|
Average
|
551
|
1.22
|
18.8
|
0.59
|
3.41
|
|
|
Dry
Cow
|
Spring
|
955
|
----
|
24.0
|
0.76
|
2.51
|
|
Summer
|
1116
|
----
|
29.4
|
0.93
|
2.63
|
|
|
Average
|
1022
|
----
|
26.2
|
0.83
|
2.56
|
|
|
Winter
|
1037
|
----
|
26.2
|
0.83
|
2.53
|
|
|
Average
|
1029
|
----
|
26.2
|
0.83
|
2.55
|
| Gain per day is based upon lamb, calf, and steer body weight change only. |
| Intake per animal includes forage eaten by lamb or calf prorated on average weight of ewe or cow, and on a 100% lamb and calf crop. |
Following are some calculations of air-dry range forage consumed by
various species of animal and by various classes of animal within species.
•
Dry cow that weighs 1000# will consume 32.7 #/day
• Wet cow and calf that weighs 1000# will consume 36.4#/day
• Year-long range cow that weighs 1000# (6 months dry and 6 months
wet) will consume 34.5#/day or 1035#/month
• During spring-summer steer or heifer weighing 667 pounds will
consume 22.3#/day or 669#/month
• Dry ewe weighing 140# will consume 5.1#/day
• Wet ewe weighing 140# with a lamb will consume 6.9#/day
• A ewe year-long grazing (6 months dry and 6 months wet) will
consume about 6.0#/day
The final calculation (Table 1) uses the pounds/acre of forage consumed
which was 497.5 pounds (total vegetation growth was 1250 pounds per
acre). Thus, this pasture had a carrying capacity of 1.91 acres/animal
unit month year-long (948# required per cow month divided by 497.5#
per acre of useable herbage).
• Tables 1 and 2 display the consumption of various animal units.
• Dry cow weighing 910 pounds is 29.8#/day x 30 = 894#/month animal
unit
• Wet cow weighing 919 pounds is 33.4#/day x 30 = 1002#/month per
animal unit
• Year-long grazing by a cow weighing 914 is 31.6#/day x 30 = 948#/month
for 6 months dry and 6 months wet
It is possible to arrive at a satisfiable grazing capacity by simply
observing the degree of use made of the dominate palatable plants. If
the dominate palatable plants are not used over 50% to 55% on the average
range areas and not used over 65% on the smaller lower meadow like areas
then the utilization is appropriate. If a person knows the plant species
and takes pains in his estimates and makes the estimates near and perhaps
again at the end of the grazing season.
It's expressed by many range managers that a reasonable approach to
carrying capacity is by means of placing what is believed to be a reasonable
number of animals of any species or a combination of any two species
on the area. The last choice, of course, complicates the system. Therefore
a number of only a single species of animals is recommended unless the
estimator has a large amount of experience.
• At various intervals during the grazing season, the actual use
on at least the major species of plants is estimated as light, medium
or heavy use. At the second interval during the season such as 75% through
the season a similar estimate of utilization is estimated for all of
the major plant species.
• If at this time, when 3/4 of the season has passed, it is hoped
that only an average of 40% to 50% utilization is made on the major
forage species. As a check, it is preferable if there is not more than
60% use on the major species by the end of the grazing season. Some
range managers use 75% as optimum use by the end of the season but this
is too intense to maintain the normal vigor for existence of the major
or more palatable species in the plant expression that is sought.
• It could be that you have a single species that is so highly
palatable and also rather sparse that you may choose to sacrifice the
species in order to fully use the major plant species for production
of export meat from the ecosystem that will optimize the expenditure
of cultural energy per unit of land production.
The classification of plant species as indicators of condition of the
range is indeed a major problem for modern range management. It is however
well known that the more palatable plants are replaced with less palatable
plants when ranges are overgrazed. However, a particular plant species
may be relatively unpalatable during a particular season or on a particular
site but may be readily eaten during another season or on another site
where plant association is different. Seldom is the presence or absence
of a single plant species used as the only indicator of range condition
or range use.
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•
A range in a deteriorated condition displays a greater abundance of
the less palatable plants; whereas, a range in good condition has a
greater abundance of plants that are known to be more palatable under
most conditions.
• The identification of plants as indicators generally is a product
of opinion based upon general observation. However, a quantitative confirmation
of such classifications of indicator species for broad range types is
desirable if the decision is obvious.
• A highly palatable plant species is sometimes called a sacrifice
plant because it is readily consumed by many animal species under various
conditions. Therefore it will disappear even with light utilization
of the entire flora present.
• In a study of the cold desert shrub areas of Utah and Nevada
it was concluded that most desert plants can be grazed in late spring
only if herbage removal is at the level of 30 percent or below. Utilization
of 50 percent or more during this season is extremely detrimental. It
was also found that forage removal during the winter and again in the
spring could not exceed 30% during any one season if satisfactory range
condition is to be maintained.
Livestock
numbers with respect to breeds and size are of primary importance:
• The European breeds of cattle that are being used in the western
United States as range animals vary in female cow weights and weaning
weights of calves. The Hereford cows may average 850# per female cow
whereas some European breeds may have cows that weigh as much as 1300#
with calves averaging 150# more weight at weaning than the common Hereford
calf of the same age.
• This appears to give the European breed a decided advantage but
this is not really true. If the range forage consumption per breed is
considered, the heavier weight is really no advantage for the European
breeds. The grazing capacity should be calculated on the total weight
of female cow and her calf that grazes beside her. The forage consumption
is approximately the same for the average per pound weight for each
breed cow and calf, which makes the production cost per pound of calf
produced the same for each breed.
• Many western ranchers have felt that the European breeds, because
they produced a bigger calf at weaning made the larger European cattle
more profitable. This is not true. After a year or two you will soon
find that you cannot run the same number of female European cattle as
with your initial Hereford herd of earlier years, because you will have
a deteriorated range from too many big cows with big hungry calves.
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•
Generally it is recognized that spring use should not be more than 30%
to 40% of the foliage cover and when a range is used for only fall or
winter use not more than 55% of the total foliage cover for the grazing
year should have been removed. These figures are taken from what we
call average overall seasonal use of rangelands which are generally
known to be dry with about 8 to 12 inches of moisture annually.
• Using these utilization figures, which have been taken from several
research studies from different range areas of the western states, appears
to be a good baseline standard.
• These figures do not apply to meadows where moisture and soil
fertility are considerably higher than normal range lands.