8 pp., 11 photos, 4 tables, 2 graphs, 1 worksheetResearch using cattle fitted with GPS collars has shed light on how cattle behave in response to different landscape features such as brush, rock cover, slope, water, forage and grazing land accessibility. This publication explains how to use aerial maps and ecological site descriptions, in combination with ground inventories, to determine how much actual acreage is available for cattle to graze. This can help producers with difficult stocking decisions

Hanselka, C. Wayne

Hohlt, Jason C.

Lyons, Robert K.

McKown, David

Texas A&M University

Estimating Grazeable Acreage
for Cattle
Jason C. Hohlt, Robert K. Lyons, C. Wayne Hanselka and David McKown*
B-6222
4-09
*Natural Resources Conservation Service; and Professor and Extension Range Specialist, 
Professor and Extension Range Specialist, and Extension Assistant, The Texas A&M System.
2Stocking decisions can be difficult to make. Forage supplies vary from season to season and from year to 
year, and with more brush on our rangelands than ever 
before, old stocking rates may be of little value. Many 
land managers are also realizing that their cattle use 
only a portion of the land available. The goal of recent 
research was to determine why cattle use some areas but 
not others and how ranchers can use such information to 
estimate the number of grazeable acres they have.
How Grazeability was Studied
Research using cattle fitted with Global Positioning 
System (GPS) collars shed light on how cattle behave in 
response to different features of the landscape. The re-
search was conducted on ranches in different regions of 
Texas to demonstrate the influence of landscape features 
such as brush density, rock cover, surface slope, water 
and forage species on livestock grazing. Test sites were in 
the Davis Mountains, Edwards Plateau and South Texas 
Plains.
Digital aerial photographs of the test ranches and over-
lays showing the various ecological sites within each 
photograph were obtained. An ecological site is an area 
of land with specific physical characteristics that differs 
from other kinds of land in the types and amounts of 
vegetation it produces. Descriptions of these ecologi-
cal sites define certain landscape features. For example, 
an ecological site designated as gravelly redland has 
36 percent or less surface rock cover. The ecological 
site maps and aerial photographs enabled researchers 
to predict which areas cattle would not use because of 
their apparent brush and rock cover, slope, or inacces-
sibility. (Landowners can contact a local USDA Natural 
Resources Conservation Service office for help acquiring 
digital aerial photographs and ecological site overlays for 
their property.) 
Next, areas that appeared ungrazeable from the maps 
were observed on the ground. Observers measured brush 
density and rock cover and determined the herbaceous 
plant species growing in each area. Again, researchers 
predicted grazeability from their ground observations.
To test the map and ground observation prediction 
methods, researchers fitted cattle with GPS collars and 
recorded their positions for 23 days. Collars were pro-
grammed to take a position fix every 5 minutes and are 
accurate to within 10 to 16 feet of the true location. As 
Table 1 shows, both map estimates and ground estimates 
were fairly accurate, as validated by the actual GPS loca-
tions of cattle on the ranches. However, ground observa-
tion is clearly the more accurate method for predicting 
the grazeability of an area. 
Table 1. A comparison of the accuracy of 
predicting grazeable areas from map and ground 
estimates, as validated by  GPS studies. 
Research area
Map estimates, 
% correct
Ground 
estimates,  
% correct
Edwards Plateau 80 93
South Texas Plains 1* 67 -
South Texas Plains 2 92 92
South Texas Plains 3 88 100
Average 82 95
*No ground observation was done at this site.
What the Research Shows
Brush Density
While aerial photographs can give a general estimate of 
the brush cover on your property, they do have limita-
tions. If photographs are taken at the time of year when 
brush plants such as mesquite have dropped their leaves, 
the extent of brush cover may not be apparent. Photo-
graphs are helpful in pinpointing areas that might be 
too brushy for cattle so that these areas can be checked 
on the ground. There is often more brush in the pasture 
than can be seen on an aerial photograph.
Actual brush density should be checked and scored 
in several areas on the property. Walk a straight line 
through each area and assign a brush density score every 
20 steps. Use Figure 1 as a guide in determining brush 
density scores.
The GPS collar research showed that, overall, only 25 
percent of areas with a brush density score of 3 were 
visited by cattle, and that cattle completely avoided 
areas with scores of 4 or 5 (Fig. 2). This relationship 
was true in both the Edwards Plateau and South Texas 
Plains, regions with very different brush species.
3Figure 1. Brush density scores.
BDS = 0
(no brush present, foreground clear to tree line)
BDS = 1
(very light brush, only a few scatted plants)
BDS = 2
(light brush, brush common, but mobility or 
access not limited)
BDS = 3
(brush thick enough to limit mobility, but cattle 
can maneuver through it)
BDS = 4
(brush thick, mobility possible only in pathways)
BDS = 5
(very thick brush, mobility through it nearly 
impossible)
4Figure 2. Average brush density scores of grazed 
and ungrazed areas at three research sites in the 
South Texas Plains.
Rock Cover
With ecological site descriptions you can identify areas 
where rock cover might be a problem and spot-check 
these on the ground at the same time you determine 
brush density scores. Walk a 300-foot line through 
the area. At 20-foot intervals, place a PVC frame on 
the ground as a guide. Examples of various rock cover 
percentages are shown in Figure 3. The frames used 
in this study had an outside measurement of about 29 
inches and were divided into quarters to make it easier 
to visualize percent rock cover. To build a frame you will 
need tubing, four 90-degree elbows, four tees and one 
cross fitting.
The GPS collar study showed that cattle tend to avoid 
areas with 30 percent or more rock cover. When deter-
mining a rock cover percentage, remember that if an area 
is uncomfortable for a person to walk on, it will also be 
uncomfortable for cattle.
Slope
Percent slope is calculated as the change in elevation 
over a 100-foot distance. Cattle prefer flat areas or broad, 
gentle slopes and are usually deterred by steep slopes. 
In the GPS collar study at the Davis Mountains site, 95 
percent of cow locations were on slopes of 11 percent 
or less. Table 2 shows how slope affects whether or not 
cattle will use an area. Ecological site descriptions can 
help you identify areas where slope could limit cattle use.
Table 2. Expected effect of slope on cattle use. 
Percent slope Percent reduction in use
0-10 0
10-30 30
31-60 60
> 60 100
Water
The distance cattle have to travel to find water affects 
their use of a pasture. In general, cattle graze within 
about 1 mile of water, as was shown in the Davis Moun-
tains study where about 73 percent of cow locations were 
within a 1-mile radius of either of the two water sources 
available.
Forage
Another consideration when determining grazeable 
acreage is what cattle will and will not eat. So you will 
need to be able to identify some of the major plant 
species, especially grasses. Grasses such as threeawn 
(Aristida spp.), red grama (Bouteloua trifida) and Texas 
grama (Bouteloua rigidiseta) are very unpalatable to 
cattle. Cattle will avoid areas dominated by these species 
if more palatable grasses are available elsewhere. Like-
wise, cattle will not use areas with heavy concentrations 
of certain perennial weeds such as goldenweed (Isocoma 
drummondii) and wolfweed (Leucosyris spinosa).  
The frequency of herbaceous species was estimated at 
the South Texas Plains locations. No dominant grasses 
emerged as attractants or deterrents in grazed or un-
grazed areas. However, at two of these locations the 
average number of herbaceous species was greater in the 
grazed areas (Fig. 4).
Figure 3. Rock cover percentages.
5Figure 4. The number of herbaceous species 
was generally greater in grazed areas than in 
ungrazed areas at the South Texas locations.
Although there was little difference in the number of 
herbaceous species within the grazed and ungrazed areas 
at site 1, the grazed areas did have less Kleberg bluestem, 
a relatively unpalatable grass. It may be that cattle avoid 
this grass when possible.
Accessibility
Some areas of pastures may have low brush density 
scores, little rock cover, adequate water, gentle slopes and 
palatable forage species, but still not be grazed because 
they are inaccessible. These areas may be surrounded by 
dense brush, heavy rock cover and/or steep slopes. Aerial 
photographs with ecological site layers and descriptions 
can be very helpful in identifying such areas. If possible, 
creating roads or trails into these areas will make them 
more accessible to cattle. 
How to Use this Information
Begin by visiting your local NRCS office and requesting 
up-to-date aerial photographs of your property. There is 
no charge for these photographs. NRCS personnel can 
also provide ecological site overlays for the aerial maps 
and help you figure out how many acres of each kind of 
ecological site there are. Use these maps and overlays 
to identify areas that might have dense brush, extensive 
rock cover or steep slopes. Then, check these areas on the 
ground; calculate brush density scores and rock cover 
percentages and make note of the abundance of various 
forage species. Also verify the accessibility or inaccessi-
bility of suspect areas on the maps.
Here are typical ecological sites, with their surface rock 
and slope characteristics, for the Edwards Plateau and 
South Texas Plains regions.
Table 3. Typical ecological sites in the Edwards 
Plateau and their rock and slope characteristics. 
Ecological site Surface rock, % Slope, %
Deep redland 7 0-5
Redland 7 0-5
Gravelly redland ≤36 1-12
Low stony hill ≤50 0-15
Steep rocky 35-65
15-45
(some 20-60)
Table 4. Typical ecological sites in the South 
Texas Plains and their surface rock and slope 
characteristics. 
Ecological site Surface rock, % Slope, %
Clay loam 0 <3
Claypan prairie 0 0
Gray sandy loam 0 <2
Lakebed 0 <1
Sandy loam 0 0-5
Tight sandy loam 0 0-3
6Use these guidelines to estimate the number of grazeable acres you have.
The Importance of Estimating 
Grazeable Acreage
It is often possible to increase the amount of grazeable 
acreage by controlling brush, improving access to certain 
areas or adding water sources. Seeding rangeland with 
desirable grass species is another option. So taking 
time to estimate your grazeable acreage might prove 
beneficial by revealing management measures that would 
improve the productivity of your land.
But the major benefit is in determining the proper stock-
ing rate. As this research shows, the number of grazeable 
acres on a ranch may be much lower than the total acre-
age, which can have a dramatic effect on stocking rate. At 
one Edwards Plateau ranch (Fig. 5), the GPS collar study 
revealed that cattle were using only 39 percent of the to-
tal area. The ranch had been stocked at 20 acres per cow 
based on total acreage, but the effective stocking rate was 
9 acres per cow. 
Total acreage available
Subtract acres with brush density 
scores of 3 or higher.
Subtract acres with 30 percent or more 
rock cover (unless already subtracted 
for brush density).
Subtract acres dominated by undesirable 
plants (unless already subtracted for 
brush density and/or rock cover).
Subtract acres with slopes of 10 percent 
or more (unless already subtracted for 
brush density, rock cover 
and/or undesirable plants).
Subtract acres 2 miles or more 
from water (unless already subtracted).
Total Grazeable Acreage
–
–
–
–
–
=
By using these guidelines, you will be able to estimate 
the number of grazeable acres on your property. Then 
you should make stocking decisions on the basis of the 
amount of forage available on that acreage. 
Figure 5. In this three-dimensional photograph, 
the green dots are cattle locations. Areas with no 
dots are mostly within the Steep Rocky ecological 
site or surrounded by this site, which had the 
most rock cover and slope and the greatest brush 
density.

Educational programs of the Texas AgriLife Extension Service are open to all people without regard to socioeconomic 
level, race, color, sex, disability, religion, age, or national origin.
Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, 
as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Edward G. Smith, Direc-
tor, Texas AgriLife Extension Service, The Texas A&M System.
8M, New
Produced by AgriLife Communications, The Texas A&M System
Extension publications can be found on the Web at: http://AgriLifeBookstore.org.
Visit Texas AgriLife Extension Service at http://AgriLifeExtension.tamu.edu.
Initial printing of this publication was supported in part by a grant from 
the Texas Coalition of the Grazing Lands Conservation Initiative.


English

Estimating Grazeable Acreage for Cattle

Texas A&amp;M Repository

Estimating Grazeable Acreagefor CattleJason C. Hohlt, Robert K. Lyons, C. Wayne Hanselka and David McKown*B-62224-09*Natural Resources Conservation Service; and Professor and Extension Range Specialist, Professor and Extension Range Specialist, and Extension Assistant, The Texas A&M System.2Stocking decisions can be difficult to make. Forage supplies vary from season to season and from year to year, and with more brush on our rangelands than ever before, old stocking rates may be of little value. Many land managers are also realizing that their cattle use only a portion of the land available. The goal of recent research was to determine why cattle use some areas but not others and how ranchers can use such information to estimate the number of grazeable acres they have.How Grazeability was StudiedResearch using cattle fitted with Global Positioning System (GPS) collars shed light on how cattle behave in response to different features of the landscape. The re-search was conducted on ranches in different regions of Texas to demonstrate the influence of landscape features such as brush density, rock cover, surface slope, water and forage species on livestock grazing. Test sites were in the Davis Mountains, Edwards Plateau and South Texas Plains.Digital aerial photographs of the test ranches and over-lays showing the various ecological sites within each photograph were obtained. An ecological site is an area of land with specific physical characteristics that differs from other kinds of land in the types and amounts of vegetation it produces. Descriptions of these ecologi-cal sites define certain landscape features. For example, an ecological site designated as gravelly redland has 36 percent or less surface rock cover. The ecological site maps and aerial photographs enabled researchers to predict which areas cattle would not use because of their apparent brush and rock cover, slope, or inacces-sibility. (Landowners can contact a local USDA Natural Resources Conservation Service office for help acquiring digital aerial photographs and ecological site overlays for their property.) Next, areas that appeared ungrazeable from the maps were observed on the ground. Observers measured brush density and rock cover and determined the herbaceous plant species growing in each area. Again, researchers predicted grazeability from their ground observations.To test the map and ground observation prediction methods, researchers fitted cattle with GPS collars and recorded their positions for 23 days. Collars were pro-grammed to take a position fix every 5 minutes and are accurate to within 10 to 16 feet of the true location. As Table 1 shows, both map estimates and ground estimates were fairly accurate, as validated by the actual GPS loca-tions of cattle on the ranches. However, ground observa-tion is clearly the more accurate method for predicting the grazeability of an area. Table 1. A comparison of the accuracy of predicting grazeable areas from map and ground estimates, as validated by  GPS studies. Research areaMap estimates, % correctGround estimates,  % correctEdwards Plateau 80 93South Texas Plains 1* 67 -South Texas Plains 2 92 92South Texas Plains 3 88 100Average 82 95*No ground observation was done at this site.What the Research ShowsBrush DensityWhile aerial photographs can give a general estimate of the brush cover on your property, they do have limita-tions. If photographs are taken at the time of year when brush plants such as mesquite have dropped their leaves, the extent of brush cover may not be apparent. Photo-graphs are helpful in pinpointing areas that might be too brushy for cattle so that these areas can be checked on the ground. There is often more brush in the pasture than can be seen on an aerial photograph.Actual brush density should be checked and scored in several areas on the property. Walk a straight line through each area and assign a brush density score every 20 steps. Use Figure 1 as a guide in determining brush density scores.The GPS collar research showed that, overall, only 25 percent of areas with a brush density score of 3 were visited by cattle, and that cattle completely avoided areas with scores of 4 or 5 (Fig. 2). This relationship was true in both the Edwards Plateau and South Texas Plains, regions with very different brush species.3Figure 1. Brush density scores.BDS = 0(no brush present, foreground clear to tree line)BDS = 1(very light brush, only a few scatted plants)BDS = 2(light brush, brush common, but mobility or access not limited)BDS = 3(brush thick enough to limit mobility, but cattle can maneuver through it)BDS = 4(brush thick, mobility possible only in pathways)BDS = 5(very thick brush, mobility through it nearly impossible)4Figure 2. Average brush density scores of grazed and ungrazed areas at three research sites in the South Texas Plains.Rock CoverWith ecological site descriptions you can identify areas where rock cover might be a problem and spot-check these on the ground at the same time you determine brush density scores. Walk a 300-foot line through the area. At 20-foot intervals, place a PVC frame on the ground as a guide. Examples of various rock cover percentages are shown in Figure 3. The frames used in this study had an outside measurement of about 29 inches and were divided into quarters to make it easier to visualize percent rock cover. To build a frame you will need tubing, four 90-degree elbows, four tees and one cross fitting.The GPS collar study showed that cattle tend to avoid areas with 30 percent or more rock cover. When deter-mining a rock cover percentage, remember that if an area is uncomfortable for a person to walk on, it will also be uncomfortable for cattle.SlopePercent slope is calculated as the change in elevation over a 100-foot distance. Cattle prefer flat areas or broad, gentle slopes and are usually deterred by steep slopes. In the GPS collar study at the Davis Mountains site, 95 percent of cow locations were on slopes of 11 percent or less. Table 2 shows how slope affects whether or not cattle will use an area. Ecological site descriptions can help you identify areas where slope could limit cattle use.Table 2. Expected effect of slope on cattle use. Percent slope Percent reduction in use0-10 010-30 3031-60 60> 60 100WaterThe distance cattle have to travel to find water affects their use of a pasture. In general, cattle graze within about 1 mile of water, as was shown in the Davis Moun-tains study where about 73 percent of cow locations were within a 1-mile radius of either of the two water sources available.ForageAnother consideration when determining grazeable acreage is what cattle will and will not eat. So you will need to be able to identify some of the major plant species, especially grasses. Grasses such as threeawn (Aristida spp.), red grama (Bouteloua trifida) and Texas grama (Bouteloua rigidiseta) are very unpalatable to cattle. Cattle will avoid areas dominated by these species if more palatable grasses are available elsewhere. Like-wise, cattle will not use areas with heavy concentrations of certain perennial weeds such as goldenweed (Isocoma drummondii) and wolfweed (Leucosyris spinosa).  The frequency of herbaceous species was estimated at the South Texas Plains locations. No dominant grasses emerged as attractants or deterrents in grazed or un-grazed areas. However, at two of these locations the average number of herbaceous species was greater in the grazed areas (Fig. 4).Figure 3. Rock cover percentages.5Figure 4. The number of herbaceous species was generally greater in grazed areas than in ungrazed areas at the South Texas locations.Although there was little difference in the number of herbaceous species within the grazed and ungrazed areas at site 1, the grazed areas did have less Kleberg bluestem, a relatively unpalatable grass. It may be that cattle avoid this grass when possible.AccessibilitySome areas of pastures may have low brush density scores, little rock cover, adequate water, gentle slopes and palatable forage species, but still not be grazed because they are inaccessible. These areas may be surrounded by dense brush, heavy rock cover and/or steep slopes. Aerial photographs with ecological site layers and descriptions can be very helpful in identifying such areas. If possible, creating roads or trails into these areas will make them more accessible to cattle. How to Use this InformationBegin by visiting your local NRCS office and requesting up-to-date aerial photographs of your property. There is no charge for these photographs. NRCS personnel can also provide ecological site overlays for the aerial maps and help you figure out how many acres of each kind of ecological site there are. Use these maps and overlays to identify areas that might have dense brush, extensive rock cover or steep slopes. Then, check these areas on the ground; calculate brush density scores and rock cover percentages and make note of the abundance of various forage species. Also verify the accessibility or inaccessi-bility of suspect areas on the maps.Here are typical ecological sites, with their surface rock and slope characteristics, for the Edwards Plateau and South Texas Plains regions.Table 3. Typical ecological sites in the Edwards Plateau and their rock and slope characteristics. Ecological site Surface rock, % Slope, %Deep redland 7 0-5Redland 7 0-5Gravelly redland ≤36 1-12Low stony hill ≤50 0-15Steep rocky 35-6515-45(some 20-60)Table 4. Typical ecological sites in the South Texas Plains and their surface rock and slope characteristics. Ecological site Surface rock, % Slope, %Clay loam 0 <3Claypan prairie 0 0Gray sandy loam 0 <2Lakebed 0 <1Sandy loam 0 0-5Tight sandy loam 0 0-36Use these guidelines to estimate the number of grazeable acres you have.The Importance of Estimating Grazeable AcreageIt is often possible to increase the amount of grazeable acreage by controlling brush, improving access to certain areas or adding water sources. Seeding rangeland with desirable grass species is another option. So taking time to estimate your grazeable acreage might prove beneficial by revealing management measures that would improve the productivity of your land.But the major benefit is in determining the proper stock-ing rate. As this research shows, the number of grazeable acres on a ranch may be much lower than the total acre-age, which can have a dramatic effect on stocking rate. At one Edwards Plateau ranch (Fig. 5), the GPS collar study revealed that cattle were using only 39 percent of the to-tal area. The ranch had been stocked at 20 acres per cow based on total acreage, but the effective stocking rate was 9 acres per cow. Total acreage availableSubtract acres with brush density scores of 3 or higher.Subtract acres with 30 percent or more rock cover (unless already subtracted for brush density).Subtract acres dominated by undesirable plants (unless already subtracted for brush density and/or rock cover).Subtract acres with slopes of 10 percent or more (unless already subtracted for brush density, rock cover and/or undesirable plants).Subtract acres 2 miles or more from water (unless already subtracted).Total Grazeable Acreage–––––=By using these guidelines, you will be able to estimate the number of grazeable acres on your property. Then you should make stocking decisions on the basis of the amount of forage available on that acreage. Figure 5. In this three-dimensional photograph, the green dots are cattle locations. Areas with no dots are mostly within the Steep Rocky ecological site or surrounded by this site, which had the most rock cover and slope and the greatest brush density.Educational programs of the Texas AgriLife Extension Service are open to all people without regard to socioeconomic level, race, color, sex, disability, religion, age, or national origin.Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Edward G. Smith, Direc-tor, Texas AgriLife Extension Service, The Texas A&M System.8M, NewProduced by AgriLife Communications, The Texas A&M SystemExtension publications can be found on the Web at: http://AgriLifeBookstore.org.Visit Texas AgriLife Extension Service at http://AgriLifeExtension.tamu.edu.Initial printing of this publication was supported in part by a grant from the Texas Coalition of the Grazing Lands Conservation Initiative.

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Estimating Grazeable Acreage for Cattle

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Estimating Grazeable Acreage for Cattle

Abstract

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