Carrying Capacity Calculator

Method 1

Carrying capacity, or grazing capacity, is the amount of forage available for grazing animals in a specific pasture or field. Understanding how much forage is available is a key principle of pasture management to balance the available forage supply with livestock demand.

Carrying capacity can be calculated using a variety of techniques and is somewhat based on trial and error. Carrying capacity can be monitored and adjusted over time to determine the long-term average.

The BCRC Carrying Capacity Calculator allows producers to determine the carrying capacity of pastures based on two methods: 1) estimates based on provincial guides and 2) field-based sampling.

Producers can use the Method 1 calculator if they wish to calculate an estimate of carrying capacity based on available provincial forage production guides. Using Method 1 is easy and works best when the pasture condition (or range health) is similar throughout the field and the forage plant community (or range type) is uniform.

Producers can use Method 2 if they plan on clipping, drying, and weighing samples collected from their pasture. Field-based sampling provides greater accuracy but requires more hands-on work. Producers may choose field-based sampling if provincial guides are unavailable for their region or if pasture types or conditions vary within their field. Forage production varies each year, so the Method 2 approach should include multiple years of sampling to estimate the long-term productivity of the pasture.

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Method #1: Using Provincial Production Guides

Instructions:
Use Tables 1 & 2 to estimate forage production.
Enter your own pasture and herd information into the yellow cells to estimate carrying capacity.

Step 1: Determine Pasture Condition

Table 1. Tame pasture condition class definitions
Category Excellent Good Fair Poor
Potential yield of the area 75-100% 60-75% 50-60% 33-50%
Production from desirable, adapted grass and legumes 95% 90% 60% Less than 50%
Production from weeds or undesirable plants Less than 5% Less than 10% 20% or more 50% or more
Fertility Program Average to above average Average Below average or non-existent No fertility program
Source: Alberta Forage Manual (Table 37, page 206) adapted from Wroe et al. (1988) Guide to Range Condition and Stocking Rates for Alberta Grasslands

Step 2: Determine Estimated Forage Yield

Table 2. Estimated Forage Production based on tame pasture condition (pounds per acre)
 
Pasture Condition
Precipitation zone
Excellent
Good
Fair
Poor
mm / inches lbs/ac lbs/ac lbs/ac lbs/ac
250-350 / 10-14 700 460 370 230
350-450 / 14-18 1,150 740 550 370
450-550 / 18-22 1,850 1,300 1,000 650
550-650 / 22-26 3,000 2,000 1,500 1,000
Irrigation 6,900 5,150 3,500 2,300
Source: Alberta Agriculture Beef Cow-Calf Manure or Alberta Forage Manual (page 205) adapted from Wroe et al. (1988) Guide to Range Condition and Stocking Rates for Alberta Grasslands

Note: This chart estimates yield production based on Alberta data for tame pastures. If you live in a different province/region or are evaluating native pastures, Method 2 (Field Sample) may provide more accurate results.

Step 3: Calculate Available Forage

Use the tables above to determine forage supply.

Enter pasture information in the yellow cells.

The utilization rate determines how much forage is used or lost to grazing, trampling, insects and wildlife. This helps determine how much forage material should be left behind to maintain future production. Utilizing pasture at a rate that exceeds the plant communities’ ability to recover can lead to lower forage production and encourage less palatable/productive forage plants to invade the pasture, including weeds. Choose a realistic utilization rate that will ensure adequate pasture recovery following grazing.

Recommended utilization rates for native pastures vary from 25-50%. For tame pastures, recommended utilization rates range from 50-75% depending on fertility. If the pasture is forested or brushed, adjust acres to reflect available grazing rather than the total acres in the field. Alternatively, the utilization rate for forested or brushy pasture can be reduced to 25% using the entire pasture size.

Table 3. Carrying Capacity
Forage Supply (lbs/acre)
Utilization Rate (%)
Available Forage for Grazing (lbs/acre)
370
Pasture Size (# of acres)
Total Forage Available in Pasture (lbs)
59,200
Total Animal Unit Month(s) (AUM)
74.7
Total Animal Unit Day(s) (AUD)
2,272
Carrying Capacity (AUM/ac)
0.47

Step 4: Determine Animal Unit Equivalents (AUE)

Different classes or sizes of cattle consume different amounts of forage. Either:

1) Use Table 4a below to find an Animal Unit Equivalent (AUE) similar to your grazing animals, or
2) Enter the average weight of your grazing animals into the AUE calculator (Table 4b).

Enter the AUE into the yellow cells in Step 5.

Table 4a. Animal Unit Equivalents (AUE)
Animal Type AUE
Cow, dry (1000 lbs) 1.00
Weaned animal (less than 800 lbs) 0.75
Yearling (800-900 lbs) 0.85
Cow with calf (900-1100 lbs) 1.00
Cow with calf (1100-1300 lbs) 1.15
Cow with calf (1300-1500 lbs) 1.29
Bull (less than 2000 lbs) 1.50
Bull (2000+ lbs) 2.00
Table 4b. Animal Unit Equivalent (AUE) Calculator
Animal Weight (lb) AUE
1.42

Step 5: Available Grazing Days

Enter the AUE determined in step 4 into the yellow cells below.

Table 5. Available Grazing Days
  Scenario 1 Scenario 2 Scenario 3
Animal Type
Animal Unit Equivalent (AUE)
Number of head to graze
Animal Grazing Days
2,673
1,761
1,976
Number of animals that can graze for one month
88
58
65
Total grazing days for your herd
53
35
40

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