Method 2

# Carrying Capacity Calculator, Method 2

Carrying capacity, also known as 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 in order 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.

## Method #2: Field-based Sampling

Field-based sampling provides greater accuracy but also requires more hands-on work.

## Sampling Instructions

Supplies needed include: a square frame that is 20 inches by 20 inches (50 cm by 50 cm) OR a circle frame that is 400 square inches, scissors to cut grass, paper bags, markers to identify location associated with each bag, a way to dry the samples and a scale (a kitchen scale works).

The goal is to take enough samples to capture the variation in the pasture (e.g. low spots with higher productivity and bare spots on top of hills with lower productivity). This will ensure samples are obtained that are representative of production across the entire field. The number of samples taken will vary with the size of the field and how uniform or uneven grass production is.

This calculator is based on a sample taken from 400 square inches (or 2500 square centimeters).

### Step 2: Dry, Sort, Weigh & Record

Grass clippings can be dried in a food dehydrator, or at room temperature over several days (do not use the microwave!). Make sure to keep sample bags contained and identified. Weigh each bag for the total from each sample and enter into the yellow cells below. Sort the litter from the green material – tweezers make this job easier but either way it is tedious. Enter the weight of the green material only in the yellow cells below (litter weight is calculated and does not need to be weighed separately).

Note the percentage of litter cover. The amount of litter left on the land can make a big difference in the performance of the pasture. Litter includes ungrazed residue from the previous year’s growth, residue from bale grazing, fallen stems, leaf material and other partially decomposed material. Litter helps to conserve moisture by reducing evaporation, improving infiltration and cooling the soil surface.

The carrying capacity or total Animal Unit Month(s) for the field sampled is calculated below.

Samples from Single Pasture #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 Average
Total dry forage weight (grams)
58
Dry Green material weight (grams)
29
Litter (calculated)
20
5
30
25
25
25
80
25
30
25
29
Percentage Litter Sampled
50%

### Step 3: Calculate Available Forage

The forage supply has been calculated using information from Step 2.

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.

Enter pasture information in yellow cells.

Pasture Information
Forage Supply (lbs of dry matter/acre)
2,064.8
Utilization Rate %
Available Forage for Grazing (lbs/acre)
20.65
Pasture Size (# of acres)
Total Forage Available in Pasture (lbs)
3,303.68
Total Animal Unit Months (AUMs) of Forage Available
4.2
Total Animal Unit Days
127

### Step 4: Calculate the number of pairs or yearlings that can graze

Different classes, or sizes, of cattle consume different amounts of forage, with bigger animals eating more available forage. Use the chart below to find an Animal Unit Equivalent (AUE) similar to your grazing animals and enter in the yellow cells.

The carrying capacity for the field sampled is calculated below.

Adjusted for Animal Unit Equivalents (AUE)
Cow-calf Pair Yearling
Animal Unit Equivalent
3.3
5
Number of pairs/yearlings that can graze for one month
98
149
Total pairs/yearling grazing days available
Animal Unit Equivalent Chart
Animal Type Animal Unit Equivalent
Cow, dry (1000 lbs) 1.0
Cow, 1300 lbs with calf to 4 months 1.3
Cow, 1400 lbs with calf to 4 months 1.4
Cow, 1500 lbs with calf to 4 months 1.5
Yearling Steer 0.85
Heifers, 700 lbs 0.80
Bulls, mature (1700 lbs. average) 1.7