Grazing Management Calculations – Answers
Forage Production – How much forage does the land produce?
Example A:
1. Average mass the five samples:
42g + 45g + 36g + 39g + 38g = 200g
200 g = 40g
(The average mass of the 5 samples is 40 grams.)
5
2. Convert average sample mass to kilograms per hectare:
Sample frame area = 0.5m x 0.5 m = 0.25m2
Mass per m2 = 40g/frame = 160 g/m2
0.25m2
(Each sample comes from a 0.25m2 area)
(every m2 of the field produces an average of 160 g
of forage)
Mass per ha = 160 g/m2 x 10,000 m2/ha = 1600 kg/ha (every hectare of the field produces
1000 g/kg
an average of 1600 kg of forage)
3. Estimate the total forage production of the entire 65 ha field:
Total mass of forage production = 1600 kg/ha x 65 ha = 104,000 kg
(The 65 ha field produces an estimated total of 104,000 kg of forage)
Example B: You would do it the same way, with 2 exceptions. Treat each plant community area as two
different areas and complete the whole process for each one separately. Then to get the total
production of the entire field, just add the total forage production results from step 3 for each of the
two processes together.
Available Forage Supply – How much forage should be allocated to livestock?
Example A:
Q1: biomass/acre × area = total forage production
800 kg/ha × 550 ha = 440,000 kg total forage production
Q2: (SUF = 45%, so you are leaving behind for conservation purposes 55% of the forage)
Total biomass × safe use (%) = total forage supply (remember that 45% is the same as 0.45)
440,000 kg × 45%= 198,000 kg total forage supply
How many animals can I feed with the available forage supply? – Part 1
Example A:
Total forage production (kg) x Safe Use Factor = Available Forage Supply (kg)
25,317 kg 0.5 = 12,665.9 kg (12,665.9 kg of forage is available to feed the animals)
Carrying Capacity = Available forage supply ÷ monthly forage required by standard animal unit
⁄
= 34.6 AUM
(The available forage supply can feed 34.6 standard animals for one
month)
Herd size = carrying capacity (AUM) ÷ # months (M)
34.6 AUM ÷ 4 M = 8.65 AU
(the field can feed 8.65 standard animals over a 4 month period)
Variation:
Carrying capacity is sometimes expressed “up-side-down” as ha/AUM or ha/AUD instead of AUM/ha or
AUD/ha. You can convert this to “right-side-up” by dividing 1 by the value. For example:
1 ÷ 0.35 ha/AUM = 2.86 AUM/ha
Example B:
Hectares of land ÷ per AUM carrying capacity rate (ha/AUM) = carrying capacity (AUM)
650 ha ÷ 1.5 ha/AUM = 433 AUM of forage supply
How many animals can I feed with the available forage supply? – Part 2
Calf herd size = carrying capacity (AUM) ÷ number of months (M) ÷ animal unit equivalent
(weaned calves/AU)
34.6 AUM ÷ 4 M ÷ 0.75 weaned calves/AU = 11.5 weaned calves
(this field can be grazed by 12 weaned calves for 4 months – in this case you would round the answer
because we are now talking about real animals)
Calculating Forage Demand and Converting Forage Requirements Based on either Animal Unit
Equivalents or Animal Size
Example A: 600 cow-calf pairs X 1.3 AU/pair X 5.5M = 4,290 AUM(these cattle require 4,290 AUM of
forage)
Example B: Demand of forage (kg) = Number of pairs x combined weight of each pair (kg/pair) x daily
forage requirement (% of body mass/day) x conversion of days to months (30.5D/M) x month number of
months (M)
150 cow/calf pairs x 630 kg/pair x 2.5%/day x 30 days/month x 6 months = 425,250 kg of forage
Example C: Forage supply:
1480 AUM
Forage demand:
# animals x standard animal unit conversion (AU/animal) x months (M)
150 cows x 1 AU/cow x 6 months = 900 AUM
(remember to get the standard animal unit conversion from the AUE table)
Balance: Supply (AUM) – Demand (AUM) = 1480 AUM – 900 AUM = + 580 AUM (there is a extra forage
in the grazing system because the supply is greater than the demand by 580 AUM)