Forage - How Grass Grows
Module 4

Lesson 1- Determining Carrying Capacity, Activity Sheet
Acknowledgement: Taken from "Living on the Land 2001"

Carrying capacity is the number of animals that a paddock or cell can accommodate without overgrazing.  Simply put, the carrying capacity is how much grass you have. Determining carrying capacity helps you determine the number of months your pastures can provide forage for your livestock. 

It’s important to recognize that grazing animals only need to eat 2.5 to 3% of their body weight each day.  An 1100-pound horse will eat 27.5 pounds of air-dry forage (hay) a day, or 825 pounds per month. 

Remember that horses damage or trample another 25% of the forage in a pasture, so we have to add 25%, making the total available forage requirement for an 1100-pound horse closer to 1000 pounds per month.  Horses will eat more than needed if given continuous access to pasture grasses.

Sample Carrying Capacity Problems

Pasture 1 consists of introduced plant species.  It is in reasonably good condition and produces a total of 4893 pounds of useable forage per growing season.  Pasture 1 is 3 acres in size.

Pasture 2 consists of native pasture or rangeland.  It is also 3 acres in size.  It produces 3495 pounds of useable forage per growing season

Problem 1:  Estimate the approximate number of months one horse can graze on Pasture 1.

 

Problem 2:  Estimate the approximate number of months one horse can graze on Pasture 2.

  

Problem 3:  Estimate the approximate number of months three horses can graze on Pasture 1.

 

Problem 4:  Estimate the approximate number of months three horses can graze on Pasture 2.

 

Pasture 3 consists of 10 acres of introduced grasses and is not subdivided with fences.  Representative areas were clipped and an average forage yield of 850 pounds per acre was calculated.  Two (2) horses graze on this property from May 1 to September 1.

Problem 5a.  What is the total production for the 10 acres?

  

Problem 5b.  What is the useable forage after harvest efficiency?

  

Problem 5c.  How many months of grazing are available for one (1) horse?

 

Problem 5d.  How many months of grazing are available for two (2) horses?

 

Problem 5e.  Is this pasture overgrazed?  In other words, are the key species grazed too short, or are plants regrazed before desired regrowth has occurred?  When this happens repeatedly, plants die, bare ground results, and weeds invade.

Answer Key

Note:  Calculations are based on calculations from the “Determining Forage Yield” activity.

Problem 1: 

4893 pounds of forage divided by 1000 pounds per month forage requirement  =  5 months 

Pasture 1 can provide five months of forage for one horse to graze.

Problem 2: 

3495 pounds of forage divided by 1000 pounds per month forage requirement  =  3.5 months

Pasture 2 can provide three and a half months of forage for one  horse to graze.

Problem 3: 

Three horses, each requiring 1000 pounds per month, brings the monthly forage requirement to 3000 pounds.

4893 pounds of forage divided by 3000 pounds per month forage requirement  =  1.6 months

Problem 4: 

3495 lb of forage divided by 3000 pounds per month  (forage requirement for 3 horses)  =  1 month

Problem 5a. 

850 pounds per acre multiplied by 10 acres  =  8500 pounds of total forage

Problem 5b. 

8500 pounds of total forage multiplied by 35% (0.35)  = 2975 pounds of useable forage

Problem 5c. 

2975 pounds of usable forage divided by 1000 pounds per month needed for one horse  = 3 months of grazing

Problem 5d. 

2975 pounds of usable forage divided by 2000 pounds per month needed for two horses  =  1.5 months

Problem 5e. 

No clear answer.  If the owners are regulating the number of hours a day the horses are grazing, the pastures will not be overgrazed.  If the horses have access to the pastures 24 hours a day, seven days a week, then it is probably overgrazed.

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