Forage - How Grass Grows
Module 4

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


Forage yield is the actual amount of forage dry matter available in a pasture or rangeland on a per acre basis.  When you calculate forage dry matter, it is comparable to cutting and drying hay.  Rather than mowing drying, and weighing all of the useable forage in an acre of your pasture, we will sample from a small area to estimate the total forage yield.

Equipment needed:          

It’s easiest to construct a cable hoop by bolting together the ends of an eight-foot-long, ¼-inch cable.  This cable is available at most farm and ranch supply stores for a cost of about $5. A hand-held spring scale is available from forestry , animal health or surveying equipment stores.  A 300-500-gram scale works best. Forestry Suppliers has the clippers and scale for reasonable prices, 1-800-647-5368, www.forestry-suppliers.com.  The Pesola Medio-Line Spring Scale, 300 grams, stock #93013 and model #40300, is $34.50.  They also have clipping shears for smaller hands, stock #81191, at $16.95.  Ben Meadows, 800-241-6401, has Corona Grass Trimmers for $21.95.

Now we have an 8-foot-circumference circle.   This circle is roughly 30 inches in diameter.  We want to check the actual forage in our pasture.  For simplicity, clip areas that are not grazed.  Use a representative area of the pasture, not the high or the low producing areas, but the moderate areas.  We also want to take three or so different samples and then average the results for greater accuracy.

As we discussed before, the best management practice for grazing is to take half and leave half.  That means grazing about half of the plant from the top down.  This leaves the plant plenty of vegetation to continue making food for itself from sunlight (photosynthesis) and plenty of cover to protect the soil from drying out during the growing season.  In order to graze either pasture or rangeland in a sustainable manner, a ‘key species’ should be pre-determined.  This is the forage specie that you manage for.  When the suggested grazed height is reached, it is time to rest the pasture.  The idea is to evenly graze the forage so that some plants are not left ungrazed.  When plants are not grazed initially, they usually are not chosen the rest of the growing season.  Instead, previously grazed plants are re-grazed after a rest, over and over. On some rangelands, shrubs may be the dominant plants.  If your grazing animals, such as cows or horses, cannot eat the shrubs, there may not be much useable forage. 

 Use your calculations from clipping or soil survey production estimates in order to apply the calculations below. A method for preliminary estimates on usable forage for rangeland and pasture is provided.

On rangeland, if we start with 100% of the plants and take half and leave half  (100% x 50%), then 50% remains usable.  Animals will not eat 25% of the available species that grow on the rangeland (50% x 75%), reducing usable forage to 37.5%.  Of the 37.5% that is left, the animals will trample or destroy another 25% (37.5% x 75%) leaving only 28%.  For ease of calculation, we’ll round the number to 25%.  The actual useable forage in a rangeland setting may be only about 25% of the available forage!  Note that the terms “available” and “usable” are two different concepts.  Available forage includes all plant material growing in a pasture that animals have access to, while useable forage is what the grazing animal will actually eat.

For pasturelands, the calculations are a little different.  First, on pastureland as on rangeland, starting with 100% of the plants and grazing half (100% x 50%) leaves 50%.  However, on pasture, animals will usually eat all of the available species unless the pasture is overrun with weeds.  If this is case, weed control measures and/or renovation may be options.  Assuming the pasture is in good condition, only about 25% will be spoiled with trampling or manure (50% x 75%) yielding 37.5%, which we’ll round to 35%.   The actual usable forage in a healthy pasture is only about 35% of the available forage.  Your goal is to rest the pasture after the ‘key specie’ has been grazed down to the appropriate height for that species.

Sample Calculation

The average clipping weight (total weight less the bag weight, averaged for several samples) is 200 grams.  To convert from 200 grams per 8-foot circumference circle (30 inch diameter) to pounds per acre, multiply 200 by 20 to get 4,000 pounds per acre of total forage. 

4,000 pounds multiplied by 25% = 1,000 usable pounds per acre for rangeland 

4,000 pounds multiplied by 35% = 1,400 usable pounds per acre for pasture

To calculate total forage in a given pasture, multiply the pounds per acre times the number of acres in the pasture.

Determining Forage Yield – Sample Problem

You have clipped three representative areas in Pasture 1.  The pasture is 3 acres in size.  The three samples you took weighed 251 grams, 191 grams and 281 grams.  The lunch bags weigh 8 grams each.

How many pounds of forage are useable in Pasture 1 per acre?  Assume Pasture 1 is a pasture in reasonably good condition.

How many useable pounds of forage per acre would Pasture 1 provide if it were a rangeland or native pasture?

How many pounds of forage are useable in the entire introduced species pasture?  How many pounds of forage are available in the entire native pasture?


Answer Key:

The actual weights of the three samples from Pasture 1, after subtraction of the 8-gram bag weight, are:

             251 – 8 = 243 grams

            191 – 8 = 183 grams

            281 – 8 = 273 grams   

The average weight of forage, in grams is:  (243 + 183 + 273)  =  699  = 233 grams
                                                           
            3                      3

Converting grams to pounds per acre:  233 x 20 = 4660 pounds total forage

Available forage = 35% of total forage or 4660 pounds x 35% (0.35) = 1631 useable pounds per acre

Available forage per acre for rangeland or native pasture would be 4660 x 25% (0.25) = 1165 useable pounds per acre

Available forage for all 3 acres of “reasonably good condition” pasture = 1631 x 3 = 4893 useable pounds of forage

Available forage for 3 acres of rangeland or native pasture = 1165 x 3 = 3495 useable pounds of forage

Total forage, available forage, and usable forage are three completely different concepts.  Total forage includes everything growing in a pasture.  Available forage is what the grazing animals have access to and could potentially eat.  Usable forage is what they prefer to eat and what they do not damage.  Forage on very steep slopes may not be available to some animals.  Usable forage that has been urinated on is no longer useable.  Total forage would include shrubs/forbs that the animals would not eat.  On range and pasture, the 25% and 35% estimates take much of these factors into account.

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