How Do You Know What You Grow?

Susan Kerr

WSU NW Regional Livestock and Dairy Extension Specialist

By now, most livestock producers have probably heard the phrase “I don’t grow livestock, I grow grass*.” Maximizing the harvest of sunlight through plant photosynthesis is indeed the goal of knowledgeable livestock producers; it is the foundation for producing the best quality and quantity of home-grown forage. Basic pasture/forage management practices include:

 

• Conducting periodic soil tests to monitor soil’s nutrient content and assess fertilizer needs.

• Selecting, planting, and managing grass or legume varieties with the best likelihood of success in the region.

• Not letting livestock graze unless grass is at least 6” tall. Ever.

• Not letting livestock graze grass to less than 3”. Ever.

• Not turning livestock out when soils are waterlogged and compaction will occur.

• Dividing forage growth areas into multiple grazing cells and rotating livestock through the cells during the grazing season and not re-grazing a cell until grass is at least 6” tall.

• Holding livestock off pasture and feeding hay when regrowth is slow.

• Letting grass plants stockpile sugar reserves for next year’s growth by not grazing pasture in the fall; grazing stockpiled but dormant grasses after a killing frost is permissible, dependent on soil conditions.

• Distributing manure or fertilizer throughout the pasture.

• Irrigating as needed depending on water availability.

 

Great! You’ve done all these things and now want to know how much forage you’ve produced so you can make decisions about how many animals your acreage can support (or how much hay you will need to buy when the grass runs out…). How can you measure pasture

productivity? Photo 1. Grazing stick.

 

You will need:

1. A one-meter-square frame made out of PVC pipe or metal (other sizes are possible; refer to reference #1). A square meter frame makes the subsequent math easier.

2. Grass shears.

3. A scale.

4. A place to dry cut forage via sunlight, microwave or oven.

5. Large bags or other containers for collecting, drying and weighing.

6. Paper and pen for recording results.

 

Procedure

1. Take the one-meter-square frame to the forage area; toss it to a random area of the field. The more uniform plant growth is in the area being measured, the fewer samples that need to be taken. Generally, 10 random samples are suggested. Five are adequate in a very uniform field.

2. Cut and collect all plant material with stems originating inside the frame. Do not collect any plant material that originates outside the frame but falls over inside the frame; do collect all parts of plants with stems originating inside the frame but upper parts falling outside. Cut all the way to the dirt and bag all this forage. You may choose to discard anything that looks like residue from last year’s production if you are only interested in this year’s production, but even old forage is potentially available for grazing.

3. Weigh what you have collected as soon as possible. Don’t forget to subtract the weight of the container you used to hold the forage during weighing. Record the forage weight. You can measure in grams, kilograms or pounds.

4. It’s time to dry: put forage overnight in a 120°F oven OR heat repeatedly in a microwave until the weight of the sample stops decreasing OR leave in a protected area in the sun until the weight of the sample stops decreasing. You can also simulate a solar oven by leaving the sample in a closed car in the sun for several days. In all cases, focus on safety and do not let the sample(s) burn. Collect all leaves and stems that fall off after drying and include in Step 5.

5. Weigh the completely dry forage, again remembering to subtract the weight of the container holding the forage. Record the weight of the dried forage.

6. Repeat for all samples and average the weights.

 

Calculations

Subtract the average weight of dried forage from average weight of fresh forage; the difference is water. To determine the moisture content of the fresh forage, divide the “water weight” (difference between fresh and dry forage weight) by the weight of the fresh forage. Moisture content can range from 10 to 95%, depending on the maturity of the grass being measured.

 

 

Conversions

 

1 pound = 453.59 grams = 0.45359 kilograms

 

1 hectare = 10,000 square meters = 2.471 acres

 

1 kilogram/hectare = 0.892 pounds/acre

 

 

Here’s the fun part: estimate how much total forage the field has produced from the start of the growing season until sample time:

o Convert the average dry weight to grams to determine grams per square meter.

o Multiply this by 10.

o The result is total forage production in kilograms per hectare

o If desired, convert to pounds per acre by multiplying by 0.892.

 

Here’s the not fun part: you’ve calculated total forage production BUT not all is available for grazing or even haying. If haying, leave 4-6” of stems, which contain plants’ stores of sugar needed for regrowth. If grazing, plan on only taking half of what is available with the understanding that livestock will trample and soil half of that, meaning about 25% will be available for consumption. Management-intensive grazing can greatly reduce forage wastage but as the name implies, it is management intensive. “Frontal grazing,” wherein livestock follow a moving hot wire and graze immediately behind it, makes the most efficient use of forage, minimizes waste, and extends the forage season.

 

A grazing stick can be used to estimate forage production (Photo 1). Instructions are available in one of the resources listed. Sticks may be available at USDA NRCS offices.

 

After estimating forage production, the next task is to determine the pasture’s livestock carrying capacity. That is a topic for another day.

 

*Using pre-passage of Washington Initiative 502 definition.

 

Resources

1. Calculating Available Forage (includes conversion formulas for smaller samples): http://extension.usu.edu/files/publications/publication/NR_RM_03.pdf

2. How to use a grazing stick (video): http://drought.unl.edu/ranchplan/InventoryMonitor/ForageRange/ForageAvailability.aspx

3. Grazing stick manual: http://www.noble.org/ag/pasture/grazingstick/

4. Pasture and Grazing Management in the Northwest (Chapter 16 details microwave drying and various measurement techniques): http://www.cals.uidaho.edu/edComm/pdf/PNW/PNW0614.pdf

 

 

 

 

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