Determining forage production and stocking rates



Determining a sustainable stocking rate for grazing livestock using the USDA-NRCS Web Soil Survey

--Tip Hudson, WSU Extension rangeland and livestock specialist

Rangeland research across the semi-arid Western United States has shown that an appropriate stocking rate is the most critical management decision in protecting the productive capacity of grazed rangelands, dry forests, and non-irrigated pastures. The second most important management input is ensuring proper distribution of livestock to avoid excessive utilization of the most preferred or most grazing-sensitive forage species and subsequent damage to soil.

The first step to deciding how many animals are appropriate for a farm or ranch property or leased grazing area is to determine how much usable forage it grows and to decide how much of this forage you plan to leave. One can obtain this information from either direct measurement through a clipping procedure or from estimates produced by the Natural Resource Conservation Service in Ecological Site Descriptions or U.S. Geological Survey Soil Survey data (). On large properties, developing reliable forage production data from a clipping procedure can be laborious and complicated, requiring stratification by soil or vegetation type and collecting a large number of samples in each stratum. Various combinations of soil type, aspect, elevation, and vegetation yield major differences in total biomass production on many Washington State landscapes.

This fact sheet will walk you through developing an initial stocking rate from both USDA-NRCS Web Soil Survey data or manually collected forage production data obtained through clipping and weighing vegetation.

CALCULATE FROM WEB SOIL SURVEY

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The USDA-NRCS Web Soil Survey application provides an enormous amount of soils data. Not all data are available for every location. The web address given above will take you to this page. From here, click on “START WSS” to begin.

Step 1: Navigate to the general area using one of the options on the left of the screen. You can use a physical address, zoom to a county, identify a site by township, range, section, etc.

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Here I have navigated to Kittitas County, Washington.

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Step 2: Once you have zoomed in to a scale at which the entire area of interest is visible, you must outline your “area of interest” (AOI) in the web soil survey application. If the property is a square, use the square-shaped AOI tool; if, as is usually the case, it is not square, use the polygon (irregular) shaped AOI tool. The shape is created by clicking on each corner or vertex and double-clicking when you’ve closed the loop.

The website has a very useful Help section accessed by clicking on the question mark icon on the top right of the screen.

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Step 3: After selecting the Area of Interest, go to Soil Data Explorer to access data about the area.

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Once in Soil Data Explorer, click on Vegetative Productivity about halfway down the left side of the page. Then select “Range Production (Normal Year)”; click on “View Rating”. It can be useful to generate reports for “Favorable Year” and “Unfavorable Year” to better understand how much variation is possible on your particular site. Some range sites have rather dramatic differences between favorable and favorable year forage production values. Keep in mind that these values are for all plants on the site, including current year’s growth shrubs. These values are also representative of the “reference state”, which means vegetative productivity based on the expected native assemblage of plant species and not a degraded state with annual grasses and weeds. A site that has been seeded with introduced perennials may well be as productive as the native state in terms of forage production. You must be the judge of whether the reference state values given by the soil survey data or Ecological Site Descriptions are close enough to use for calculating a stocking rate. If not, choose the “unfavorable year” values or spot check the values with actual measurements from clipping data.

Portions of the Area of Interest may be inaccessible or unaccessed by livestock due to distance from water, geographic barriers, high slope, etc. The simplest way to account for this is to adjust the AOI to exclude that area so that this is reflected in the range production values. If a pasture/range area has fairly homogeneous vegetation you can also simply reduce the final stocking rate by the percentage of unavailable area.

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Step 4: Open the WSU Extension Stocking Rate Calculator spreadsheet available at range . . .

Step 5: Copy the table values for range production by dragging the cursor over the entire table except the column headings and the totals at the bottom, then right-clicking and selecting “copy”. Open the WSU Extension Stocking Rate Calculator spreadsheet, click on the top-left empty cell (A4), select “paste special”, and paste unformatted values into the spreadsheet.

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The important data are the rating for each soil type (in either pounds per acre per year or in Animal Unit Months) and the acres of that soil type inside the Area of Interest, but the whole table should be imported for the right values to go in the right columns. There are 30 rows available in the spreadsheet. If your AOI is large, it may have more than 30 rows of soil types with production data. If this is the case, you have a few options, depending on your level of comfort with Excel.

1. Preferred option: Add rows to the spreadsheet by right-clicking on the last row in the table and selecting “insert”. When you’ve added enough rows, you must copy the formula in the last column to the right by selecting the last cell that has the formula in it at the former bottom of this table, then clicking on the “handle” on the bottom right corner of the cell and dragging the formula to the new cells in this column.

2. Split the AOI into two pieces – this requires re-drawing the AOI in the web soil survey. Run the stocking rate calculation separately and combine the values.

3. Duplicate the worksheet in the Excel spreadsheet and split the range production table values across the two worksheets.

Sidebar: If you wish to calculate a stocking rate from these data without using the spreadsheet you multiple the “Rating (pounds per acre per year)” by the “Acres in AOI” for each “Map unit symbol”, which corresponds to a soil type. The sum of these subtotals by soil type results in total pounds of range production for the Area of Interest. Multiply this by a harvest coefficient (HE) to obtain total grazable pounds of forage. A 25-35% harvest coefficient represents a sustainable conservative level of annual consumption. Once you have total forage production, use the procedure described in the Stocking Rate Calculations section of this fact sheet to arrive at a stocking rate for the Area of Interest.

On the right side of the spreadsheet is table labeled “Stocking Rate”. This calculates total grazable forage for a range of harvest coefficient values as well as AUMs (which is Grazable Forage / 900) and Animal Days (which is AUMs multiplied by 30). These are two different ways of expressing the stocking rate values based on a 1,000-pound animal. You then enter the size of the herd in the yellow box labeled “Herd size” and enter an Animal Unit Factor from the table below, based on the type of livestock being grazed and their weight relative to the 1000-pound equivalent, and values for Herd Days will be calculated in the far right column of the Stocking Rate table. This represents the total number of days your herd could graze the Area of Interest before consuming the total grazable forage. Keep in mind total forage production is not usually available May 1. If a site has 50 herd days, one could conceivably graze it 20 days in May and 30 days in August but not April 10-May 30. The growing season is not completed.

You can save the Web Soil Survey report for your AOI by clicking on the Soil Reports tab; enter a name for the site and click on Generate Report to create a PDF document you can save to your own device.

Save the spreadsheet to your device, renaming the file to something that will be descriptive of the Area of Interest. You may have a Web Soil Survey report and spreadsheet for several grazing areas, particularly if they are not contiguous.

CLIP & WEIGH METHOD

Stocking rate from clipping & weighing procedure

1. Use a soils map (paper or Web Soil Survey report), aerial photograph, or satellite image to identify different soil/vegetation combinations which produce different plant communities or forage yields.

2. Develop a sampling procedure to collect forage clipping data from each of the strata. Plan to collect at least twenty samples from each using some method to randomize plot location.

3. Collect clipping samples, preferably after dominant forage species have reached the peak of growth or have produced seed in order to accurately measure the actual total growth for that growing season. For example, clipping on May 1 will fail to capture growth that will likely occur for another 60 days on Eastern Washington rangeland, depending on soil moisture and temperatures.

a. A coated cable hoop with a circumference of 93 inches works well for sampling and easily converts to pounds per acre. Be sure to keep the hoop as close to a true circle as possible because the area decreases as a circle becomes more oval.

b. Clipping involves cutting all the forage within the hoop at almost ground level (~1/4”), weighing the sample in grams. In locations where grasses have not been grazed in several years there may be an accumulation of previous year’s stems; these also have negligible nutrient value and will generally be avoided by livestock and should be removed from the clipped sample prior to weighing.

c. Weigh the sack first so you know what to subtract from the total weight, then weigh the bagged sample.

4. Use a dry matter table (below) to estimate the percentage of water in the samples (this may be different across strata if there are differences in maturity). Multiply the fresh sample weight by the estimated dry matter percentage to arrive at grams of dry forage (roughly a hay equivalent) per sample. Average the samples for each strata.

a. Purists advocate removing the moisture instead of relying on estimates by placing paper sample sacks in a warm oven for several hours to arrive at a dry matter weight.

5. If using the 93” hoop, multiply the average dry matter weight per sample by 20 to get pounds per acre per stratum. Multiply the stratum per acre value by the estimated percent of the total area represented by that stratum. Do this for each.

6. Use the sum total of all strata to multiple by a harvest coefficient and obtain grazable pounds of forage for the entire area. Divide this by 900 to obtain a stocking rate in AUMs.

Estimate dry matter content using Table 1. For irrigated grasses use 25% dry matter before heading.

|Table 1. |Before heading (initial |Headed out (boot stage|Seed ripe |Leaves dry (stems partly |Apparent |

|% dry matter of grasses|growth to boot stage) |to flowering) |(leaf tips |dry) |dormancy |

| | | |drying) | | |

|Cool-season grasses |35 |45 |60 |85 |95 |

(from National Range Handbook at )

STOCKING RATE CALCULATIONS

Once you have total grazable forage production (after multiplying total forage production by a harvest coefficient), whether through the USDA NRCS Web Soil Survey or clipping/weighing data, all that remains is to determine how long the Area of Interest will support your herd. There are countless reputable documents available on the internet that explain the concept of Animal Unit Months and using these as a basis for stocking rate, but here is a workable summary.

A ruminant animal will consume a quantity of (dry matter) forage equal to 2.5-3% of its body weight every day. Most stocking rates are based on 1000-pound equivalents. Therefore, a 1000-pound animal consumes ~30 pounds of forage on a dry matter basis per day. The Animal Unit Month is a standardized measure of forage quantity that represents the amount of dry matter forage necessary to support that 1000-lb animal unit for one month. That value is given as 786-900 lbs, depending on the quality of the feed. Feed intake decreases with forage quality. 900 pounds is a workable figure that builds in some conservatism and assumes that animals will select individual plants and plant parts that will be higher in forage quality than the average of all standing forage for a given site.

Total grazable forage divided by 900, then, gives you the number of Animal Unit Months (AUMs) available for a site; again, this is another way to express forage quantity independent of herd size, animal type, length of grazing period, etc. Conceptually, a grazing area with 1000 AUMs (or 900,000 pounds of forage) could support 1000 mature cows for one month provided the grazing period is late enough in the season that forage yield has peaked, 500 cows for two months, 333 cows for three months . . . you see the pattern.

The Animal Unit factor is often used to account for differences in animal size. A cow with calf would be 1.3 AUs, a bull 1.35, a ewe .2, etc. So if your herd is 250 mother cows, 10 bulls, and 250 calves at 300 pounds, we have a total of (250*1.3)+(10*1.35)=325+13.5=338.5 Animal Units. Using this example, the 1000 AUM grazing area would support the herd for (1000/338.5)=2.95 months or 89 days.

Damage to primary forage species is still possible using a sustainable stocking rate based on accurate assumptions or direct measurement of forage production combined with a conservative harvest coefficient if efforts are not made to distribute livestock properly. Animals will tend to favor areas that are near water, have low slope or easy topography, have favorable thermal cover, etc., especially if they are present in the same pasture or grazing unit for an extended period of time.

Periodic growing season rest is critical for most bunchgrasses native to the Intermountain West. Yearly grazing during the active growing season, especially without opportunity for regrowth, will cause desirable perennial grasses to decline in vigor and abundance. Bunchgrasses should be allowed to set seed roughly every other year. This can be accommodated through spring grazing and removing livestock while soil moisture exists to support full regrowth or by grazing after plants have set seed. Dormant grasses can sustain somewhat heavier utilization rates, and grazing after seed shatter helps overcome the greatest limiting factor for seed germination, seed-to-soil contact.

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This is the selection you will copy into the spreadsheet.

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