Vegetation Measurement and Monitoring

11.1 Utilization Introduction

Video Presentation

Watch this video presentation for an overview and discussion of utilization concepts and methods.

To add captions to this video click the CC icon on the bottom right side of the YouTube panel and select English: Corrected captions.


Learning Guide

Introduction to Utilization

Utilization is defined as the proportion of the current year’s forage production that is consumed or destroyed by grazing animals. Utilization is determined by comparing the amount of herbage left with the amount of herbage produced during the year. The term utilization may be used in reference to use of individual species or to the amount of use on the vegetation as a whole.


Why Do We Measure Utilization?

Utilization provides information that is central to the concept of sustainable range management:   proper use of the forage base is essential to ensure that sufficient aboveground plant material remains after grazing to support regrowth and to protect the soil. The idea is simple – the manager wants to ensure that the level of use is sustainable, and that excessive and unsustainable levels of utilization are not occurring.  The application of this idea is difficult – how do you measure the proportion of a plant that has been removed? How do you measure something that no longer exists?


Important Considerations When Measuring and Interpreting Utilization

Utilization is arguably one of the most controversial of all vegetation measurements because methods to measure utilization may be applied incorrectly or inconsistently. There have been a variety of perspectives on how utilization data should be interpreted and used to inform management decisions. A full discussion of this complex issue is not practical in the context of this lesson, and several excellent publications about utilization measurements and their interpretation relative to rangeland management have been published, such as “Principles of Obtaining and Interpreting Utilization Data on Rangelands” by Smith et al 2007.

Note: It is very important to understand the distinction between utilization and relative use, sometimes called seasonal utilization. Recall that utilization is the proportion of annual forage production that is consumed, destroyed, or otherwise removed by grazing animals. Annual forage production is difficult to measure, and is usually estimated as peak standing crop. However, this assumes that production is measured once the growing season is over, and plants have stopped growing. If utilization is estimated during the growing season, and is calculated as a proportion of forage that was produced before the growing season ended, this is an estimate of relative use, not utilization.

For example, if the growing season spans between April 1 and October 1, and you base your estimate of utilization on the amount of production and removal of forage on July 15, this overlooks plant growth that occurs between July 15 and October 1. It is perfectly acceptable to estimate relative use in July, but be sure that you don’t report relative use as utilization. A true measure of utilization must compare the amount of forage removed to the total amount produced during the entire growing season.


Where Do We Measure Utilization?

We often use a selective sampling approach when we decide where to measure utilization. Measurements are usually made in key areas or critical areas.  A key area is a sampling site that is specifically selected because of its location, use, or value as a monitoring site specific to grazing. Key areas are representative of the larger management unit, both in terms of the mix of species that grow there, as well as how the site responds to management.

Key areas should:

  1. Be representative of the pasture or management unit;
  2. Be capable of showing a response to management actions; and
  3. Contain the key species

 

Key areas should not be located in areas where livestock congregate, such as near watering points or fences, because these areas are not “representative” of the entire management unit. In addition, key areas should not be located in remote locations that are inaccessible or receive little use by grazing animals.

We may choose to evaluate utilization in critical areas. Critical areas are areas that receive special consideration, from a management perspective, because of inherent site factors, sizes, locations, conditions, values, or significant potential conflicts among uses. Critical areas may include riparian areas, sites that support known populations of species of concern, or culturally significant sites, among others.


Which Plants are Evaluated?

Utilization can be measured on the forage vegetation as a whole, including all species that are available for consumption and that would be selected by the kind and class of grazing animal being considered. Utilization is often measured by focusing on key species, or those species that are selected by the grazing animal of interest. Key species are forage species whose use serves as an indicator of the degree to which associated species are also used. In addition, key species include those species which must, because of their importance, be considered in the management program. It is common to focus our measurement efforts on selected key species as indicators of the response of the unit to management.

Utilization is reported as an average value. For example, if you have an area where each of ten plants has 50% utilization, the average utilization for that area will be 50% (Figure 1a). However, if you have an area with five plants that are ungrazed (0% utilization) and five plants with their biomass completely removed (100% utilization), the average utilization for that area will also be 50% (Figure 1b).

Figure 1. Ten plants are shown with 50% utilization per plant (a), and ten plants are shown with five having 0% utilization per plant, and the other five having 100% utilization per plant (b). Both (a) and (b) have an average utilization of 50%.


Methods to Measure Utilization

A variety of methods have been developed to measure utilization. Utilization can be determined through direct comparison between grazed and ungrazed areas, techniques that apply knowledge of the relationships between plant height and the weight of removed forage, and ocular estimation approaches. This discussion focuses on applications specific to utilization measurements on herbaceous forage, but several of these methods can be adapted to estimate utilization of browse on shrubs. Details of estimating utilization of browse on shrubs may be found in Chapter 4 of the National Range and Pasture Handbook and in Utilization Studies and Residual Measurements.


Direct Comparisons

Utilization can be estimated by directly comparing grazed and ungrazed areas. The difference between these sample units represents the amount of forage consumed or destroyed during the grazing season. This method usually requires the use of cages or exclosures to protect areas from grazing.

 

Advantages
  • No need to estimate what is “missing”
  • Generally perceived as the most accurate and reliable utilization measurement method

 

Limitations
  • Potential for different growth rates between protected and unprotected areas
  • Cages may modify micro-environment causing increased growth within the cage
  • Expense and time associated with constructing and placing cages on the landscape

 

Procedures

Direct comparisons are usually accomplished by clipping standing crop in areas protected from grazing by cages and comparing that to standing crop harvested from similar, uncaged areas that were grazed. Cages are typically positioned in multiple locations in a pasture prior to grazing, and carefully secured to the ground to prevent them from being moved (Figure 2). At the end of the grazing period, the cages are removed and biomass is harvested, dried, and weighed using techniques described in the module on plant biomass. Standing crop is also harvested at the same time to determine the amount of biomass remaining after grazing. This approach may be applied to key species, or to forage vegetation as a whole.

Figure 2. Five cages have been placed in a field with grazing and are secured in place with steel t-posts and steel welded wire mesh panels.

Comparisons using caged plots may be done using paired plots or unpaired plots. For the unpaired approach, cages are positioned in the pasture of management unit without considering the location of the “grazed” plots that will be harvested. Utilization is determined by calculating the difference between the mean biomass on grazed plots and mean total production, and dividing that by the total annual production.

With the paired plot method, pairs of plots are selected that are similar in the amount and type of plants prior to grazing: one of the plots is caged, and the exact location of the uncaged plot is recorded so that it can be identified after the grazing period is over. The difference between biomass harvested in each pair of plots is determined, and the average difference in biomass from all pairs is compared to total standing crop to determine utilization. Compared to unpaired plots, paired plots take more time to set up initially, but require a smaller sample size to determine utilization.

The microenvironment inside cages is different than in uncaged areas, and so cages are best used to monitor utilization over short periods (e.g., several weeks). Fenced exclosures can be used to protect areas from grazing throughout a lengthy growing season if utilization over longer periods is desired. However, fencing is expensive, and the time and costs needed to build exclosures and cages can be a disadvantage of this method.


The Height-Weight Method

The height-weight method involves measuring the heights of both grazed and ungrazed plants to determine the relationship between grazed-plant height and percent utilization.

Note: The percentage of plant height does not equal the percentage of utilization because most of the weight in grass plants is concentrated near the base of the plant! There is a tendency for untrained people to equate 50% removal of height to 50% removal of weight and thus 50% utilization. This is incorrect. In Figure 3, we see that the top 21cm of a purple threeawn (Aristida purpurea) comprises only 30% of the plant’s weight, while the bottom 21cm comprises 70%.

Figure 3. An image of a purple threeawn clipped into sections; the weight (in grams) of each section is listed in black with the corresponding percent weight in purple.

 

Advantages
  • Provides accurate, uniform estimates of utilization for grasses and grass-like plants
  • Height-weight curves and utilization gauges available for numerous key species

 

Limitations
  • Cannot be used for forbs or shrubs
  • Ungrazed plants must be available to ensure appropriate comparisons
  • Height-weight curves may not be available for selected key species

 

Steps of Height-Weight Method

This technique involves sampling both grazed and ungrazed plants at selected intervals along a transect to determine the average ungrazed plant height and the average percent utilization.

  1. At each sampling point on the transect, locate the nearest key species.
  2. Measure the average height of the plant to the nearest ¼ inch. Use a measuring tape or ruler is to obtain plant height by placing it in the middle of the plant where it rests firmly on the crown for bunchgrasses, or in the center of a 2-inch circle for turf grasses.
  3. Record the height in the appropriate column on the data sheet indicating whether the plant was grazed or ungrazed, and calculate the average utilization (Figure 4). For partially grazed plants, the general rule is to count a plant as grazed if >50% of the tillers have been grazed, and ungrazed if <50% of the tillers are not grazed.
    Figure 4. An example Height-Weight data sheet with corresponding calculations for average ungrazed plant height and average utilization (from Utilization Studies and Residual Measurements).

     

  4. The method requires that at least 20 ungrazed plants be measured to obtain a reliable representation of ungrazed plant heights.
  5. A “utilization gauge” for the species being sampled is used to determine percent utilization based on average height of ungrazed plants. Utilization gauges are developed from previous height-weight investigations (Figure 5).
  6. Figure 5.  Utilization gauge developed by the USDA Forest Service (from Utilization Studies and Residual Measurements)
  7. Height-weight curves and conversion tables can be developed as needed when utilization gauges are not available for selected key species. This is accomplished by tightly wrapping a string in a spiral around a plant, harvesting the wrapped plant, and cutting it into segments of known length. Each segment is dried and weighed. This procedure is repeated for multiple plants of similar height. The average weights are plotted against the height of the segment to develop a height-weight curve, and percent utilization represented by different grazed heights is calculated. Conversion tables can be created that reflect the amount of utilization based on the average height of ungrazed plants and the measured height of grazed plants.

  8. Grazed -Class Method

    The grazed-class method uses photo guides of key species to help the observer classify plants into a utilization category. Photo guides can be general for a plant type or they can be species specific (Figure 6). Actual photo guides are available for many key species and observers can also develop photo guides for additional species or areas. A detailed description of how to develop photo guides can be found in the Interagency Technical Reference, Utilization Studies and Residual Measurements on page 111-112.

    Figure 6. A photo guide showing utilization classes of switchgrass (Panicum virgatum).

     

    Advantages
    • Rapid method that is easy to learn and use
    • Gives picture of amount removed as well as residual forage
    • Once developed, guides can generally be applied to multiple sites and across different years

     

    Limitations
    • Guides do not distinguish between poor growth and use
    • Takes time to develop guides if not available

     

    Steps of Grazed Class Method
    1. At each sampling point on the transect, select the nearest plant of the key species and compare it with the photo guide of that species.
    2. Classify the plant according to the six grazed-classes representing 0, 10, 30, 50, 70, or 90% use.
    3. To calculate utilization, multiply the number of plants sampled in each grazed-class by the grazed-class percent. Divide the sum of these products with the total number of plants observed on the transect to get average utilization for the study site (Figure 7).

     

    Figure 7. An example Grazed Class data sheet with corresponding calculations for average utilization (from Utilization Studies and Residual Measurements).


    Grazed Plant Method

    This method involves developing a regression equation between the proportion of plants that have been grazed (irrespective of degree of grazing) and the average utilization level of the management unit. This relationship can be developed by visiting similar sites with different grazing intensities or by revisiting a single site multiple times during the grazing season. Another, independent method of determining utilization must be used to describe the relationship between the proportion of plants grazed and the actual percent utilization of the key species.

     

    Advantage
    • Field sampling is rapid once the regression relationship is established

     

    Limitations
    • Requires some ungrazed plants
    • Loses sensitivity when plants are grazed more than once, assumes that plants are not regrazed
    • The relationship developed is species-specific.

     

    Steps of the Grazed Plant Method
    1. At each sampling point on the transect, locate the nearest plant of the key species.
    2. Record whether the plant has been grazed or ungrazed. This determination can be adapted to rhizomatous species by examining tillers instead of entire plants.
    3. Measure 100 plants.
    4. Calculate the percentage of grazed plants, and estimate percent utilization based on the regression relationship previously developed for the site.

    Visual Rating

    This technique uses an ocular rating of utilization for an area of rangeland. The method is very useful for large-scale surveys of the degree of use that areas have received, and is used to map “use zones”. This method does not require individuals to estimate utilization, but to look for indicators of the relative amount of use on key forage plants compared to low value forage plants.  The degree of use is classified into one of 6 use classes based on written descriptions of the classes (See Table 1). Separate classes exist for herbaceous plant species and browse plants.

    Table 1.  Description of herbaceous use classes for the visual rating procedure. Adapted from Anderson and Currier 1978.

    Use Class Degree of Use (%) Description
    None 0-15 Very little or no use of key forage plants.
    Light 16-35 Key forage plants lightly to moderately used. Practically no use on low value forage plants.
    Safe 36-65 Key forage plants used about right for the season of grazing and sites involved. Some use of low value forage plants.
    Heavy 66-80 Key forage plants closely cropped. Low value forage plants generally being grazed. Trampling damage may be evident.
    Severe Over 80 Key forage plants grubbed. Low value forage plants carrying the grazing load and are closely cropped. Some trampling damage may be evident.

     

    Advantages of Visual Rating
    • Rapid assessment of use allows a few observers to cover large area
    • Classification into categories facilitates greater consistency between

     

    Limitations of Visual Rating
    • Estimates are only as good as the training and experience of the observer
    • Visual rating is a qualitative measurement with no way to assess accuracy

     

    Steps of Visual Rating Method

    At each sampling point along the transect, estimate the degree of use based on the descriptions from Table 1 for all vegetation in the immediate area. Record the data and calculate by multiplying the number of observations in each use class by the midpoints of the class intervals. Average utilization is obtained by totaling the products and dividing by the number of sampling points.


    Ocular Estimates

    Ocular estimation is a visual estimation of percentage by weight of forage removed for a key species. Ocular estimation can be done for individual plants of the key species or on all the plants of the key species within small quadrats. The approach to ocular estimation is similar to that used to estimate production, with the added step of estimating weight for various portions of the plants.

     

    Advantages
    • Rapid assessment of large areas
    • No disturbance of vegetation required
    • Reasonably accurate for individual plants or small quadrats

     

    Limitations
    • Accuracy cannot be determined
    • Requires intensive training for observers
    • May require exclosures, cages, or fenced areas as reference areas for training

     

    Steps in Ocular Estimation

    Training – examiners must first compare ocular estimates against actual weight values obtained by clipping and weighing. Training may require the use of cages or exclosures to ensure there is non-grazed forage available for comparison.

    1. Clip portions of individual plants (or all plants of the key species), to simulate foraging (sample A)
    2. Estimate the percentage of weight removed.
    3. Clip the current year’s growth of the remaining forage (sample B)
    4. Weigh sample A and B separately.
    5. Calculate percent use: (Sample A / (Sample A + Sample B)) * 100
    6. Compare estimates with actual forage removed. Continue training until observers become proficient.

     

    Sampling Process
    1. Record the percent utilization of the key species plant nearest each sampling point on the transect.
    2. If using a quadrat, estimate utilization of all key species within the quadrat.
    3. Calculate the average percent utilization by totaling all utilization estimates and dividing by the number of sampling points or quadrats observed.

    Residual Measurements

    While utilization measurements are focused on quantifying the amount of forage removed, residual measurements are focused on measuring the amount of vegetation remaining after grazing. Some managers believe this approach better reflects the fundamental reasons for rangeland management and monitoring because it addresses the ability of plants to regrow through remaining photosynthetic material, and the ability of vegetation to protect the soil surface and nutrient status. Residual biomass is easy to grasp because it is focused on the remaining vegetation instead of trying to estimate what is no longer there. Several methods used to estimate biomass can be used to estimate plant residue. We will present a method commonly used to estimate residual biomass, the stubble height method.


    Stubble Height

    This method is simply an average height of remaining vegetation following the grazing period. Its ease of use and accuracy are making it a well-accepted description of rangeland use. However, stubble height can only be meaningful if it is measured for specific plant communities that have developed stubble height standards. Application of the method to areas without these standards has led to much controversy with the method and it use for monitoring utilization. Application of stubble height in the absence of standard guidelines is basically an observer trying to estimate what portion of plants are no longer there by looking at what is left. This would be roughly equivalent to estimating how many cookies were on a plate at the start of a potluck dinner simply by observing how many are left on the plate at the end of the dinner.

     

    Advantage
    • Simple, quick and accurate measurements can be obtained with little training

     

    Limitations
    • Best suited for riparian areas as more research is needed before it can be reliably applied to upland plant communities
    • Often misused as a management goal instead of a monitoring activity by applying to areas where no developed stubble height guidelines exist

     

    Steps of Stubble Height Method

    At each sampling point along the transect the height of the key species nearest the point is measured. The reported value is the average stubble height of all plants measured on the transect.


Self-Check Activity

The following questions and activity are designed to test your knowledge and understainging of utilization concepts and data analysis.

1. Why do we select key areas that are representative of the larger management unit to set up monitoring points for grazing use?

2. Assume the growing season for an area is April 1 to October 1.  If we estimate utilization in July relative to the forage produced by that point in time, is this an estimation of relative use or utilization?