EVALUATION OF A SIMPLE METHOD
FOR MEASURING PASTURE YIELD1
Dennis Cosgrove - Extension Forage Agronomist - UW- River Falls
Dan Undersander -Extension Forage Agronomist - UW- Madison
Knowledge of the amount of forage available to grazing livestock is critical in determining stocking rates. A number of methods exist aimed at determining available forage. Actual clipping and weighing of forage from a given area is the most accurate method but is time consuming, requiring drying and weighing of clipped forage. Pasture probes are available which very rapidly measure available forage by simply placing the tip of the probe below the canopy. These tools are expensive however and not widely utilized in Wisconsin. Metal disc meters have also been developed which measures height of forage depressed with a weighted plate thereby taking density into account and improving accuracy . These are also relatively expensive and cumbersome to use. Less expensive methods include simply measuring height of existing forage or measuring both height and density using calibrated rulers. While these methods have been used with varying success in many states, few have been investigated for accuracy in Wisconsin.
Rayburn reported research on the use of an acrylic pasture plate for estimating pasture dry matter yields (1). This plate has been used extensively to estimate yield of cool season grass pastures in West Virginia. Based on the results of a study conducted in 1997, Rayburn developed the following regression equation for estimating dry matter yield using the pasture plate:
Dry Matter Yield (LB./A) = 432 * Height (In.)
The regression coefficient (R2) was 0.78 with a standard error of 322 lb/a.
These plates are inexpensive and easy to use and, based on the above data, appear accurate in the area in which they have been tested. The goal of the current study was to determine if this method of estimating pasture yield was valid in Wisconsin.
Materials and Methods
The design of the pasture plate is shown in Fig. 1. The plate is made from 0.22 inch acrylic plastic sheeting cut in an 18-inch square. A 1.5-inch hole is cut in the center of the plate. A yardstick is used to measure the height of the plate above the ground when it is set on the sward. In addition, 24-0.125 in. holes may be drilled along five lines set at 3-inch intervals. These holes can be used for estimating ground cover. These holes were not drilled in the plates used in this study. The yardstick is connected to the plate by the use of a string tied at each corner and looped so the yardstick and plate can be carried as one unit. The unit is used by placing it gently on the forage until it supports the plate. The height of the plate's top above the ground is then measured.
In order to test the plate in several different environments, locations were selected in 7 Wisconsin counties: Fond du Lac, Marinette, Price, Polk, Buffalo, St. Croix and Waupaca. Locations consisted of pastures which were part of a well managed, rotational grazing system. A uniform 30-ft. by 50-ft. area was selected in each pasture. Heights were measured and samples clipped from the area beneath the plate in five places once during the growing season in order to calibrate the plate. These samples were taken by constructing a wire frame that just fits over the plate. The frame was set over the pasture plate and the plate was removed. The forage was then separated so the frame was in contract with the ground. Forage was then clipped as close to the ground as possible. The clipped forage was dried and weighed.
Species included in this study ranged from some pure grass pastures (bromegrass, orchardgrass or bluegrass) to mixed cool season legume/grass pastures. Legumes included mainly alfalfa, white clover and red clover.
Rayburn states that a logical model for rotationally grazed pastures with short residual height is a calibration equation which passes through zero. In pastures with significant residue a regression model using a Y intercept is most appropriate. As all of the pastures in this study were under rotational grazing a 0 intercept model was used. Results of the plate calibration are shown in Table 1. The equation generated from this data is:
Dry Matter Yield (LB/A) = 390 * Height (IN)
In other words, every inch of growth represents 390 lbs of forage dry matter. This value agrees well with Rayburn's which predicts 432 lbs dry matter per inch of growth. The correlation coefficient is 0.96 indicating a significant relationship between pasture height as measured by the pasture plate and dry matter yield
Table 1. Regression of pasture height and dry matter yields from cool-season, grass-legume pastures in Wisconsin.
X Coefficient 390
Y Constant 0
No. of Observations 70
The use of a pasture plate for estimating forage dry matter yield appears to a be a reliable method in the northern and central parts of Wisconsin. Additional data is being collected to investigated the reliability in southern Wisconsin.
1The authors would like to acknowledge the assistance of Greg Blonde ,Waupaca County Crops Agent, Mike Rankin, Fond du Lac County Crops Agent, Scott Reuss, Marinette/Oconto County Crops Agent, Mark Kopecky, Price County Crops Agent, Carl Duley, Buffalo County Crops Agent and Tim Jergensen, Polk County Crops Agent as well as that of all the farmers cooperators in these counties.
1. Rayburn, E.B. 1997. An acrylic pasture weight plate for estimating forage yield. West Virginia University Extension Service. www.caf.wvu.edu/~forage/pastplate.htm