Determining Pasture Yield

Direct and indirect methods of determining pasture yield.
Determining Pasture Yield - Articles
Determining Pasture Yield

Assessing pasture dry matter (DM) yield is important in budgeting feed and making management decisions such as evaluating different pasture mixtures and stocking rates, estimating forage inventory, determining fertilizer needs and cost benefits, and calculating net return on investment. In contrast to cash crops, yield is rarely evaluated in pastures for three reasons:

  1. measuring yield on pastures is more difficult because the forage is not gathered;
  2. it is time consuming; and
  3. the potential of the pasture is sometimes neglected.

Many different methods have been developed to estimate the amount of forage in a pasture without collecting all the forage. These methods can be divided into direct and indirect methods. Direct methods involve collecting a sample of the pasture and then estimating yield, while indirect methods rely on one or more plant characteristics to provide yield estimation.

Direct Methods

Hand Clipping

This method involves clipping, drying, and weighing samples. The precision of this method depends largely on pasture variability and sampling efficiency. Hand clipping is precise but time consuming, which makes routine use impractical.

Indirect Methods

Many indirect methods have been developed to estimate yield. Among these methods, the pasture ruler and the rising plate meter are used most often. Both the pasture ruler and rising plate meter need to be calibrated to specific situations in which precision is important. The calibration methods for the pasture ruler and the rising plate meter are slightly different; both methodologies are simple but time consuming. Calibration procedures are explained in the "How to Calibrate Indirect Methods" box.

Pasture Ruler

This method works on the principle of relating plant height to yield. Therefore, you need to calculate the average pasture height. First, walk through the paddock in a W pattern and record the plant height at a 25-footstep interval (Figure 1). It is important to take the measurement at the same space interval regardless of the spot. The height of both bare spots and dense (manure effect) spots must be recorded because avoiding spots will lead to a biased average height value and a miscalculated yield. Once the height data are collected, simply average the height (sum of all heights recorded divided by the number of samples). In Pennsylvania, DM accumulation averages about 300 pounds per inch of plant height. To convert the average plant height to yield, simply multiply it by 300 pounds DM per inch. While the pasture ruler method is fast, simple, and cheap, it is not as precise as hand clipping.

Rising Plate Meter

This method relies on both plant height and density, which are combined into one measurement referred to as "bulk density." To estimate yield with the rising plate meter, you follow a procedure similar to that of the pasture ruler. Before data collection, set the sample counter to zero. Then record the initial rising plate value on the accumulative pasture meter. Next, follow a W pattern in the pasture to estimate the average plate meter. At a fixed interval of 25 footsteps (see Figure 1), push the pasture meter vertically into the sward while clicking the sample counter once. The rising plate reading must be taken at the set pace interval to avoid bias in the plate meter estimation. Repeat this operation at each sampling point and collect at least 20 rising plate readings regardless of the paddock size. When you are finished collecting the rising plate values, record the final plate number. Calculate the average plate meter by subtracting the initial value from the final value and dividing by the number on the sample counter (number of times the plate meter was pushed into the sward). This average plate meter is correlated with forage bulk density and then converted to yield using a calibration equation. The rising plate meter method is more precise than the pasture ruler, but requires a greater investment in both time and money.

  • The accumulative pasture meter is connected to the rod by a gear.
  • The weighted disc slides on the rod as it is gently lowered into the sward.
  • The rod makes the accumulative pasture counter spin by turning the gear as the rod is pushed into the sward.
  • When the rod is pushed into the forage, the plants need to exert enough force to sustain the disc.
  • The greater the pasture meter number, the greater the yield.
  • The pasture meter number correlates with the forage bulk density.

How to Calibrate Indirect Methods

Pasture Ruler

Necessary Elements

  • Hand/electric shear
  • Ruler
  • Pasture ruler
  • Big cloth or plastic bag
  • Scale

Procedure

  1. Walk through the paddock in a W pattern.
  2. At a fixed interval--20 to 35 footsteps--collect a hand clipping to ground level in an area of 3 feet by the shear width.
  3. Insert clippings in the bag.
  4. Measure and record sward height.
  5. Repeat steps 2 to 4 throughout the W pattern. If paddock is small, at least 15 samples are needed.
  6. Weigh the bag. This is the total wet weight (TWw).
  7. Obtain an approximately one-pound subsample from the bag.
  8. Weigh the subsample. This is the wet weight (SubWw).
  9. Dry subsample. (For information on how to dry forage, refer to fact sheet I-106: Determining Forage Moisture Content with a Microwave Oven.)
  10. Weigh subsample. This is the dry weight (SubWd).
  11. Calculate the percent DM content (%DM) of subsample: %DM = SubWd ÷ SubWw x 100
  12. Obtain total DM: Total DM = TWw x %DM.
  13. Calculate the clipped area: Shear width (ft) x 3 feet x number of samples ÷ 43,560
  14. Calculate pounds of DM per acre (DM/A): Total DM ÷ clipped area
  15. Calculate average pasture height: Sum of individual heights ÷ number of samples
  16. Calculate average pounds of DM per plant height: step 14 ÷ step 15

Rising Plate Meter

Necessary Elements

  • Hand/electric shear
  • Ruler
  • Rising plate meter
  • 15-20 bags labeled with numbers
  • Scale

Procedure

  1. Walk through the paddock in a W pattern.
  2. At a fixed interval--20 to 35 footsteps--collect a hand clipping to ground level in an area of 3 feet by the shear width.
  3. Insert clippings in individual bags and label the bags according to the order in which they were collected.
  4. Push down the rising plate meter and record the accumulative pasture number.
  5. Repeat steps 2 to 4 throughout the W pattern. If paddock is small, at least 15 samples are needed.
  6. Weigh each bag. This is the bag wet weight (BagWw).
  7. Randomly select three bags.
  8. Weigh selected bags. This is the wet weight (SubWw).
  9. Dry selected bags. (For information on how to dry forage, refer to fact sheet I-106: Determining Forage Moisture Content with a Microwave Oven.)
  10. Weigh selected bags. This is the dry weight (SubWd).
  11. Calculate percent DM content (%DM) of selected bags: SubWd ÷ SubWw x 100
  12. Calculate average %DM.
  13. Obtain bag DM for each bag: BagDM = BagWw x average %DM
  14. Calculate the clipped area: Shear width (ft) x 3 feet ÷ 43,560
  15. Calculate pounds of DM per acre (DM/A): BagDM ÷ clipped area
  16. Download this chart and follow instructions (mouse over the red marks on cells to see instructions).

Examples

To estimate how much forage is consumed by the grazing animals, you need to estimate the DM before and after each grazing event. Subtracting the post-grazing DM from the pre-grazing DM will give the actual forage consumed.

Pasture Ruler

Table 1. Pasture ruler.
Pre-grazingPost-grazing
Sample Number>Plant Height (inches)Sample NumberPlant Height (inches)
1511.5
21022
31235
4743
5652
61163
71373.5
8982
96.591.5
109102
1110112.5
128121
1311133.5
147142
1512153
1613164
177.5172
189181.5
195.5191
209202
Sum = 180.5 inchesSum = 48 inches
Average height = 180.5 / 20 = 9.025Average height = 48 / 20 = 2.4
Pre-grazing yield = 9.0 * 300 lb DM/in/A = 2,700 lbs DM/APost-grazing yield = 2.4 * 300 lbs DM/in/A = 720 lbs DM/A

Total forage consumed = Pre-grazing DM - Post-grazing DM

Total forage consumed = 2,700 lbs DM/A - 720 lbs DM/A = 1,980 lbs DM/A

Thus, 1,980 pounds DM per acre were consumed during this grazing event.

Rising Plate Meter

Table 2. Rising plate.
Pre-grazingPost-grazing
Initial Plate Meter57,89360,245
Final Plate Meter58,94360,670
Counter2525
Final - Initial58,943 - 57,893 = 1,05060,670 - 60,245 = 425
Average RPM1,050 / 25 = 42425 / 25 = 17
Yield*42 * 69 - 560 = 2,338 lbs DM/A17 8 69 - 560 = 613 lbs DM/A

*This equation was developed for grass-based pastures (less than 30 percent legumes).

Total forage consumed = Pre-grazing DM - Post-grazing DM

Total forage consumed = 2,338 lbs DM/A - 613 lbs DM/A = 1,725 lbs DM/A

Thus, 1,725 lb DM/A were consumed during this grazing event.

Recommendation

The equations used in this fact sheet were calibrated to be robust enough to adjust to most situations encountered in Pennsylvania pastures. However, to increase precision, both the pasture ruler and the rising plate meter should be calibrated to the particular situation in which they are intended to be used. The rising plate meter will underestimate yield in pastures covered by heavy dew, pastures in flowering stage, and extremely tall pastures (more than 14 inches tall). Accurate DM determination will aid in the decision-making process of pasture management.

Prepared by Marvin Hall, professor of forage management, and Atila Deak, Ph.D. candidate in agronomy. The authors would like to thank peer reviewers Greg Roth, professor of agronomy, and Mena Hautau, extension educator in Berks County, for their recommendations.

Authors

Forage Crop Establishment, Management and Utilization Perennial Warm-Season Grass Establishment and Management

More by Marvin Hall, Ph.D.