Measuring grass, particularly for estimating yield, is a common practice in agriculture and land management. One specific method for measuring grass quantity involves physically sampling an area and determining its dry matter content.
You can measure grass using a method that uses a quadrat and shears to determine the dry matter yield per unit area.
Method 1: Using a Quadrat and Shears
This technique provides a practical way to estimate the amount of grass available in a specific area, often a paddock. It focuses on calculating the Dry Matter (DM) weight, which is the weight of the grass after all moisture has been removed, as this is typically what is most valuable for feed or analysis.
Here's a breakdown of the steps involved:
- Select a Representative Area: Choose a spot within the paddock that accurately reflects the average grass cover you wish to measure.
- Place the Quadrat: A quadrat, specifically a 0.5m x 0.5m frame, is placed on the ground in the chosen area. This defines a specific, measurable section for sampling.
- Harvest the Grass: Using shears, carefully cut all the grass within the boundaries of the quadrat down to ground level or a consistent cutting height relevant to how the grass will be utilized (e.g., grazing height).
- Weigh the Sample: Collect the cut grass and weigh it. It's crucial to use a bag or container for this, and you must remember to subtract the weight of the empty bag to get the true weight of the fresh grass sample.
- Determine Dry Matter (DM): Freshly cut grass contains a significant amount of water. To find the dry matter weight, you need to know the grass's DM percentage. This is often determined by drying a subsample in an oven until all moisture is gone and comparing the dry weight to the fresh weight. The reference provides an example where the Grass DM% = 16% (0.16).
- Calculate DM Yield per Area: Use the fresh weight, the DM percentage, and the knowledge of how many quadrats of that size fit into a larger area (like a hectare) to calculate the DM yield per hectare (kg DM/ha).
Example Calculation (Based on Reference)
Let's walk through the calculation provided in the reference:
- Fresh Weight of Sample (after subtracting bag weight): 0.200 kg
- Grass Dry Matter Percentage (DM%): 16% or 0.16
- Area of Quadrat: 0.5m x 0.5m = 0.25 square meters (m²)
- Number of Quadrats per Hectare: A hectare is 10,000 m². Since the quadrat is 0.25 m², there are 10,000 / 0.25 = 40,000 quadrats in a hectare.
The calculation to find kg DM/ha is:
(Fresh Weight in kg) x (DM% as a decimal) x (Number of Quadrats per Hectare) = kg DM/ha
Using the values from the reference:
0.200 kg x 0.16 x 40,000 = 1,280 kg DM/ha
This calculation shows that, based on the sample taken from the 0.25 m² quadrat, the estimated dry matter yield for the entire hectare is 1,280 kilograms.
Summary Table: Quadrat Method Calculation
Measurement/Value | Example Value | Description |
---|---|---|
Fresh Weight of Sample (kg) | 0.200 | Weight of grass cut from quadrat, minus bag weight. |
Grass DM% (as decimal) | 0.16 | Percentage of the grass sample that is dry matter (16%). |
Number of Quadrats per Hectare | 40,000 | How many 0.5m x 0.5m quadrats fit into one hectare (10,000 m² / 0.25 m²). |
Calculated DM Yield (kg/ha) | 1,280 | Estimated dry matter yield per hectare. |
This method provides a standardized way to assess grass availability and inform decisions regarding grazing management, fertilization, or harvest timing.