Methods for measuring forage yield

Visual estimation relies on the farmer’s experience to visually assess the amount of grass in the field and estimate the yield.

While less accurate than other methods, it can be useful for quick assessments or when no other method is available.

Equipment required: None (or pasture cover sheet).

Pros: Instant and free, no equipment needed.

Cons: Highly subjective and varies by operator.

How to measure:

1. Walk the field and assess the height, density and appearance of the sward
2. Based on your visual inspection, estimate the usable fresh or DM yield. Some farmers use their experience with typical yields in similar conditions

A sward stick is a tool used to measure the height of grass in each area. The method estimates grass yield based on sward height and a set of pre-established conversion factors.

Equipment required: Sward stick (order from AHDB).

Pros: Simple, good for grazed paddocks.

Cons: Subjective to species within ley.

How to measure:

1. Select the plot area: Choose a representative area in the field to measure.
2. Measure grass height: Use a sward stick to measure the height of the grass in the chosen area. Multiple readings across the field will increase accuracy.
3. Use the conversion factor: Based on the average height, use the conversion factor on the sward stick that translates sward height to biomass yield (based on regional or field-specific data).

Example calculation:

• Average grass height = 17 cm (target residual grass is 5 cm, therefore utilisable grass = 12 cm)
• Conversion factor = 250 kg of DM per cm of height
• Yield estimate = 12 cm × 250 kg = 3000 kg (DM) per hectare (3 t DM/ha)

This method involves counting the number of bales harvested from a field, estimating the average weight of each bale and calculating the total yield.

It can be adapted depending on whether the forage is conserved as silage, haylage or hay, as moisture content has a significant impact on both weight and DM yield.

Bale weight varies depending on moisture content, forage species and the machine used to harvest and pack the bales.

Measure the DM content of a sample of forage from the bales to get the most accurate estimate of DM yield. Bale size and type significantly affect your yield calculations because bale weights vary according to their dimensions, density and moisture content.

Equipment required: Bale counter, weight estimates and DM analysis.

Pros: Simple, practical for conserved forage and useful for feed budgeting.

Cons: Bale weights can vary. Only able to estimate yields post-harvest.

How to measure:

1. Accurately identify bale size and type.
2. Weigh bales or estimate bale weight.
3. Measure or estimate DM content.

Bale types and approximate fresh weights

*Based on a DM content of 30%

Example calculation:

45 large round silage bales, from a 4-ha field. Average weight 900 kg per bale. DM content 30%.

DM yield (kg/ha) = (Number of bales × average fresh weight (kg) × (DM % / 100)) / Field area (ha)

DM yield (kg/ha) = 45 × 900 × 0.30 = 12,150 kg

DM yield (kg/ha) = (12,150 / 4) = 3,037.5 kg DM/ha (3.0 t DM/ha)

Manual rising plate meters have a round plate that slides along a central shaft, resting on the top of the grass sward as it is placed vertically into the pasture.

The compressed height of the sward is recorded in centimetres, and this is converted to an estimate of DM yield (kg DM/ha) using a standard or farm-calibrated equation. Plate meters are suited to weekly pasture walks and grazing rotation planning.

Electronic pasture meters use ultrasonic or capacitance sensors combined with GPS mapping to automatically measure sward height and estimate DM yield across a field.

The device is walked or driven across the field, collecting real-time data points which are then processed through a paired app or software platform. The data is visualised on a digital map.

Equipment required: Plate meter (manual or digital), calibration chart or software.

Pros: Quick and easy, common for grazed paddocks, portable.

Cons: Accuracy is affected by sward species, density and calibration.

How to measure (manual plate):

1. Use a standard formula or calibrate your own based on grass species and growth stage. A typical conversion is: DM yield (kg/ha) = (Compressed height × 140) + 500.
2. Walk the field in a ‘W’ or zigzag pattern. Take 30–40 readings per field, pressing the plate vertically down until it rests on the sward. Record the compressed height in cm at each point (either manually or with a digital counter).
3. Add up all readings and divide by the number taken to get an average compressed height.
4. Apply the conversion formula to the average compressed height to calculate kg DM/ha.
5. Some plate meters will calculate an average for you and convert directly to kg DM/ha.

Example calculation:

Let’s say you take 40 readings, and the average compressed height is 15 cm.

Using the standard formula:

DM (kg DM/ha) = (Compressed height × 140) + 500

• (15 × 140) + 500
• = 2,100 + 500 = 2,600 kg DM/ha

So, the estimated cover is 2,600 kg DM/ha. The available yield (which excludes the residual) is 2,100 kg DM/ha.

DM yield (kg/ha) = (Compressed height × 140) + 500 is a standardised formula used. It produces an estimate – actual DM can vary depending on species, clover content, soil moisture, time of day, etc.

Compressed height reflects how much bulk forage is available, factoring in leafiness and sward density. The × 140 multiplier estimates the amount of DM per cm of compressed sward height (14%). The + 500 constant accounts for the residual DM (e.g. base tillers and stems).

How to measure (electronic plate):

1. Calibrate the device by entering the pasture type, species mix and typical DM% into the device or app. Optionally calibrate for specific field conditions (e.g. dense swards, clover-rich pastures).
2. Walk or tow the meter across the field in a W-shape or grid pattern to capture a representative sample. The device automatically logs sward height at each GPS point.
3. The software uses pre-programmed equations (based on height-to-DM relationships) to estimate:
   1. kg DM/ha at each GPS point
   2. Average yield across the paddock or field
4. Use the app to export results (Excel, PDF, etc.) for farm planning or reporting.

The cut-and-weigh method involves selecting a representative sample of grass, typically from a 1 m² area or similar, and cutting it to grazing or cutting height (typically 4–6 cm height). The fresh weight (FW) of the sample is then recorded.

This gives an estimate of fresh weight yield, which can be scaled up to tonnes per hectare and then converted to DM yield using the forage’s DM content.

Equipment required: Quadrat, shears or scissors, scales and method of drying (oven, microwave or air fryer).

Pros: Accurate and good for calibration.

Cons: Time consuming and impractical for routine use.

How to measure:

1. Define the area: Mark out a 1 m² plot in the field to harvest a representative sample.
2. Cut the grass: Use shears/scissors to cut the grass within the marked area to grazing or cutting height.
3. Collect all the cut grass in a bag.
4. Weigh the grass: Weigh the collected sample (fresh weight).
5. Calculate the yield (t/ha): Following the steps outlined below.

Example calculation:

• Weight of cut sample from 1 m² = 1.5 kg
• Field area = 1 hectares = 10,000 m²
• Number of 1 m² plots in the field = 10,000 m²/1 m² = 10,000 plots
• Total fresh yield = 1.5 kg × 10,000 m² /ha = 15,000 kg FW/ha or 1
• If DM content = 18%
• 15 t x 0.18 = 2.7 t DM/ha utilisable yield

Weighbridge involves weighing a vehicle (e.g. tractor and trailer) when full and empty to work out the weight of the harvested grass (either fresh or after baling). Fresh weight values can then be converted into dry matter. This is suited to silage fields, contracting jobs or total crop assessments.

Equipment required: Access to weighbridge, trailer counts and DM analysis.

Pros: Accurate for total yield, great for contractor jobs or large fields.

Cons: Requires weighbridge access, not suitable for grazing yield, only calculated after cutting.

How to measure:

1. Weigh the vehicle empty: Drive the empty vehicle onto the weighbridge and record the weight.
2. Weigh the vehicle full: After loading the vehicle with harvested grass (e.g. bales or chopped grass), drive the full vehicle onto the weighbridge and record the weight.
3. Calculate the grass yield: Subtract the empty weight from the full weight to determine the weight of the harvested grass.
4. Convert fresh weight to DM yield: To understand the actual feed value, fresh weight needs to be adjusted using the DM content.

Example calculation:

• Empty vehicle weight = 3,000 kg
• Full vehicle weight = 8,000 kg
• Grass yield = 8,000 kg – 3,000 kg = 5,000 kg fresh weight
• If DM content = 20%, then: 5,000 kg × 0.20 = 1,000 kg DM yield
• In this scenario, we had a 5 ha field, where 15 trailers loads came off, this means it had a yield of 15 t DM from the field, or 3 t DM/ha

Yield prediction modelling/software-based estimation

Yield prediction modelling uses historical data to predict the yield. Weather data, soil condition and historical growth rates are added into software which will then output a fresh weight or dry matter prediction for grass yield based on provided conditions.

Depending on the software, there may be decision support offered to the user. Accuracy of outputs will rely on the data going in and the quality of calibration.

Drone imagery and remote sensing

Drones with cameras or sensors and satellite imagery can be used to estimate biomass. Large areas can be covered, but progress is limited by poor weather.

The cost and complexity of this method means it is often limited to innovation or research farms, or larger commercial operations.