How to measure forage crop yields

How you can undertake a quadrat test to identify DM% and estimate its impact on any farm.

What you’ll need

• 3.54m length of alkathene bent into a circle to create a 1m² hoop
• Alternatively use four 1m sticks joined with a hinge to create 1m²
• One empty PGG Wrightson Seeds grass seed bag
• Knife or machete
• Handheld scales (fish scales can hook into top of bag for weighing)

Method

Walk the paddock to assess variation as soil, topography, pest, weed and disease pressures will create variation in yield. Choose sampling areas representative of the paddock, avoid throwing randomly as this often picks bare areas or the tallest plants. 

Use the table below to determine the recommended number of quadrats or samples per paddock: 

Any crops sown in rows, measure a calculated row length to give a 1m² sample:

• 500mm row spacing, yield one 2m row length
• 600mm row spacing, yield one 1.7m row length 

Any precision drilled crops (e.g. swedes and fodder beet), yielding a 2m² area per-sample greatly increases accuracy. For 500mm spacing, yielding two rows side by side, 2m row length = 2m².

Harvest the plant material that falls within the quadrat. For bulb crops, pull the plants out of the ground and scrape off any dirt and weigh leaf and bulb components separately. For kales, forage rapes, leafy turnips and raphno, cut stems off at about 50-100 mm above ground level.

While weighing quadrats, collect smaller sub samples to submit for laboratory dry matter % (DM%) testing.

Dry matter sampling

When sending a sample away for a laboratory dry matter % testing, a minimum of 200g of sample is required.

For kales, rapes and raphno, a minimum of two plants (leaf and stem) representative of the paddock are required per sample.

For leafy turnip, swede or fodder beet tops, take half the leaves from 4 separate plants. For bulbs take 20 cores from representative plants in the paddock, if you don’t have a corer take a minimum of four bulb quarters.

Example calculations

Once sampling is completed and the DM% results have been received, follow the steps below to calculate crop yield: 

1. Calculate average sample weights per m². If sample area was 2m², divide by 2 to give kg/m².
   g. average of 10.6, 11.2, 10.8, 11.8, 11.6 = 11.2kg/m² 

3. Multiply by 10,000 to convert kg/m2 to kg/ha
   g. 11.2kg/m² x 10,000 = 112,000kg/ha 
4. Multiply by laboratory dry matter % test result
   g. If laboratory test result was 13%DM - 112,000kg/ha x 13% = 14,560 kgDM/ha

Impact of dry matter% on yield

Getting a laboratory dry matter test is highly recommended over using generic book values. As seen in the table below, a 1% change or error in DM% can have a significant effect on yield calculations - and ultimately crop allocations. Estimating DM% should be a last resort.