Estimating Crop Yields and Crop Losses

Note Number: AG0104
Published: January 1998
Updated: May 2012

Accurate, early estimations of grain yield and crop loss are important skills in grain  production. Farmers require accurate estimates for:

• crop insurance purposes
• forward marketing and delivery planning
• planning harvest and storage requirements
• cash-flow budgeting.

Extensive personal  experience is essential for estimating yields at early stages of growth. As crops near maturity, it becomes easier to estimate yields with greater  accuracy.

Estimation methods

A simple but  accurate formula for estimating cereal grain yield is based on the number of heads per 500 mm of drill row, the number of grains per head and the size of the grain.

Formula for estimating grain yield

   Average number   X    Average number of heads
     of grains per head         per 500 mm of row                    = tonnes/hectare
                        Known constant (K)

The "known constant"  is that number of grains in the half metre of row at 175 mm row spacing that is  equivalent to 1 tonne per hectare.

It is more difficult  to count the number of grains per 500 mm of row for trailing crops such as  field peas and lentils, but if managed, the average number of grains per 500 mm  of row divided by the appropriate known constant will provide a yield estimate.   Alternatively, you can use the formula  for estimating losses, by counting the number of grains of these crops per 0.1  sq m and divide by the known constant "X" from table 3. This will also give a  yield estimate.

The value of the  "known constant" varies according to the grain weight, which differs for each  type of crop. Even within the same crop it may be necessary to adjust the  "known constant" to compensate for a heavier or lighter grain weight. For  example, in seasons of heavy rust infection the "known constant" for wheat is  generally decreased to compensate for lighter grain weights.

A range of "known  constants" for different grain weights is shown in Table 1.
  Estimation accuracy,  regardless of method, depends on the accuracy of observations taken in the  field. Counts of grains per head and heads per length of row must be accurate  and taken randomly at enough locations (at least 10) to provide an average  count representative of the whole field.A length of steel rod or  light timber, cut or clearly marked in half-metre segments, is a useful  measuring aid. Another useful aid is a pre-ruled form for recording of counts.  This is used for calculation and a permanent record of the yield estimate.

Table 1. "Known constants" for various grain weights

Note: The "known constant" (K) is the number of grains per 500 mm of row that is equivalent to a yield of 1 tonne per hectare at 175 mm row spacing.

Compensation for row spacing

The calculations used  in these formulas assume there is row spacing of 175 mm. As there is a range of  row spacings in the different forms of modern sowing equipment, it is necessary  to allow for row spacing when estimating yield by the head and grain count  method.

The most convenient  procedure is to carry out a yield estimation according to the Formula for  estimating grain yield (175-mm row spacing) and then multiply the result by one  of the conversion factors in table 2 which adjusts yield estimates for  different row spacings.

Table 2. Conversion factors that adjust yield estimates for different row spacings

Row counts are not  practical for broad leaf crops which branch or sprawl. Yield estimates for such  crops are more easily taken on a seed per unit area basis (usually 0.1 square  metre).

The information on  the assessment of crop loss and table 3 is equally applicable to yield  estimates of broad leaf crops. Instead of counting seeds on the ground, the  seed is rubbed out of the standing heads and pods within an area of 0.1 square  metres.

Assessment of crop loss

Having estimated  potential yield for a given area of crop it is often necessary to assess grain  losses. These could be the result of environmental factors (hail or wind) or  mechanical factors at harvest.

In the case of hail  damage it is often appropriate to substitute an estimate of the average grains  per head missing for grains per head in the calculation outlined above. This  would produce an estimate of the loss due to hail.

Losses that are the  result of other factors are more appropriately estimated by the number of  grains per unit area spilt on the ground.

Formula for estimating yield loss

Grain count/unit area
Yield loss (t/ha) = Known constant (X)

Where "X" is the  number of grains per unit area equivalent to 1.0 tonne/ha (see table 3)

As is the case in the  formula for estimating grain yield, the known constant will be different as the  grain weight varies.  

Table 3 provides  values for known constant "X" for a range of 100-grain weights. These  values are recorded for a unit area of 0.1 square metre; it is usually  impracticable to count larger areas unless grain loss is very slight.

A simple measure of  0.1 square metre can be formed from a square of light steel rod or square  tubing with inside measurements of 316 by 316 mm. A fully formed quadrat may  prove difficult to place in the crop: an "L"-shaped device may be easier to  use, with the missing sides represented by imaginary lines.

Similarly to yield  estimation, you should take a number of random counts that are representative  of the loss problem and use an average figure in the final calculation. Again,  a pre-ruled page for recording counts and calculation is a valuable aid.

Example 1

After a number of  counts the average number of wheat grains on the ground in a standing crop was  recorded as 147 per 0.1 square metre.

We already know that  wheat usually has a 100-grain weight of 3.4 grams. The known constant  "X" for this particular calculation is therefore 294 (from table 3).

Yield loss (t/ha) = Grain count/unit area = 147 = 0.5 t/ha
                              Known constant "X"      294

Estimates of header losses

The formula for  estimating crop loss can also be used to estimate losses at harvest as an aid  to correct header adjustments.

Losses due to  environmental factors will have occurred before the header passes and should be  subtracted from machine losses. Machine losses can occur at the front of the  machine (gathering losses) and behind the machine (walker losses).
  Gathering losses can  be assessed by stopping the header and backing it up to expose the cut stubble  before the walkers have passed over. Gathering losses are the grains under the  header minus the environmental losses. "Walker"  losses are the total losses behind the straw walkers minus environmental and  gathering losses divided by a factor to account for the walkers being narrower  than the full width of the machine.

This factor is equal  to the cutter bar width divided by the walker outlet width. The straw spreader  (where fitted) should be disconnected during this assessment.

The following example  will clarify this procedure:

Example 2

A count of grain  loss due to environmental causes of 147 grains per 0.1 square metre was  established in example 1. A count of 162 grains per 0.1 square metre was  recorded "under" the header.
 
  The gathering losses are:

Gathering loss = 162 – 147 = 15 grains/0.1 sq m

Gathering yield loss =  15 = 0.051 t/ha
                                   294

The header had a  comb width of 7.2 m. After it had passed, the average number of grains on the  ground behind the 1.5 m straw walkers was 234.

Therefore walker  losses

         = 234 minus 162 divided by width factor = 72 divided by 7.2 m = 72 divided by 4.8
                                                                                                1.5 m
         = 15 grains/0.1 sq m

Therefore walker yield loss = 15 = 0.05 t/ha
                                              294

Total yield loss is therefore: Environment losses 0.5 t/ha + Gathering losses 0.05 t/ha + Walker losses 0.05 t/ha = 0.60 t/ha

Table 3. Values of known constant "X" for various 100-grain weights

Note: The "known constant" (X) is the number of grains per 0.1 square metre that is equivalent to a yield of 1 tonne per hectare.

*Faba bean weight can vary from 35g per 100 grains to 70g per 100 grains

Record of grain yield (example form)

Name..............................................................................
Date................................................................................
Crop type........................................................................
Variety............................................................................
Location..........................................................................
Anticipated 100-grain weight..................................grams
Therefore K = ................................................................

Calculation

Yield (t/ha) = (H) .................................................... X(GH)......................................................................... 
                                                                               "K" ....................................................................
Correction for row spacing

Yield (t/ha) =   Above estimate X conversion factor (Table 2)
                  = ..................................X ...................................      =    .......................................... t/ha