Calculating harvesting losses

Page last updated: Tuesday, 5 April 2022 - 7:16am

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Serious grain losses at harvest reduce the profitability of crops. Grain can be lost at a number of places during harvest. The loss should be assessed at each place so that the corrective action can be taken.

When adjusting a harvester to reduce losses, make only one adjustment as a time, checking the losses after each change.

Where losses occur

Grain can be lost:

  • pre-harvest, due to natural shedding or weather damage
  • at the front, due to front type (open front or modified knife guards) or set-up
  • from the threshing system of the machine, due to the drum or rotor, concave, wind and sieve settings.

All these losses may be reduced by resetting the machine and changing the harvesting technique.

Assessing losses using a drop tray

Drop trays or drop pans are the fastest and easiest way to measure machine losses. A magnet holds a tray on the header and once the header is operating normally a button is pressed to release the magnet and the tray falls to the ground. The tray can be placed under the rear axle to measure through the machine losses (Figure 1) or behind the side of the front to catch rotor losses.

The tray is emptied into a battery powered fan cleaner and the grain weighed and the weight can be added to a phone app to calculate the losses.

There are several companies making drop trays. Make sure the release system is reliable or time can be wasted in the field, usually in hot weather. The cleaning system also needs to have a good fully charged battery as several assessments in bulky crops like canola can use a lot of fan time. Over-threshing of canola in hot weather should be avoided by harvesting in the cooler part of the day or changing machine settings as broken up pod makes cleaning more time consuming.

Figure 1 Example of a drop tray mounted on a header rear axle. The tray is shown in the released position

Assessing losses without a drop tray

Follow these steps to check all the loss areas without starting and stopping the machine several times.

  1. Harvest a typical area.
  2. Allow the machine to clear itself of material.
  3. Back the harvester about 10 metres and stop the machine.
  4. Sample grain losses in each of the three areas (pre-harvest, front and machine) with at least 10 samples in each area. Average the 10 samples in each area.

Sampling with a quadrat

Use a sampling quadrat with an area of 0.1 m2 (see Figure 2). Count the number of seeds lying within each quadrat.

Quadrats can be made from any material, for example, round bar or flat mild steel. For measuring pre-harvest losses, use a three-sided quadrat.

Line drawing of quadrats used for recording seed loss
Figure 2 Four and three-sided quadrats with areas of 0.1m2

The number of seeds in a quadrat of this size which correspond to a loss of 100 kilograms per hectare for different crops can vary by plus or minus 20%. Table 1 is a useful guide.

Table 1 Average seed number per 0.1 m2 for 100 kg/ha loss
Crop type Average seed number
Wheat 27
Barley 23
Oats 29
Lupins 7
Chickpeas (desi) 6.5
Lentils, red 33
Lentils, green 28
Yellow lupins 7.5
Field peas 5
Faba beans 2.5
Chickpeas (kabuli) 2
Canola * 267

*For canola, an easier measure is 60 seeds in the area of a square 2L ice cream container (0.022 m2).

Sampling a canola crop

The easiest method of assessing the losses in canola is to use square 2L ice cream containers. Put eight containers in the uncut crop in front of the harvester, four either side of where the harvester will go, to measure the front losses and four in the middle of the machine, to measure the front plus machine losses. Then harvest the crop over the top of where the containers are and count the number of seeds in each container. Remember the concentrating factor when you calculate machine losses (see Figure 2). For example, if the quadrat is behind the sieves and the sieves are 1.5m wide and the front is 9m wide then the losses are 1.5/9 times the measured quadrat seed number.

If harvesting swathed crops and the swath is formed by a 11m wide swather and the sieves are 1.5m wide then the losses are 1.5/11 times the seed number in the quadrat behind the sieves.

Pre-harvest loss

Sample Area A (Figure 3) for pre-harvest loss, that is, losses before harvest due to natural shedding. Take at least ten samples from an area 2-3 metres into the standing crop, using the three-sided quadrat. Count the seeds in each quadrat, including any seeds in pods or heads on the ground and find the average number of seeds per quadrat, then use the following method to calculate pre-harvest loss.

Pre-harvest loss (kg/ha) = (average number of seeds per quadrat divided by the corresponding crop type average seed number from Table 1) times one hundred.

Worked example for an average of five lupin seeds per quadrat in Area A:

Pre-harvest loss = (5/7) x 100 = 71 kg/ha

Front loss

Sample Area B (Figure 3) to estimate front loss, plus pre-harvest losses. Take at least 10 samples across the full width of the machine to within 0.5m of the edge of the cut.

Front loss (kg/ha) = [(average seeds per quadrat divided by the corresponding crop type average seed number from Table 1) minus pre-harvest losses]

Worked example for an average of 10 lupin seeds per quadrat in Area B:

Front loss = [(10/7) x 100] - 71 = 72 kg/ha

Machine loss

Sample Area C (Figure 3) to estimate machine loss, plus front loss, plus pre-harvest loss.

Line drawing of sampling places for estimating losses
Figure 3 Sampling places for estimating pre-harvest (A), front (B), and machine losses (C)

The machine losses may be concentrated in a stream, that is, the width of the sieves if there is no chaff spreader, so the losses must be related to the width of the cut.

Take at least 10 samples across the full width of the sieves. First, carefully take the straw off the header trail, then count the seeds, including the seeds in the unthreshed heads.

Machine loss (kg/ha) = [Average number of seeds per quadrat x 100] divided by [the corresponding crop type average seed number from Table 1] minus [front loss - preharvest loss] multiplied by [sieve width divided by width of cut]

Worked example, for an average of 22 lupin seeds per quadrat, with sieves 1.5 metres wide, 9 metre wide cut and using 0.1 m2 quadrat:

Machine loss = [(22 x 100 ÷ 7) - 71 - 72] x (1.5 ÷ 9) = 28 kg/ha

If the harvester has a chaff and straw spreader, it is more difficult to measure the machine loss. Measure the average number of seeds in equally spaced quadrats across the full machine width after the spreader has passed over and subtract the front loss and the pre-harvest loss.

Worked example, if an average of 12 seeds per quadrat is measured across the harvester width, the machine loss is:

Machine loss = (12 x 100 ÷ 7) - 71 - 72  = 28 kg/ha

Total harvest loss

Total harvest loss = pre-harvest + front loss + machine loss.

Based on the above example;

Total harvest loss = 71 + 72 + 28 = 171 kg/ha

Acceptable losses

Generally, harvest losses range from 0 to 3% of the yield in cereal crops and from 5 to 20% in lupins. What is acceptable varies with the operator; some are not concerned with losses of 3% in wheat and 20% in lupins but others do not want any losses.

If a contractor is being used, an agreement should be reached on acceptable losses before the crop is harvested and losses should be checked at the start of harvest. For example, if a lupin harvesting contractor does not have any header improvements, the losses may be expected to be at the high end of the loss range.

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