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The how's and why's of yield estimation

April 2016

The how's and why's of yield estimation

The current production year has been one of the most difficult in over thirty years with only some provinces and districts receiving rain critical to establishing a maize crop at the planned optimum planting window.

Early plantings in some provinces suffered enormous heat stress from the intermittent heat waves only to use up all the available moisture reserves. The possible final yield of the final crop estimate will only become clearer during March and April 2016 due to the mixed planting dates and loss of the mainly white maize production in the Free State.

It will be extremely useful to be able to assess you own maize crop so as to estimate final yield and possible income which extend from May to August 2016.

Reasons for making a yield determination

Predicting possible yields will enable you to define the harvesting plan, transport and storage capacity required. Certainty of yield will help in making the relevant decisions such as Safex marketing positions, your future financial position and being able to communicate with financial institutions or co-ops as to possible crop income to be expected. Pricing will be dependent on Safex futures and the Rand/Dollar exchange rate. Please consult Safex to see the current and future prices to be able to work out your possible crop income after a yield assessment has been completed.

White maize futures are trading at about R4 850/ton with yellow maize futures being derived from import costs or parity at R3 450 per ton. For some farmers a yield of 2 t/ha to 2,5 t/ha might be at breakeven or profitable levels.

Farmers who do manage to produce a crop of over 2,5 t/ha of white or yellow maize this season will do very well financially. In some cases the crop will be best turned into silage or grazed off if the estimated yield is deemed to be at a level that is not worth harvesting.

Production parameters

Many factors that influence production and critical plant growth stages, for the many cultivars available, will influence a yield estimation. This will be especially important in a dry hot year with very varying conditions of drought and then intermittent thunder showers that provided crop saving moisture and further development of the plants.

Variable climatic conditions can change the published benchmarks regarding days to physiological maturity or harvesting date for specific cultivars. The best policy is to critically examine the crop before and make an accurate estimation as possible depending on its actual stage of development.

Maize cultivars are available in South Africa that take from 105 to 145 days to physiological maturity and 155 to 180 days from planting to harvest. You should note when these benchmarks occur for the maize cultivars planted on your farm. Conservation tillage practices will extend these estimates by several days. Normal planting populations used for the new cultivars in dry land production vary from 15 to 20,000 plants per hectare for lower potential soils to 18 to 36,000 plants per hectare in higher potential soils.


During the later growth stage V12, which occurs about 42 days - 46 days after emergence the number of kernel rows, number of potential kernels, and the size of ear is determined. Moisture and heat stress at this stage will result in curtailed ear development and final yield. After this stage the development of the ear proceeds rapidly. At the first reproductive stage known as R1 the first silks are visible outside the husks and the actual kernel number and kernel size is determined. Moisture deficiencies at this stage can result in the loss of 7% of the potential yield per day. Kernel weight is determined at reproductive stage R6.

Assessing yield by measuring cob size
The main factors to be measured in determining yield are the number of ears per unit area, the kernels per ear and the average mass of the kernels. As can be noted above, keep in mind whether or not the crop was stressed at the critical points described. Cobs can best be examined at the earliest, at the soft dough stage to enable the number of rows and portion of rows that have been successfully pollinated and developed. Remember to cut cobs in half or carefully count the number of rows which can vary from between 8 and 20. The usual number would probably be between 12 and 16. Commercial maize with good cobs would average about 600 kernels.

If the crop is at physiological maturity a more accurate determination can be made.

Calculation steps
Count the number of cobs per 10 metres in many representative areas of the land under consideration, count the potential kernels that will harden to form harvestable grain in the small, medium and large cobs, using a mass of 0,28 grams per kernel work out the mass of the small, medium and large cobs. As a general guideline small cobs have a mass of about 120 grams, medium about 150 grams and large cobs about 180 grams. If you can preferably work out an accurate mass from the number of kernels per cob or alternatively estimate the average size of the cobs counted in the 10 metres or row.

Most maize would be planted in 0,92 or 0,75 metre rows or tram lines. There are 108 by 100 metre rows or 10 800 metres of plants in a hectare planted at 0,92 widths and 133 by 100 metre rows in 0,75 row widths.

Thus for every 10 metres of points counted to determine number of cobs the multiplication factor for our calculation will be 1 080. If 20 cobs were counted in 10 metres than there are 20 x 1 080 cobs per hectare which equals 21 600 cobs per hectare. At a medium cob mass of 150 grams there are thus 21 600 x 150 divided by 1 000 to show kilograms (kg’s) per ha (1 000 g/kg) divided by 1 000 (1 000 kg/ton) to show the tons per hectare of yield expected. The answer will be 3,24 t/ha. One can be conservative and take off 10% for over estimation and harvesting losses to arrive at a final yield rounded off estimation of 2,9 t/ha. As can be calculated 180 gram cobs would give a yield of about 3,5 t/ha.


Keep in mind any very dry or hot conditions experienced at critical growth stages that your maize crop experienced when making a considered and conservative estimate of potential yield prior to the actual harvesting of your crop.

Article submitted by a retired farmer.

Publication: April 2016

Section: Pula/Imvula