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Sunflower seed rates and spacing, can growers adjust their planting specifications to improve returns?

April 23, 2019

Plant seed and you harvest a crop, plant more seed and you get more yield, plant too much seed and yield will start to decline.

This article looks at some of the interactions and mechanics of seed rates and suggests ways to improving and maintaining optimum yields.

Seed rates

Seed is an expensive input, sunflowers seed represents about 30% of your variable costs, so using the right seed rate matters.

Get your seed rate right and you’ll gain through;

  • optimising your yield
  • meeting your product quality target
  • being cost efficient with your inputs
  • avoiding harvest complications

But there are ‘Field Factors’ that might make an adjustment worthwhile, such as;

  • the specific soil conditions for your site
  • predictions about the upcoming seasonal weather
  • early or late sowing

Nuseed will be able to suggest a suitable seed rate for your chosen variety, based on trial data for varietal characteristics, product market specification and your growing region and climate, but it’s useful to understand the principles behind seed rates.

Understanding seed rates

There are basically two ways of determining how much seed to plant, either by numbers or by weight.

Planting by numbers is used when planting seeds using a precision planter, where each seed is placed equal distance apart.

Planting by weight is more appropriate when planting with a metred unit where seed is trickled or blown down the coulter.

Either way the first issue to decide is what plant population or Target Plant Population, do you want to establish?

Target Plant Populations

The Target Plant Population, usually given as the number of plants per hectare or per square meter, is the final number of plants that are established and growing in the field and is important for optimal economic yield and quality.

Nuseed trials have established the final plant population that will give you the optimum economic seed quality and yield for a given variety.

As plant population increases competition between plants will increase and individual yield per plant will decrease.

However, yield per hectare will increase because the community of plants is able to make more efficient use of the resources.

This is particularly important for the ability of the crop canopy to capture light.

As the population increases the yield response starts to diminish and no further response to population can be achieved.

Plant density affects the partitioning of assimilates, with higher densities tending to give a lower reproductive or storage component per plant.

In sunflowers it’s the reproductive components we want as the marketable product.  This makes achieving the optimum plant population very important.  Go over the optimum plant population and you will reduce seed size.  Go under, and you will lose valuable consistency in head size.

And if the adverse effect of partitioning away from the economic component is not outweighed by the advantage of more plants per hectare then yield will also decline beyond a certain point.

Other effects of a dense crop to consider are;

  • the advantage of synchronising and accelerating ripening – lower plant populations will increase head size and large heads ripen more slowly
  • the disadvantage of high competition between plants weakening stems or roots – important in exposed or droughty areas where strong stems and effective roots will be crucial


Not all seeds planted will result in plants, a small percentage will fail to germinate because the seed is damaged or is infertile or is diseased.

Commercial seed has a guaranteed high germination percentage and it is a false economy to sow anything but seed of high germination and vigour, such as seed supplied by Nuseed.

It’s also worth checking the date on the seed bag as the germination percentage will start to drop for old seed, particularly if the seed has been treated.

Field Factor

This is where seeds that have been planted fail to develop into plants because soil conditions were unsuitable, too dry or too wet, too cold and so on.

To some extent The Field Factor depends on your judgement and expertise, you are estimating the allowance you must make for field losses due to adverse seedbed conditions.

If conditions are poor and you expect only 50% of the seeds that germinate to establish, your Field Factor is 0.5.

If conditions are good, and you expect 90% of germinated seeds to establish then your Field Factor is 0.9.

Plant population becomes more critical with later sowing in spring sown crops.

Increasing the seed rate can have the advantage of achieving earlier 100% ground cover, followed by a quick rise in Leaf Area Index, and the opportunity to absorb as much sunlight energy as quickly as possible.

If you’re sowing late this can be very useful.

Nuseed will advise a seed rate, but remember, for best results you should apply your understanding of your site and season to adjust this according to soil conditions.

Available moisture will be particularly important.

Calculating seed rates by weight

Seed rates in kilograms per hectare can be calculated using this formula:

Seed rate (kg/ha) = Target Plant Population (no./m2) × TSW (g)
Germination (%) × Field Factor

Target Plant Population

Your Nuseed Fact Sheet will give you the target plant population for your chosen variety.

  • For confection varieties it is often between 40,000 and 55,000/ha, so that’s 4.0-5.5 plants/m2.
  • For oil varieties it may be higher, at 55,000 to 65,000/ha, or 5.5 – 6.5/m2.

Thousand Seed Weight (TSW)

Varieties, and seed lots within a variety, vary in their seed weight so the seed rate must be calculated for the seed lot you are using, this will be quoted in grams.


Nuseed varieties are sold with a guaranteed minimum germination of 85%.

Field Factor

Normally, you can expect 10% losses in good conditions, that’s a Field Factor of 0.9; in less good conditions you might lose 20% during establishment, that’s a Field Factor of 0.8.

Look carefully at the Seed Rate table for your variety and make adjustment for the establishment losses you can expect as a result of your unique site and season.

Calculating seed rates by numbers

Once you have determined your seeding rate by taking the Target Plant Population and factoring in germination and your Field Factor, you will need to calculate the in-row spacing.

There are several ways to calculate the in-row spacing but the easiest is to calculate the length of one-hectare row and divide the target plant population into it.

The length of one-hectare row is one hectare expressed as 10,000 m2, divided by the width of one row (e.g. 75cm).

  • For example, 10,000m ÷ 0.75m = 13,333m
  • The length of one-hectare row, in this case, is 13,333m
  • Then divide the length of one-hectare row by the target plant population (e.g. 60,000 per hectare).
  • For example, 13,333m ÷ 60,000 = 0.22m or 22cm.

This means that if you set up your planter to place seeds 22cm apart on 75cm row widths you will achieve a seeding population of 60,000 seeds per hectare.

Plant Spacing

Plant spacing is generally determined and fixed by the combine header and planters are set up to fit with that configuration.

Consequently, we see many sunflower planters set up on 70-75cm row widths.

With typical 50-60,000 plant population, that means plants are 22-26cm apart down the row but 75cm across the row.

Ideally plants would be equal distance apart from its neighbours giving each plant the same amount of space in which to grow.

The table below shows the in-row seed spacing for a range of row widths and plant populations and highlights configurations that gives the closest to equal amount of space between each plant.

This could be achieved with planters set up on 40-55cm row widths, too close for a precision row crop planter (there’s a limit to how close you can get the seeding units together), but achievable with conventional type grain planters.

Row width (cm)
30 40 45 55 75 90
Plants/ha Seed spacing within row (cm)
40,000 83.3 62.5 55.6 45.5 33.3 27.8
45,000 74.1 55.6 49.4 40.4 29.6 24.7
50,000 66.7 50.0 44.4 36.4 26.7 22.2
55,000 60.6 45.5 40.4 33.1 24.2 20.2
60,000 55.6 41.7 37.0 30.3 22.2 18.5
65,000 51.3 38.5 34.2 28.0 20.5 17.1


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