Why use this calculator
Plant density, not kilograms, is what drives yield. Two seed lots of the same variety can differ by a third in seed size, so sowing the same kilograms per hectare out of habit gives you a thin crop one year and a crop that runs out of water at grain fill the next.
Working back from the plant population you actually want, through the germination of the lot in the shed and the establishment you realistically expect in your seedbed, is the only way to get the population you planned. It also stops two expensive mistakes. Sowing light leaves gaps that weeds fill and costs yield you cannot buy back with nitrogen.
Sowing heavy wastes seed, raises the risk of haying off on a dry finish, and in canola or pulses the seed bill alone can move a gross margin by fifty dollars a hectare.
How to use it
- Enter the sown area of the paddock from your farm mapping software or paddock records rather than the title area, since headlands and tree lines are not sown.
- Enter the target plant density from your agronomist or your state sowing guide for that crop and district.
- Enter the thousand grain weight of the actual lot: weigh 200 seeds on kitchen scales and multiply by five if you do not have a seed test.
- Enter the germination percentage from the current test on that lot, not a certificate from a previous season.
- Enter the field establishment you expect, which is the share of germinable seed that becomes a plant, typically 80 to 90 per cent into good moisture and lower into a dry or cloddy seedbed.
- Enter your row spacing so the rate can be converted into seeds per metre of row for calibration.
- Enter the delivered seed price per tonne, or the value of retained seed plus cleaning and dressing.
What each result means
- Seeding rate
- Kilograms of seed per hectare to sow. Calibrate the air seeder to this figure and check it in the paddock.
- Seed lost to germination and establishment
- The share of the seeding rate that is sown but never becomes an established plant, in kilograms and dollars per hectare. It is what a poor seedbed or a weak seed lot costs you.
- Each extra plant per square metre
- How many more kilograms of seed per hectare, and how many more dollars per hectare, it takes to raise the target density by one plant per square metre.
- Seeding rate in imperial
- The identical rate in pounds per acre, useful when comparing with older records or overseas agronomy.
- Total seed required
- Tonnes needed for the whole paddock. Round up to the next bulk bag and keep some back for a re-sow.
- Seed cost per hectare
- What the seed alone adds to your variable costs, before dressing, freight or the sowing pass.
- Total seed cost
- The seed bill for this paddock, the number to compare against a quote for graded and treated seed.
- Seeds sown per square metre
- Seeds that actually go in the ground, always above the target because germination and establishment losses are added back.
- Seeds per metre of row
- Count seeds in a metre of open furrow behind the bar to confirm the machine is delivering the rate you set.
- Target established plants
- The population you are aiming for. Count plants at two to three leaves to see if the assumptions held.
The formula
Seeding rate in
kg/ha = target plants per square metre x 10,000 square metres per hectare, divided by the establishment factor, multiplied by the thousand grain weight in grams, divided by 1,000,000 to convert grams per thousand seeds into kilograms
The establishment factor is germination percentage divided by 100, multiplied by field establishment percentage divided by 100.
Total seed in tonnes = rate in kg/ha x area in hectares divided by 1,000
Seeds per metre of row = seeds sown per square metre x row spacing in millimetres divided by 1,000
How this number is used in practice
Target plant density is a district decision before it is a crop decision. In a low rainfall Mallee or upper Eyre Peninsula paddock, wheat is commonly targeted at 100 to 120 plants per square metre because a thicker crop simply runs the profile dry before grain fill.
In a high rainfall zone in the south west of Western Australia, the south east of South Australia or southern Victoria, the same variety is pushed to 180 or 200 plants to compete with ryegrass and use the extra water. Sowing date shifts it again: crops sown late are usually sown thicker to make up for lost tillering time, and crops sown very early are sown thinner to slow canopy closure and save water for spring.
Field establishment is the number most people guess wrong, and it is worth measuring rather than assuming. Into good moisture with a well set up press wheel system, 85 to 90 per cent of germinable cereal seed will come up. Dry sowing into a cloddy seedbed, deep sowing to chase moisture, a heavy stubble load or a fertiliser toxicity problem beside the seed can pull that below 60 per cent, and small-seeded canola is the most exposed of all.
Counting emerged plants in a fixed length of row at the two to three leaf stage, then comparing with the seeds per metre of row this calculator gives you, tells you your real establishment percentage for that paddock and that machine. Two seasons of those counts are worth more than any published table.
Do the arithmetic once per seed lot, not once per season. Retained seed from a hot, dry finish can be substantially smaller than the same variety from a kind season, and a thousand grain weight that drops from 38 grams to 30 grams means the same kilograms per hectare delivers a quarter more seeds.
That is how growers accidentally sow a crop 25 per cent thicker than they intended in exactly the season when the profile cannot support it.
Worked examples
Wheat at 120 plants per square metre, 34 g thousand grain weight, 95 per cent germination, 85 per cent establishment, 150 ha
50.53 kg/ha, 7.58 t of seed for the paddock and a seed bill of $4,168
Hybrid canola at 35 plants per square metre, 4 g thousand grain weight, 90 per cent germination and only 60 per cent establishment, 100 ha
2.59 kg/ha and 0.26 t of seed, but at hybrid seed prices that is still $57.04 a hectare
Faba beans at 25 plants per square metre with a heavy 520 g thousand grain weight, 92 per cent germination, 80 per cent establishment, 80 ha
176.63 kg/ha and 14.13 t of seed, which is why big-seeded pulses need bin capacity planned before sowing
When people use it
- Converting an agronomist's target plant density into a rate to set on the air seeder
- Ordering and budgeting seed before sowing, including retained seed cleaning and dressing
- Adjusting the rate when a seed test comes back with lower germination than expected
- Checking in-paddock calibration by counting seeds in a metre of open furrow
- Comparing the seed cost of hybrid canola against an open pollinated variety at the same plant density
Tips
- Weigh 200 seeds from the lot you are sowing and multiply by five. It takes two minutes and is the single most valuable input here.
- Run a fresh germination test on retained seed every season. Storage over a hot summer in a shed can cost 10 to 20 percentage points.
- Count established plants in a metre of row at two to three leaves and work backwards to your real establishment percentage for next year.
- Sow thicker when you are sowing late and thinner when you are sowing very early, because tillering capacity changes with sowing date.
- Keep a re-sow reserve of seed on hand until the crop is established, particularly for canola where a wind or slug event can wipe a paddock.
- Watch fertiliser placed with the seed. Toxicity beside small seed can cut establishment badly enough to undo careful rate arithmetic.
What it assumes, and where it stops being accurate
- Assumes the thousand grain weight and germination you enter belong to the lot you are actually sowing. Old certificates are the single largest source of error here.
- Assumes field establishment is uniform across the paddock. Sand hills, hard setting flats and headlands often establish very differently and average them out to a figure that flatters the worst areas.
- Not accurate for pelleted or coated seed unless the thousand grain weight you enter is for the coated product, since the coating can be a large share of the weight.
- Ignores seed dressing and inoculant, which add cost and can slightly reduce flowability and metering accuracy on some machines.
- Assumes precise metering. Air seeder delivery can drift by 5 to 10 per cent between the chart and reality, which is why the seeds per metre of row check exists.
- Does not account for insect, slug, mouse or frost damage after emergence. Those losses hit after establishment and are a separate re-sow decision.
Terms used on this page
- Thousand grain weight
- The weight in grams of 1,000 seeds from a particular lot, the standard Australian measure of seed size.
- Target plant density
- The number of established plants per square metre the crop is being sown to achieve.
- Field establishment
- The percentage of germinable seed that emerges as a plant in the paddock, always lower than laboratory germination.
- Germination test
- A laboratory or tray test measuring the percentage of seed in a lot capable of producing a normal seedling.
- Calibration
- Checking and adjusting the seeder so the rate delivered in the paddock matches the rate set on the controller.
Questions people ask
- What is thousand grain weight and why does it matter more than seeds per pound?
- Thousand grain weight is the mass in grams of 1,000 seeds from the lot you are sowing. It is the Australian standard because it converts directly between plant numbers and kilograms per hectare. Seed size varies by season and variety, so the same kilograms can deliver very different plant populations.
- How do I measure thousand grain weight without a seed test?
- Count out 200 seeds from a well mixed sample, weigh them on accurate kitchen scales in grams, and multiply by five. Repeat with a second sample and average the two. If the two counts disagree by more than a few per cent, mix the sample again and repeat.
- What field establishment percentage should I use?
- Into good moisture with press wheels, 85 to 90 per cent is realistic for cereals. Dry sowing, deep sowing, heavy stubble, cloddy seedbeds or fertiliser toxicity beside the seed can drop it to 60 per cent or less, and canola is the most sensitive. Measure your own by counting plants.
- Does a higher seeding rate always mean a higher yield?
- No. In lower rainfall districts a thick crop uses the stored profile before grain fill and can yield less than a thinner one. Higher rates mainly buy weed competition and compensate for late sowing. Match the density to your rainfall, sowing date and weed pressure.
- Why is my seeds per metre of row figure useful?
- It is the only check you can do in the paddock. Stop after a short run, expose a metre of furrow, and count the seeds. If the count is well below the calculator figure, the metering, the roller or the fan speed needs adjusting before you sow the rest of the paddock.
- Can I use this for pasture or forage seed?
- Yes, provided you enter the thousand grain weight for that species and a realistic establishment percentage. Small pasture seeds sown shallow into a firm seedbed often establish poorly, so figures below 50 per cent are common and should be measured rather than assumed.
Use this with
Seed Rate Calculator answers one part of the question. These pick up where it stops: Fertiliser Application Calculator, Cost Per Hectare Calculator, Crop Yield Estimator, Harvest Yield Estimator and Crop Rotation Nitrogen Credit Calculator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- Grains Research and Development Corporation. Crop specific target plant densities, establishment research and sowing rate guidance
- NSW Department of Primary Industries. Winter crop variety sowing guides and thousand grain weight data
- Agriculture Victoria. Sowing rate and crop establishment advice for cereals, pulses and canola
Seed Rate Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.