A legume seed lot whose germination falls below 70 percent should not be sown. Between 70 and 80 percent, Agriculture Victoria asks for a laboratory test before the lot is used, and above 80 percent the lot is judged fit to sow. That threshold can be checked on the farm, on damp paper, in about ten days and with no special equipment. The result is then used to decide whether the lot goes to the field, and to correct the seed rate per hectare to make up for what will not emerge.
The germination test, step by step
The method Agriculture Victoria describes for pulses and cereals transfers directly to cowpea. It takes ten minutes to set up and a single count.
- Draw a representative sample of the lot by mixing several small subsamples taken from different points in the bag or the heap. A sample taken off the top tells you nothing about the bottom of the bag.
- Run the test after the lot has been cleaned if you want an accurate result. If you suspect germination is badly poor, test before paying to clean seed you will not use.
- Count three lots of 100 seeds. Three replicates, not one: it is the average that counts.
- Lay each lot out in a 10 by 10 grid on four sheets of damp paper towel, on a tray or a plate. Alternative for large seed: sow at 2 to 3 cm deep in a tray of sand or seed raising mix.
- Hold the temperature at about 20 to 25 degrees Celsius, with as little fluctuation as possible. A shaded, covered spot indoors will do.
- Water gently, without drenching. Cover the tray or seal it in a bag to limit evaporation, check regularly and lightly re-moisten.
- Keep paper towel tests in the dark, or under a dark cloth.
- Count at 7 to 10 days for pulses.
What counts and what does not
This is where most on-farm tests give an over-optimistic result. Agriculture Victoria is explicit: count only normal, vigorous, healthy seedlings.
Leave out of the count any seedling that is diseased, discoloured, twisted or deformed, and any with no root. A seed that has swollen and pushed out an aborted shoot is not a germinated seed. Nor should you wait for the late ones: seedlings that emerge after the counting date are usually weak, and in the field they will not make a productive plant.
The arithmetic is immediate. Out of 100 seeds, 87 normal seedlings give 87 percent germination. Take the average of your three replicates.
Reading the result: the decision thresholds
The thresholds Agriculture Victoria publishes for pulses and cereals:
- Above 80 percent: the lot is suitable for sowing.
- Between 70 and 80 percent: have the lot tested in a laboratory before committing it.
- Below 70 percent: do not sow this lot.
One point worth knowing for a cowpea lot harvested recently: if the test was done soon after harvest, repeat it closer to the sowing date. Germination capacity shifts over the first weeks of storage, and it is the figure at sowing time that decides.
Turning germination into a seed rate
A lot at 80 percent germination sown at the normal rate puts 20 percent of its seed in the ground for nothing. The correction is made through pure live seed, as Dan Undersander sets it out for the Division of Extension at the University of Wisconsin-Madison.
The calculation has two steps. First, the share of the bag that is genuinely alive:
Pure live seed (%) = germination (%) x purity (%) / 100
Purity is the proportion of the bag weight that is actually seed, the rest being trash, dust and foreign seed. The example given by the University of Wisconsin: 95 percent germination and 66 percent purity give 62.7 percent pure live seed, which is 62.7 kg of live seed per 100 kg bag.
Then the corrected rate:
Corrected rate = target rate / pure live seed (%) x 100
The example from the same source: at 70 percent pure live seed and a target of 10 units of weight per unit of area, you need to sow 14.3. Without that correction the stand comes in about 40 percent below target. The University of Wisconsin advises applying the correction as soon as the recalculated rate drops below 85 percent of the recommended rate.
Applied to cowpea, using the 20 kg per hectare that IITA gives for a semi-erect cowpea at 75 by 20 cm: a lot at 85 percent germination and 98 percent purity gives 83.3 percent pure live seed, so 24 kg per hectare. The same target with a lot at 70 percent pure live seed calls for 28.6 kg per hectare. That calculation is ours, applied to the rate published by IITA and the formula from the University of Wisconsin.
Base cowpea seed rates by plant type
The farmers’ guide to cowpea production in West Africa published by IITA, by Dugje, Omoigui, Ekeleme, Kamara and Ajeigbe in 2009, gives rates and spacings by plant type. These are the figures to start from, before correction.
| Cowpea type | Maturity | Spacing | Base seed rate |
|---|---|---|---|
| Erect | Extra-early | 50 x 20 cm | 25 kg/ha |
| Semi-erect | Early to medium | 75 x 20 cm | 20 kg/ha |
| Prostrate | Medium to late | 75 x 30 cm | 16 kg/ha |
| Prostrate | Late | 75 x 50 cm | 12 kg/ha |
The guide sums the general range up as 12 to 25 kg per hectare, depending on variety, seed size, cropping system and seed viability. That last item is the reason the test exists: viability is the only one of the four you can measure yourself, on the farm, in ten days.
The logic is the same as for vegetative planting material, where the density is also worked out before the order goes in. We covered it for another crop in our article on the number of cassava cuttings per hectare.
What the eye can see on a lot
The test measures what the eye cannot see, but it does not replace visual sorting. The IITA guide asks you to select good seed, free of damage holes and free of wrinkles. A round hole marks a weevil’s passage, and a wrinkled seed has either lost water or been harvested immature.
Two further reference points complete the sorting:
- Storage moisture content should stay below 10 percent according to the same guide. A lot held wetter than that loses germination and goes mouldy.
- The guide recommends treating seed before sowing to improve emergence and protect young plants from soil insects and fungi.
In practice, sort first, test the sorted lot, and calculate the rate on the sorted lot’s result. Testing an unsorted lot and then sowing the sorted one means underestimating germination, and sowing too thick.
After the test: sowing depth and density
A corrected rate is worth nothing if the sowing itself spoils emergence. The IITA reference points:
- Sowing depth of 2.5 to 5 cm. Beyond 5 cm, emergence is delayed, seed rots more readily and the stand becomes uneven.
- Three seeds per hill, then thin to 2 plants per stand two weeks after planting.
- Spacings: 50 cm between rows and 20 cm within the row for erect varieties, especially the 60 to 70 day types. For semi-erect types, 75 cm between rows and 25 to 30 cm within the row in the body of the guide, 20 cm in its seed rate table. For prostrate types, 75 by 50 cm, which is also the spacing used in intercropping or relay cropping with a cereal.
- Cycles: 60 to 69 days for an extra-early variety, 70 to 79 days for an early one, 80 to 89 days for a medium one, 90 to 120 days for a late one.
On sowing dates, the guide works from the onset of the rains rather than by agroecological zone. Where the rains start in May, it places erect cowpea sowing in the second week of August and semi-erect in the first week. Where they start in June or July, it recommends the end of July for both erect and semi-erect types, and the first week of August for prostrate types, which need dew.
After sowing, keeping moisture in the seedbed decides how even emergence will be, as much as depth does. That is the job of mulch, whose laying and thickness we set out in our article on mulching. For crops that go through a nursery before transplanting, the same testing logic applies to the trays: see our guide to raising a nursery under shade net.
Ordering your seed and inputs
JOGOO Agriculture supplies seed, nursery trays, substrates and inputs for food and vegetable crops in West and Central Africa. See the Agro-inputs & Supplies page for the range, and the Contact page for a quote based on your area and your varieties.
Frequently asked questions
How many seeds does a germination test need?
Three lots of 100 seeds, according to Agriculture Victoria. The three replicates smooth out variation, and it is their average that gives the lot’s germination rate.
After how many days do you count cowpea seedlings?
Between 7 and 10 days, which is the counting window used for pulses. Do not count seedlings that emerge after that date: they are usually weak.
At what germination rate should a lot no longer be sown?
Below 70 percent, the lot should not be sown. Between 70 and 80 percent, Agriculture Victoria asks for a laboratory test. Above 80 percent, the lot is suitable.
How do you correct the seed rate after a germination test?
Work out pure live seed, which is germination multiplied by purity and divided by 100, then divide the target rate by that percentage and multiply by 100. Example applied to cowpea: a 20 kg per hectare target at 83.3 percent pure live seed gives 24 kg per hectare.
What is the cowpea seed rate per hectare?
Between 12 and 25 kg per hectare according to IITA, depending on plant type, seed size, cropping system and the viability of the lot. A semi-erect cowpea at 75 by 20 cm starts from 20 kg per hectare.
How deep should cowpea be sown?
From 2.5 to 5 cm according to IITA. Beyond 5 cm, emergence is delayed and the stand becomes uneven.
Sources
- Agriculture Victoria, Testing crop seed for sowing
- Dugje I. Y., Omoigui L. O., Ekeleme F., Kamara A. Y., Ajeigbe H., Farmers’ Guide to Cowpea Production in West Africa, IITA, Ibadan, 2009
- Undersander D., Pure live seed, Crops and Soils, Division of Extension, University of Wisconsin-Madison
