Plant population for Queensland

Wheat growers should aim for a plant population of about 1 million plants per hectare or 100 plants per square metre.

Crops are often too thin, resulting in lower yields and delayed harvest. In Queensland, over the last 20 years, plant populations of 1 million plants per hectare gave the best wheat crops under most conditions. Higher populations did not adversely affect yield, whereas populations fewer than half a million plants per hectare yielded significantly less (see Figure 1).


A graph showing plant population per hectare and wheat grain yield
Figure 1: Grain yield response in wheat to plant population averaged over a wide range of growing conditions in Queensland

Advantages of 1 million plants per hectare

This plant population helps:

  • compensate for poor emergence from deep sowing (> 40mm) and reduced coleoptile length in early planted crops
  • improve dry matter production of early and late crops, as high dry matter production is necessary for good grain yields
  • varieties produce fewer tillers, which varieties tend to do under high plant populations (higher planting rates)
  • suppress weeds by increasing crop competition - less herbicide is needed
  • reduce lodging due to the high number of small heads
  • escape grain damage from wet weather that causes downgrading of quality by enabling earlier flowering and harvest
  • facilitate earlier harvest by reducing the incidence of late 'green' heads.

Wheat seeding rate guide

Seed size, germination and vigour

Factors to consider in establishing a wheat population are seed size, seed germination, vigour and establishment rate. Seed size, germination and seedling vigour will vary between seed batches and among varieties. Planting low-quality seed can lead to poor establishment, especially under marginal planting conditions. Replanting operations and lost planting opportunities are costly and are irrecoverable losses to farm profits.

Rain prior to wheat harvest can cause weather damage to the grain. Weather damage can greatly affect germination and seed vigour. Although weather-damaged seed can look quite normal and may germinate, it will have poor seedling vigour and result in poor establishment.

Seed suspected of weather damage should be tested for germination and seedling vigour several months after harvest because although weather-damaged seed may have normal germination and seedling vigour when tested soon after harvest, it deteriorates rapidly in storage.

Causes of poor quality seed

The major causes of poor quality seed are:

  • high moisture and temperatures, which often cause seed to deteriorate during storage. Seed with less than 11 per cent moisture content held below 20°C is desirable if stored for 12 months. When using grain dryers, ensure maximum temperatures are not exceeded
  • stored grain insect pests, which will cause damage. The use of aeration and/or a grain protectant is recommended
  • mechanical damage through harvesting, handling and cleaning, which can cause cracks in seedcoats, and splits and breaks, which reducing seed quality
  • seed sourced from crops that have suffered nutritional disease, environmental stress or weathering, which may result in seed viability dropping quickly with prolonged storage.

Establishment

Establishment depends on seedbed conditions, soil moisture, insect pests and climate. Establishment percentage is the percentage of seed planted that establishes on planting moisture. Establishment may be as high as 95 per cent under ideal conditions or drop to as low as 40 per cent with rough seedbeds, early planting and limited moisture. As a general guide, Table 1 shows the expected establishment percentage on three Queensland soil types.

Poor quality seed with low laboratory germination will give poor establishment; for example, weathered seed that has been stored in high temperature and humidity or seed that has been attacked by insects.

Table 1. Expected establishment % on three Queensland soil types
Soil type Expected establishment %
No presswheels Presswheels
Heavy clay 45 60
Brigalow clay 55 70
Red earth 70 80

The seeding rate formulae to achieve target plant population are:

  1. Target plant population (plants/ha) ÷ Germination % ÷ Expected field establishment % = Seeds/ha
  2. Seeds/ha ÷ Seeds/kg = Planting rate (kg/ha)

Example:

  1. 800,000 ÷ 0.9 ÷ 0.75 = 1,185,185
  2. 1,185,185 ÷ 27,500 = 43 kg/ha

Alternatively, to calculate planting rate use the following formula to calculate planting rate:

Target plants/ha = Seeds/kg x Establishment % x Germination %

Germination percentage and seeds/kg information can be found on bag labels, or you can do your own germination tests and/or seed counts. Divide 100 into the percentage rates to produce a metric fraction (e.g. 90% = 0.9).

Planting rates can be calculated as shown in Table 2.

Table 2. Planting rate to establish a plant population of 1,000,000 plants/ha
Seed size
(seeds/kg)
Planting rate kg/ha
Expected field establishment %
50 70 80 95
25 000 80 57 50 42
27 500 73 52 45 38
30 000 67 48 42 35


How to measure your plant population

Here is a simple way to check the plant population in your wheat crop:

  1. Cut to size a 1 m length of steel rod or wooden stick. While the crop is still young, preferably no later than day 20 after sowing (to easily identify individual plants), place the 1 m rule along a row and count the number of plants along this row. Do this 10 times at different locations to get a representative count and calculate the average.
  2. To achieve 1 million plants/ha (100 plants/m2), the number of plants in the 1 m row should equal your row spacing in centimetres. For example, if your row spacing is 25 cm, the number of plants in the 1 m section of row should be 25 plants for an establishment of 1 million plants/ha. For a 30 cm row spacing, your plant number should be 30 plants and so on.

If this is the case, you have achieved the desired plant population for maximum yield potential. In addition, your crop will be more uniform, flower and mature earlier, suppress weeds more effectively and lodge less than crops of lower populations.

Further information

Contact:

Peter Keys
Principal Technical Officer
Biloela
Ph: +61 7 4992 9109
Email

Ann-Maree Bach
Wheat Industry Development
Toowoomba
Ph: +61 7 4688 1267
Email

Last updated 28 July 2010