Planting time
Timing of the wheat planting is critical. Planting early generally increases the yield potential but also results in higher frost risk. Note that high soil temperatures can reduce establishment. Different varieties have been bred to suit different planting times.
Plant population
Chances of optimal yields are improved by establishing at least 700,000 plants/ ha (70 plants/m2) even in seasons of low rainfall. With irrigation, high yielding dryland conditions or very early and very late plantings, populations of at least 1,000,000 plants/ha are recommended. Plant populations below 600,000 plants/ha may result in a reduction in yield and increased weed competition.
Establishment in the field can be affected by a number of factors such as:
- seedbed moisture
- high temperatures (Central & Western Queensland)
- disease
- soil insects
- depth of planting
- certain seed treatments which reduce coleoptile length
- germination and vigour of the seed.
An establishment figure of 70% means that for every 10 seeds planted, only seven will emerge to produce a plant. Planting rate to achieve 700,000 to 1,000,000 plants/ha is normally in the range of 30-50 kg/ha.
A guide to likely field establishment, when good quality seed with a laboratory germination of 90% or better is planted at a depth of 5-7 cm and emerges without the assistance of post-planting rains, is set out below.
Planting depth
Optimum planting depth varies with planting moisture, soil type, seasonal conditions, climatic conditions, and the rate at which the seedbed dries. The general rule is plant as shallow as possible provided the seed is placed in the moisture zone but deep enough so that the drying front will not reach the seedling roots before leaf emergence. Optimum planting depth is for wheat is around 50-70 mm. Coleoptile length will be reduced with warm seedbed conditions.
Planting depth will therefore be more critical with early planting (e.g. April and early May) and varieties with short coleoptile length (e.g. Kennedy) should be avoided at this time. Planting on time is important in determining optimal yield. In trials, although sowing deeper (10-15 cm) reduces establishment rate, the gains from maximising yield potential by sowing at the optimal time tended to compensate for the lower plant population. Thus, it was better to sow deeper and on time and chance a lower plant population than to wait for another rainfall and plant outside the optimal window.
Planting Technique & Rates
The benefits of conservation farming - increased fallow stored moisture, less soil erosion, opportunity cropping and lower tillage costs - have resulted in a swing away from finely worked seed beds and bare ground to less soil pulverisation and more stubble retention. Even with zero tillage or reduced tillage planting, a “seed-bed” is required in the zone into which the seed is to be placed.
In some seasons the opportunity to sow into old crop stubble where soil moisture replenishment is adequate, or to sow up to six weeks after rain allows crops to be sown at their optimum time. Management decisions that allow for “opportunity cropping” include:
- maintenance of stubble cover
- the substitution of herbicides for tillage to control weeds
- having planting machinery capable of handling zero till and deep sowing.
Calculating a planting rate
Because seed sizes may vary depending on production years and variety type, a fixed quote for the weight of seed needed to plant one hectare is not always a true or accurate measure of obtaining a desired plant population per hectare. An actual seed count is required to calculate a more accurate planting rate. A two part method of calculating the planting rate of a crop in kg/ha given a designated plant population is:
- Target plant population (plants/ha) ÷ Germination % ÷ Expected field establishment % = SEEDS/ha
- Seeds/ha ÷ Seeds/kg = PLANTING RATE (kg/ha)
Alternatively, to calculate planting rate use the following formula:
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).
