Calculating seed rates for drilling

13.10.2017

Seed rates should be based on the calculation:

Seed rate (kg/ha) = {Target plants (/m²) x Thousand Grain Weight (g)} / % Establishment

For crops sown in late September/early October, the minimum plant population sufficient to achieve high yield potential is usually only 70 to 120 plants/m2.  However, targeting a population of 200 plants/m2 for this drilling window provides insurance against unpredictable factors, whilst not creating an excessive risk of lodging.  A good plant/tiller population is needed to ensure yield potential is achieved, but it also provides useful weed suppression once the crop is well established.  In general, any varieties with a low lodging resistance should not be sown before the end of September.

The target plant population should be increased by about 10 plants/m2 for each week that drilling is delayed after late September.  Percentage establishment decreases from around 70% for sowings in September to less than 50% for sowings in November or later, and on average, establishment on sandy soils is 90% compared with 65% on loams and clays.  If using farm saved seed, you could test the germination and take this into account when estimating expected % establishment.

It will generally take around 150 °C days from drilling to emergence.  This is equivalent to about 11 days in September, 15 days in October or 26 days in November.

In second and subsequent wheat positions, optimal plant populations are 30 – 40 plants/m2 lower that in first wheats, unless take-all is controlled by a seed treatment, in which case the optimal plant populations are likely to be 20-25 plants/m2 lower than for first wheats.