Winter wheat drilling, seed rates and establishment
Germination and mergence require moisture and warmth. A proportion of viable seeds fail to emerge due to pests, diseases and soil conditions. Establishment declines if sowing is delayed after mid-October. It takes 150C days from sowing to emergence, which is equivalent to about 11 days in September, 15 days in October and 26 days in November. Varieties susceptible to diseases such as eyespot should not be planted early and any varieties with a low lodging resistance should not be sown before the end of September.
Seed rates should be based on the calculations:
Seed rate (kg/ha) = Target plants/m x 1000 grain weight (grams) / %establishment
The target plant population can be adapted to a practical target to take into account variations within the field situation. A minimum of 70 plants/m2 is required for September sowings, while a practical target of 120 plants/m2makes some allowance for field situations. Fields with a high black-grass burden are likely to require a greater seed rate, along with other measures such as delayed sowing. If you are using farm saved seed it is important to test the germination and to take this into account when calculating expected% establishment.
ADAS has analysed the results of 25 seed rate response experiments in wheat since 2001, with the majority sown in October. In 20 out of the 25 experiments the economical optimum plant population was between 70 and 170 plants/m2. In only two trials, which were affected by severe spring droughts, was the optimal plant populations above 250 plants/m2. Several factors which can affect the optimum plant number are described below:
Sowing Date – Crops drilled in September or early October can be drilled at lower seed rates because of improved establishment conditions and greater tillering potential. Establishment falls by around 10% for crops sown in lateOctober and a further 10% for November sowings. This is because the plants have less time to produce compensatory tillers. In September sown crops,drilled in good conditions, establishment rates of 60-70% can be expected. However, if cloddy seedbeds are produced, 50-60% may be more likely. 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. The optimum plant population should be increased by about 10plants/m2 for each week that drilling is delayed after late September.
Latitude (south vs north) – The effects of latitude on optimal plant population are not consistent,although high optimal populations are observed more frequently at more northerly sites. This is because the colder temperatures mean there is less time for compensatory tillering. Statistical analysis has suggested that seed rate should be increased by 11-28 seeds/m2 per degree of latitude. It is likely that higher seed rates will also be required on high altitude sites, again because cooler temperatures limit compensatory tillering.
Rotational position – In second and subsequent wheat positions optimal plant populations are30-40 plants/m2 lower that in first wheats, due to take-all having a greater yield impact on high plant populations compared to low plant populations. This partly due to closer proximity of plants which increases the risk of take-all spread from plant to plant. If take-all is controlled by a seed treatment in second and later position wheats, the optimal plant populations are only 20-25 plants lower than for first wheats.
Nitrogen nutrition – AHDB project 361 showed no interaction between seed rate and the timing of early N on yield, and low plant populations did not require earlier N to help with compensatory tilling. This was because widely spaced plants have an inherently high tillering capacity because each plant receives more light (than closely spaced plants). However, low plant populations do have a reduced susceptibility to lodging, so N may be applied early without the same level of lodging risk which would occur in denser crops.
Reduce seed rates for early own crops to minimise the lodging risk. Aim to establish around 120 plants/m2from September sowings (with a minimum of 70 plants/m2), but assess each field for likely establishment.
Use known thousand grain weights, reliable germination rates and your best estimate of % establishment to calculate seed rates.