To ensure a successful sowing operation and accurately determine the appropriate seeding rate for your seeder, it is crucial to calculate the seeding rate. Achieving an optimal seeding rate depends on considering various factors related to sowing conditions, such as soil type, crop variety, sowing date, and prevailing climatic conditions, among others.

A new campaign is coming up, and you are wondering how much wheat to sow in your seeders. First, a quick math refresher:

Seeding rate = (Seed density _(grains/m²) x TGW _(g)) / 100

TGW (thousand grain weight) rate varies depending on the variety and the growing season. Seed rate is the number of seeds sown per square meter. For wheat, seed rates typically range from 150 to 300 seeds/m2. This number varies depending on soil type, planting date, weather conditions, and variety.

Excessive plant density does not necessarily enhance crop yields.

Consider favorable sowing conditions: good moisture, absence of large clods, sowing at 2 or 3 cm depth… In this case, the estimated losses at emergence are on average 15%. A sowing density of 220 to 250 grains/m2 would therefore be sufficient to achieve the maximum yield of wheat. However ideal conditions are not always met! In that case, what is the most accurate method for calculating?

First, it goes without saying that undervaluing seeding rates can lead to a decrease in yield, especially below 150 plants/m2. However, and this is less intuitive, too high seeding rates do not improve yield. Too many grains per unit area can lead to lodging, prevent the wheat from growing tall enough, and favor the appearance of foliar diseases.

Sowing density depends on soil type

Same variety of wheat sown on the same date may require a very different seeding density depending on soil type. Wheat emergence and tillering are penalized in difficult soils. In waterlogged soils, lack of roots oxygen leads to aborted tillers and slowed growth. Also, a soil with low water reserves is detrimental to tillering and spike development.

Another example is the excess of stones on the surface, which reduces the volume of soil available for root development. All these constraints increase yield losses.

According to Walloon Centre for Agronomic Research Center (CRA-W) and Gembloux Agro-Bio Tech University, sowing density in difficult soils should be increased by 20 to 50 grains/m2.

Sowing more densely in late sowing

On average, to ensure optimal wheat growth under favorable conditions, it is recommended to accumulate approximately 130°C-days between sowing and emergence. For an October sowing, this corresponds to a delay of about ten days. However, if sowing is postponed by a few weeks, soil temperatures decrease, potentially extending the delay to as much as a month. It's important to note that as the time for wheat emergence lengthens the risk of increased yield losses also rises.

Therefore, remember this mantra: the later the wheat is planted, the higher the densities should be. For example, the Chamber of Agriculture of Normandy recommends increasing the density by 30 seeds per square meter for every two weeks of delay.

Adapting to climate context

In addition, seeding rate depends on climatic context. In difficult conditions, losses at emergence and in winter can reach 30 to 40%. Under a continental climate (Champagne, Bourgogne, etc.), wheat tillering will be slowed down by colder autumns and winters. It is therefore advisable to sow wheat earlier or at a higher density in these areas. On the other hand, the absence of frost in an oceanic climate favors emergence of tillers. Seeding rate can therefore be reduced.

Another important factor is seed quality. In case of lower germination power, it is necessary to increase seeding density.

What about hybrid wheat?

Hybrid wheat seeds have a very high tillering ability. Recommended seeding rate is therefore lower than for lines and can be reduced by 30%. Thus, Arvalis successfully conducted trials with hybrids sown at 140 seeds/m2.

SOURCES

• Centre Wallon de Recherches Agronomiques (CRA-W) et de l’université Gembloux Agro-bio Tech : livre blanc « La densité de semis des céréales »
• La Chambre d’agriculture de Normandie : « semis des céréales »
• Perspectives agricoles : « Déterminer au plus juste les dates et densités de semis »
• Arvalis : « Céréales : régler les semoirs à la bonne densité »