Sowing date
As is the case for any crop, the sowing date must be chosen to gain benefit from all the features of the climate in the region concerned.
• The sowing date is chosen by weighing up the need to make swift and optimal use of the scope of weather to achieve the full potential of the variety chosen.
• The soil must be warm, but not dried out. Maize germinates when the soil temperature is between 6 and 8°C ; in the northern hemisphere, this is usually from April 1st on. It should be noted that soil temperature increases gradually and steadily throughout the spring, regardless of variations in the air temperature. The soil profile must not be allowed to dry out as the seed needs water to germinate, and when a soil surface horizon dries out, emergence will be uneven with a severely negative impact on yield.
• The risk of frost must be taken into consideration, but when seed is sown at the right depth (> 5 cm) in moist soil (with air providing insulation) it is protected from any passing surface frost.
• Soil has a great effect on emergence: in spring, dark soil warms faster than light-coloured soil because of surface absorption of sunlight.
• Depending on the colour of the soil, growth stages in the same field can differ by 2 to 3 leaves, with the discrepancy continuing all the way through to harvest.
• In soil with higher clay content, sowing must not be done with a disc seed drill with a “sculpting” effect (the furrow is not filled in and there is a risk of seed drying out). Preparation (e.g. strip-tillage in autumn or spring) can help produce fine earth along the row and improve establishment.
• To make the most of weather conditions and increasingly warm spring seasons, countries with warm weather in spring (e.g. France, Italy and Portugal) are now bringing their sowing dates forward. This strategy (referred to as “esquive” in France, i.e. bypassing) makes for more stable yield levels.
Sowing depth and technique
The goal is for quick, even emergence. The seed must be in contact with moisture and have sufficient air for germination to occur. Sowing must:
• be sufficiently deep (≥ 4-5 cm) for protection from frosts, birds and any drying of the soil surface.
• not be too deep (< 9-10 cm) so as not to exhaust the seed’s reserves as happens when the coleoptile is elongated, to shorten the period of emergence and to avoid parasites (both animal and vegetable).
Depending on the type of soil and the sowing date, the seed should be sown at a depth of 4 to 7 cm, paying special attention to ensure that:
• the soil is moist
• there is enough fine soil around the seed for proper soilseed contact
• sowing is preferably in surface mounds for friable soil, maize being able to emerge well in cloddy and stony soils
• seed is evenly covered for seedlings to emerge at the same time. An even sowing depth is more important than even seed spacing along the row.
Sowing rate and the effect
• Modern hybrids offer a guarantee of quality seed which is the primary prerequisite for quick, uniform emergence. Successful emergence thus depends, first and foremost, on the farmer.
• Many trials have shown that the rate and efficiency of the seed drill and the soil preparation affect the uniformity of the sowing.
• The same trials have shown that proper, even covering and packing of the soil are the keys to simultaneous emergence: a plant emerging a few days late, and two leaves in growth behind the adjacent plants may lose 30% in yield because of competition from neighbouring plants. A difference in growth between plants in the same row is more critical than a difference between two adjacent rows, although an “edge” effect was still found in the trials.
