The move from a conventional to a direct seeding system influences four important crop growth factors:
access to nutrients
competition with weeds
access to available moisture
use of sunlight
These growth factors affect decisions on seed row spacing and seeding rates.
Row spacing is the distance from the centre of one seed tube outlet to the centre of the next outlet (Figure 1). Row spacing is related to nutrient placement, weed competition and sunlight. Seeding rate is primarily related to weed competition and available moisture. As a result, row spacing usually changes and seeding rates sometimes change with the move to direct seeding.
Under the older conventional tillage-based seeding methods that used broadcast fertilizer, the best yields resulted from very narrow row spacings at optimum seeding rates for the crop and climate zone. Narrower spacings gave the crop a competitive advantage over weeds for access to nutrients, moisture and sunlight.
In direct seeding, the ground between the seed rows is left undisturbed. As a result, fewer weed seeds germinate because they are left in the crop residue at the soil surface. Consequently, there are fewer annual weeds competing with the crop for moisture and nutrients.
In conventional systems, nitrogen fertilizer is either broadcast and worked into the soil, or it is banded in fall or spring, often with about 1 foot (30 cm) between bands. With either application method, the crop and weed seedlings usually have about equal access to the fertilizer.
With most direct seeding systems, fertilizer is banded. Usually, nitrogen is precision-placed at seeding time, about 1 or 2 inches (2.5 to 5 cm) to the side of the seed row or between two paired rows. Thus, the nitrogen is more available to the young growing crop than to weeds, which are further away. This approach gives the young crop a competitive advantage over the weeds.