Canola is a flexible crop because wide variations in plant densities will often produce similar yields.
If plant density is low, around 60 plants per sq. metre, then individual plants grow larger, bear bigger stems and produce more pods.
As plant density increases, plants produce thinner stems, fewer branches, fewer pods and less seeds per plant.
Research studies at the University of Alberta and Agriculture Canada’s research centres in Saskatoon, Melfort, Sask., and Beaverlodge, Alta., have shown that plant populations ranging from 60 to 200 plants per sq. m have resulted in similar yields for Argentine and Polish species. Yields decline slightly when plant populations exceed 200 plants per sq. m.
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In Alberta, researchers examined 35 canola fields that had uniform stands and averaged 40 bushels per acre. Plant populations at the time of swathing varied from 76 to 235 plants per sq. m. The average for all fields was 140 plants per sq. m.
The best crops examined in the study yielded about 50 bu. per acre. Those crops had plant densities ranging 76 to 170 plants per sq. m.
Choosing a seeding rate
When planting, producers should minimize gaps in the seed row. Gaps of a third of a m or less between plants can result in costly yield reductions.
Ideally, growers should aim for plant populations of 80 to 180 plants per sq. m. With a 15 centimetre drill row spacing, this would require a stand of 12 to 27 plants per m in each drill row.
To calibrate the drill, producers should collect seed from a drill run over a 10 m distance. With a two kilogram per acre seeding rate of B. napus, 190-200 seeds will be collected.
With a 15 cm drill row spacing ,this would represent 125 seeds per sq. m.
The number of established plants per sq. m will be greatly influenced by seedbed firmness, moisture and temperature, soil texture, depth of seeding, germination, fertilization, seed treatment, insects and diseases. As a result, the seeding rate selected will depend on seeding conditions.
In above-average growing conditions, about 60 to 80 percent of the seeds planted will produce viable plants. The exception is the Peace River region where 40 to 60 percent of seeds will produce a viable plant on summerfallow.
Under average conditions, those rates drop to 40 to 60 percent and even less in Peace River area.
If only 40 percent of the seed is expected to produce viable plants, then a two kg per acre seeding rate of 125 seeds per sq. m will produce approximately 50 viable plants.
Under these conditions, a higher than recommended seeding rate will be needed to ensure the minimum plant density of 80 plants per sq. m.
Seeding rates below recommended levels could be considered when seeding conditions are excellent and late summer drought is not a concern.
Keep in mind that a low plant population takes longer to close over the rows and allows more weed growth and moisture evaporation.
Higher rates may be desirable to compensate for low-germination seed, cold soil and deep seeding or where more fertilizer than recommended is applied with the seed.
Higher-than-recommended rates can also be justified when seeding on soil that tends to crust.
Some growers in northern areas use higher rates for the B. napus varieties to hasten maturity and swathing by reducing plant height and the amount of straw. In those cases, heavy stands may result in excessive early lodging and difficult harvest conditions.
If poor emergence occurs, the plant stand should be carefully evaluated. If weed levels are low, a healthy plant stand of 40 plants per sq. m might still be adequate.
An Agriculture Canada study in Melfort found that reseeded plots yielded less than half of those that were not reseeded. Although the reseeded plots had higher plant populations, the increased stands did not compensate for the later seeding dates.
If weeds and poor emergence occur simultaneously, reseeding may be the best option.
