Plant Population

Last Update: Wednesday, September 25, 2013 4:07:17 PM

Table of contents

    Important tips for best management

    • Target a population of at least 70 healthy surviving plants per metre square (7 per square foot) to maintain yield potential for canola. The ideal range is 70 to 140 plants per square metre (7 to 14 per square foot) to achieve maximum yield potential. If economics permit, targeting the middle of this range will allow for some plant mortality due to post-seeding stresses (e.g. frost, seedling disease, wind, water, insects, weed competition) without population dropping below the optimal range.
    • The critical level is about 50 plants per metre square for hybrid canola. At this level variability in yield performance increases, and below 50 plants/m2 (5 plants/ft2) the chance to achieve maximum yield potential is generally lost. [3] However, depending on the date reseeding threshold may be much lower than this based on maturity advantages of the already established plants, reduced yield and quality potential of later seeded crops and costs of reseeding.


    Plant architecture

    Canola is often called a flexible or “plastic” crop because individual plants can change the number and size of branches and pods in response to available moisture, light and nutrients. Therefore, canola normally compensates for variations in plant population over relatively wide ranges with very little effect on final yield. [1]

    At 70 to 140 plants per square metre, canola plants normally produce 3 to 5 secondary branches in addition to the main stem. At low densities of 20 to 30 plants per square metre, canola plants will produce up to four times more branches than plants in crops with populations of between 70 and 140 plants per square metre. [2]

    Canopy architecture

    As the canopy becomes more dense (plant population increases), each plant produces less dry weight, thinner stems, fewer branches and fewer seeds per plant due to increased competition from adjacent plants. However, fewer seeds per plant are offset by a higher number of plants, resulting in similar seed yield per unit area compared with lower plant populations. This is why canola can produce similar yields over plant populations ranging from 50 to 200 plants per metre square (5 to 20 per square foot).

    A canopy with too few plants simply does not have enough plant density to fully utilize the available light, moisture and soil nutrients. [3]

    In a canopy with more than 200 plants per square metre (or 20 per square foot), stems are very thin and pods are concentrated at the top of the plants. These stands are more likely to lodge, which can create a better microenvironment for diseases like sclerotinia and make harvest more challenging. Severe lodging during bolting or early flowering stages may also directly impede the efficient uptake of moisture and nutrients through stem crimping, leading to lower yields.

    When overall plant populations are lower than the ideal, stand uniformity becomes more important for overall yield potential.An AAFC study conducted in the brown soil zone at Swift Current, Saskatchewan, found that if canola populations are at 40 per square metre (4 per square foot) or less then uniform stands are more likely to out yield non-uniform stands. [4]

    The same study also found that when moisture is limited, stands with fewer than 40 plants per square metre (4 per square foot) yield much lower than dense and more uniform stands.

    Crop competitiveness

    Moderate to high plant densities can improve crop competitiveness against early season weed growth because with more plants, the canopy closes more quickly. With low plant populations, canola crops are slower to cover the ground and provide less competition to weeds in the early growth stages. With soil bare longer, this also increases evaporation of soil moisture. 

    Moderate to high plant densities in early growth stages can also reduce yield impact of damage from insects, disease, frost and hail because the stand can afford some plant mortality and still maintain its yield potential.

    Low plant densities can produce viable crops, but the management of thin stands is more challenging, due to more variable maturity and lower tolerance for additional plant losses.

    Maturity

    Because canola plants in a low population density situation grow larger and branch more, they tend to mature later. Secondary branches account for up to 80% of the yield for canola crops with 20 plants per square metre (2 per square foot). [1] These secondary branches flower later and mature later than main stems, meaning thin crops will mature later overall 

    These plants will also have a broader range of maturity, creating yield and quality losses simply due to inability to time applications properly and harvest limitations. Pods on the main stem may be dry with completely mature seeds while pods on secondary branches may still be green with watery seeds inside. Top pods may start to shell out before pods on side branches are ready to swath. Yield losses will occur no matter when a grower decides to swath this crop.

    Extra branching in very thin stands (20 plants per square metre) can delay seed maturity up to 21 days depending on environmental conditions. [2]

    Yield

    Canola crops need 50 plants per square metre (5 per square foot) or more to maintain yield potential. Plant populations lower than this will almost always have yield loss. [3]

    Canola Council of Canada research found that stands of 50-60 plants per square metre (5-6 plants per square foot) yielded about 5 bushels per acre more than stands that averaged 20-30 plants per square metre (2-3 per square foot). [1]

    The ideal is 70 to 140 plants per square metre (7 to 14 per square foot) to maintain maximum yield potential while providing some buffer for plant loss due to post-seeding stresses like frost, seedling disease, wind or water, insects, weed competition or others. A stand already at the minimum for yield potential cannot afford to any additional plant mortality.

    Quality

    Later crops are more likely to encounter a fall frost that arrests the development of immature seed and leads to shriveled seed and higher chlorophyll counts. This may result in lost yield or grade.

    Plant population can also influence oil quality. Research has occasionally found highest oil contents and lowest free fatty acid levels at lower seeding rates. But in cases where high seeding rates decrease maturity enough to avoid fall frost, this can result in reduced green seed and chlorophyll content compared to low seeding rates.


    References

    [1] Canola Council of Canada Crop Production Centre research, 2003 and CCC factsheet “Plant Populations for Profitability, April 26, 2005”

    [2] Canola Council of Canada Canopy Manipulation Trials

    [3] Steve Shirtliffe, University of Saskatchewan, “Determining the economic plant density in canola,” published in 2009 and based on summary data from 35 experiments.

    [4] “Yield Adjustment by Canola Grown at Different Plant Populations under Semiarid Conditions” S. V. Angadi,* H. W. Cutforth, B. G. McConkey, and Y. Gan. Published in Crop Science in 2003.