Evaluating the Stand

Table of contents

    Important tips for best management

    • Take a number of counts throughout the field 20-25 days after seeding and assess whether the number of plants per square metre (or square foot) or plants per metre (or foot) of row match the target goal. If plant counts are not within reason given the seeding rate, look for causes and determine ways to improve the success rate.
    • Stands with plant densities below 40 to 50 plants per square metre (4 to 5 per square foot) are unlikely to achieve the full yield potential that could be achieved with higher plant densities under the growing conditions for that season. However, they may still outperform reseeded crops in most cases, due to the ability of individual plants in thin stands to compensate with additional branching and pod production, and the typical trend to lower yields from later seeded crops.
    • Consider the calendar date when assessing whether to reseed. A thin stand of 20 plants per square metre (2 per square foot) in the last part of May or early June often will usually have greater yield and profit potential than if the same field is reseeded. Reseeding adds to production costs and the reseeded field will have a much shorter growing season (and yield potential) because it was seeded so late. However, if fields can be reseeded before the third week of May with good soil conditions for rapid emergence, reseeding may be profitable. (Canola growers are encouraged to discuss reseeding policies with their insurance carriers and seed company reps, and look at weather data for their region.)


    Taking counts

    An accurate average plant stand count will determine whether crop establishment measures achieved the target goal. How many plants did you expect based on the seeding rate? And is the plant stand in line with this expectation? Answers to these questions will help growers improve stand establishment practices for next year, and help with management decisions for the current crop.

    To do counts, use a 50 cm by 50 cm square or a hoop with an inside diameter of 56 cm. Both are the equivalent of 0.25 of a square metre. Count the number of plants inside the square or hoop, and multiply by 4 to get plants per square metre. (for plants per square foot use a one foot square or divide the plants per square metre by approximately 10).

    Another method is to use a metre stick and count the seedlings per metre of row. Take that number and multiply by 100 then divide by the seed spacing in cm to get plants per square metre. For example, 25 plants per metre multiplied by 100 then divided by 25 cm (10” row spacing) is 100 plants per square metre (approximately 10 per square foot).

    For all methods, several counts per field are required to get a good average.

    Fields should be monitored and walked starting 10 days after seeding if conditions are good, or 15 days if conditions are cool. If the plant stand is less than expected for the given seeding rate, seed size and estimated seedling survival, then check equipment settings, seed characteristics and field conditions to identify why the ideal plant population was not achieved. The cause or causes may relate to the seeding operation, such as inconsistent depth, excess fertilizer placement with seed, or mechanical issues. Frost, wind or flooding, insects or disease, or herbicide residues could also be factors.

    When stands are spotty and thin, careful management is required to preserve the plants present. More conservative thresholds may be warranted for insects, weeds and diseases.

    Diagnosing poor stands

    Canola fields with low plant densities and with unhealthy plants are more vulnerable to losses from insects, weed competition and environmental stresses such as frost. When established stands of canola are below optimum plant densities of 70 to 140 plants per square metre (7 to 14 plants per square foot), growers need to consider:

    1. Yield potential. Research shows that the critical level for plant populations is somewhere between 40 and 50 plants per square metre. Stands below 40 plants per square metre do not tend to reach their full yield potential [1] but reseeding this stand will not necessary improve profit potential because seeding late tends to produce lower yields [2 through 7] and may not mature before a fall frost.
       
    2. Insect pressure. Insect feeding can have a more dramatic impact on yield potential when plant densities are low because the insect population is spread over fewer plants. At low plant densities, it takes fewer plants lost or damaged to have a more dramatic impact on yield. Therefore, the lower the plant density, the fewer insects needed to reach the action threshold for control. [8] For this reason, crops with low plant densities will need more frequent and intensive scouting for insects at all plant growth stages.
       
    3. Weed pressure. Because canola is not a fierce competitor until it covers ground, weeds can dramatically reduce yields through competition with the crop for light, moisture and nutrients. This competition reduces canola plant growth and leaf area resulting in increased flower, pod and seed abortion. Depending on weed type, density and stage of development, weeds can reduce canola yields by 5% to over 50%. [9]

      Weed management is essential to preserve the yield potential of a thin stand. A thin stand takes longer to fully cover the ground, giving weeds more room to grow and compete. Sequential applications of system herbicides may be required until full canopy closure is achieved.
       
    4. Fewer plants mean bigger, later plants. At recommended plant densities, canola plants normally produce 3 to 5 secondary branches, not including the main stem. At low densities of 20 to 30 plants per square metre (2 to 3 plants per square foot), canola plants will branch up to 4 times more than those within plant stands of 70 to 140 per square metre. This extra branching can delay seed maturity up to 21 days depending on environmental conditions. [10]

      Extra branching also calls for careful harvest management. Swathing when seed color change is at 50% to 60% on the main stem will reduce quality concerns and improve yield potential on areas with marginal increases in secondary branches (2 times the normal). If there is a substantial increase in secondary branches (3 to 4 times the normal), then the main stems will represent a smaller proportion of total yield, so it will be more important to evaluate the whole plant to determine maturity. Look for an average of 30% to 40% seed color change total for the whole plant. In this case, the main stem may have seed color change well above 60%.  With normal stands (50 plants per square meter and higher), the main stem and the primary branches are the major contributors to yield but as populations thin, the main stem becomes a minor contributor and the majority of seed will be on the primary and secondary branches. Make sure as much of the immature seed in the side branches as possible is at least firm and dark green to help reduce yield and quality losses.


    Is reseeding necessary?

    Each year fields or portions of fields suffer from poor emergence or from damage after emergence by frost, hail or insects. The crucial question to answer is whether reseeding will result in greater net profitability without significantly increasing risk? To aid in this decision, carefully consider the cause and severity of the damage, the percentage of the field affected, soil moisture, weed competition (amount, stage and type), reseeding costs and calendar date.

    An accurate assessment of crop injury is essential. Take the time necessary to assess plant survival. Damage may not be as bad as it seems at first. And yield potential of the crop may be better that you think. Canola seedlings damaged by frost, wind or hail need several days to recover before accurate assessments of survival can be made. Severe damage to cotyledons and true leaves that cause yellowing, browning or blackening of outer leaf tissue does not mean that seedlings are dead.

    If the growing points and hypocotyls (the stem from the seed to the above-ground growing point) remain intact and turgid, those plants should survive. Recommendations are to re-assess those plants 4 to 10 days after the damage has occurred. At this time, small leaves will begin to emerge from the growing points, indicating that the plants will survive. If no new leaves emerge, survival is unlikely.

    With successive nights of frost, the recovery period may be one week or more from the initial injury before new leaves begin to be noticeable. Seedlings that have been mortally injured will have either darkening of the growing point itself or pinching and browning of the stem below the growing point. In some cases, only the top of the stem is pinched causing the top to lean. Look for new growth out of the growing point several days after the last damaging event. In other cases, the whole stem withers and the seedling collapses on the ground. If that occurs, the plant is likely dead.

    Even if many plants have been killed, reseeding may not be the best response. During early growth, most canola stands can sustain substantial seedling mortality without significantly affecting yield due to compensatory branching of surviving plants. Make random plant density counts from the damaged field. The larger the area of damage, the more counts made.

    Ensure the majority of the field contains the minimum plant number that can produce an acceptable yield. If not, then reseeding part of the field or the whole field may be the best economical response.

    As a reasonable guideline, plant densities of 20 to 40 plants per square metre (2 to 4 plants per square foot) can be adequate to produce a viable crop, provided weed competition can be effectively controlled.

    Thin stands such as these probably will yield 90% of a normal stand seeded at an early date, and could mature later. This is still an advantage over crops replanted late in the season, which typically yield less than earlier plantings with poor stands. Although some reseeded crops manage to yield the same as very thin crops not reseeded, they incur more costs due to reseeding and, therefore, result in lower net returns.

    Canola Council of Canada agronomists collected the following two examples of crops with thin stands that were not reseeded:

    • Frost hit May 23, 2004, in a canola field seeded April 29. In this particular case, half the field was reseeded on May 25 and was compared to the original frost-damaged portion of the field. The original frost-damaged half the field yielded 2,570 kg/ha (45.8 bu./ac.) compared to the reseeded half which yielded 2,140 kg/ha (38.4 bu./ac.)
       
    • Farmers in central Alberta experienced a record string of killing frosts in late May to early June 2000. Seedling canola was severely injured by these frosts and significant reseeding occurred. However, many growers overestimated the mortality and probably reseeded unnecessarily. In one case, a grower requested the local extension crop specialist to look at a frozen canola stand. The field was seeded in early May to a Roundup Ready B. napus canola, had good straw cover and suffered severe visible damage from the late spring frosts (Figure 3).

    The grower was not sure if there were enough survivors to produce a reasonable crop and had already reseeded another damaged field. Close inspection of the damaged seedlings indicated that the surviving density was on average 32 to 43 plants per square metre (3 to 4 plants per square foot) and about 11 plants per square metre (1 per square foot) in the worst areas. He was encouraged to leave the field considering the fact that good weed control could be obtained with the herbicide-tolerant system.

    Continuing frosts hampered the recovery of the canola seedlings but eventually new growth appeared after 10 days. The canola stand began to recover slowly over the next few weeks but initially looked quite poor (Figures 4 and 5). By flowering, the stand began to fill in but differences in maturity were evident between areas that suffered different amounts of stand thinning (Figure 6). The stand continued to improve through flowering and pod fill (Figures 7 and 8).

    The crop matured in September with the thin areas maturing later than the better areas by one to two weeks. Plant counts at harvest averaged 43 plants per square metre (4 plants per square foot). The thin crop that was questioned in spring eventually yielded 2,128 kg/ha (38 bu./ac.) gross with 2% to 3% green. Although this crop probably would have yielded higher if frost had not occurred, the yield was satisfactory and equivalent or better than reseeded B. rapa canola. 

    References

    [1] Steven Shirtliffe, University of Saskatchewan, “Determining the economic plant density in canola,” 2009. Prepared for Saskatchewan Canola Development Commission. Meta-analysis of 35 research projects with data on canola plant population density and its effect on yield.

    [2] Data from the Manitoba Agricultural Services Corporation show that from 1998-2008, canola seeded in the first two weeks of May had higher yields than canola seeded later. The only exception in Manitoba was the Interlake region, where canola seeded the second and third weeks of May had higher yields than canola seeded the first week of May.

    [3] AFSC Alberta crop insurance data for 2001-10 show that canola yields start to fall after the middle of May. The graph is a ratio of the average yield on the day to the average yield for the year.

    [4] Kondra, Z.P., 1977 “Effects of planted date on rapeseed.” Can. J. Plant Sci. 57: 607-609

    [5] Degenhardt, D.F., and Kondra, Z.P.,  1981 “The influence of seeding date and seeding rate on seed yield and yield components of five genotypes of Brassica napus.” Can J. Plant Sci., 61: 175-183

    [6] Kirkland, K.J., and Johnson, E.N., 2000 “Alternative seeding dates (fall and April) affect Brassica napuscanola yield and Canadian Journal of Plant Science quality.”  Vol. 80 No. 4 pp. 713-719

    [7] Canola Council of Canada Canola Production Centre (CPC) trials in the late 1990s compared yields for fields seeded early, mid and late May. Seeding within the first 10 days of May produced the highest yields 70% of the time. Overall, the CPC trials found that canola seeded in late May yielded about 88% of canola seeded in early May.

    [8] Canola Council of Canada Canopy Manipulation Trial, 2002, Yorkton, Sask. In this trial, canola experienced moderate to severe flea beetle pressure. Treatments with an average of 20 to 30 plants per square metre (2 to 3 plants per square foot) reached the action threshold of 25% defoliation 8 days after emergence compared to treatments with 80 to 90 plants per square metre (8 to 9 plants per square foot), which reached the action threshold at 20 days after emergence.

    [9] Personal comment from John O’Donovan, AAFC

    [10] Canola Council of Canada Canopy Manipulation Trial