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Troubleshooting low emergence and assessing plant stand densities in canola, flax, and mustard

By Matthew Bernard, PAg, Provincial Specialist, Oilseed Crops

Assessing emergence and plant stand densities

Canola seedlings
Poor emergence of canola seedlings due to
deep seeding in dry conditions.
Photo credit to Canola Council of Canada.
Take plant counts after emergence is complete, which is about 10 days after seeding canola and mustard or seven to 14 days after seeding flax, under average conditions. Count plants in a square foot (ft2) or square metre (m2), and conduct counts at several locations (and away from the headlands) in each field to get a better idea of what is happening across the field. Yield potential is significantly reduced once plant stand densities fall below 30 plants/ft2 (300 plants/m2) in flax, four to five plants/ft2 (40 to 50 plants/m2) in canola, or below seven plants/ft2 (about 70 plants/ m2) in mustard. But remember to compare the counts in the field with the expected targeted stand densities that you decided on when seeding that field (repeat these counts again in the same fields at harvest to know if a significant number of plants have been lost through the season). If the average plant stand densities are lower than expected, there are several causes to consider when troubleshooting.

Seed quality

Seed quality will impact emergence. Information obtained from a seed test for germination, vigour and plant pathogen levels prior to seeding will help guide seed treatment application and seed lot selection decisions and will be useful for troubleshooting later in the season. When selecting seed, use certified or hybrid seed that has a high germination rate and consider submitting a sample to a seed testing laboratory for a disease test. Weathered, cracked, or diseased seed, as well as seed with residual herbicides, can reduce germination rates. Even if germination is okay, seedlings might have injured root tips, broken cotyledons, split hypocotyls, trapped radicles, or distorted roots. Shiny black flax seed could be an indication that the seed was frozen, which can result in reduced germination rates. If seed quality is suspected to be the cause of low emergence, refer to your spring seed test or submit a sample of seed (from the lot used for seeding) to a laboratory to test germination rate, seedling vigour, and the presence of pathogens.

Keep good records to be sure there was not an error in seeding rate or depth. Seeding too deep, whether by accident or to chase moisture, especially with flax, can reduce emergence rates and delay emergence, even if germination is normal.

Temperature and frost

Field conditions prior to and at seeding will influence emergence. Zero-tillage operations should be particularly diligent, as patchy residue distribution will cause soil temperature variation and reduce emergence uniformity. Seeding early into cooler soils can delay or reduce emergence. Check your soil temperature records from seeding when troubleshooting. Shallow roots make flax relatively susceptible to extreme soil temperature changes; however, temperatures as low as -3 C (for cotyledons) to -8 C (two leaf) are tolerable in flax, if they have hardened off. Canola seedlings may show damage after exposure to temperatures around -3 C to -4 C, but environment, growth stage, and rate of growth can all influence frost susceptibility in each of these crops. Mustard will still germinate in soil temperatures as low as 4.4 C, and seedlings are relatively more tolerant to late spring frosts than either canola or flax seedlings.

Frost-damaged seedlings may have browning at the stem, darkening of the growing point, or pinching on the stem or below the growing point, or, in severe cases, may cause the plant to collapse. Light frosts causing wilting might cause yellowing or whitening. Damage at the growing points will be most destructive. It is important to wait several days before assessing the actual severity of the frost damage, as visible signs of recovery may take a week or more to appear.

Moisture

Extremely dry conditions can increase the risk of carryover of herbicides from previous applications; be sure to check your field records and product labels for restrictions, particularly when dry conditions exist (extra precautions might be required in extremely dry conditions). If this is a potential cause of low germination, consider submitting a soil sample to a qualified laboratory for herbicide residual testing. Maintaining a firm seedbed, reducing tillage and using on-row packing can help maximize soil-to-seed contact while keeping moisture close to the surface. Rain in the middle of a dry period can result in crusting and, when this occurs early enough, can physically obstruct emergence. Flax is relatively sensitive to moisture levels due to its shallow root system. Poorly drained soils can therefore reduce emergence, resulting in low or patchy germination. In extremely dry conditions, higher seeding rates in flax can have a negative influence, as competition (per seed) for limited moisture is increased. Mustard is relatively more tolerant to extremely dry conditions than other oilseeds, but will still be negatively affected by extended dry periods, especially in sand or sandy loam soils. Making sure crop residue has been distributed evenly through the field improves even moisture distribution and will also contribute to improved emergence uniformity.

Fertility

Soil testing is important to optimize fertility with all crops. Excessive fertilizer other than starter phosphorous applied at a safe rate placed with or very near the seed, especially in dry conditions, could negatively impact emergence. Starter phosphate is essential for establishing uniform crops whether conditions are dry or moist. The relatively small size of canola, mustard, and flax make them particularly sensitive to seed-placed fertilizer.

Rotations can impact fertility management, as well; flax responds well to soil phosphorous, and similar to other broadleaf crops, does best following cereals.

Disease

Fusarium wilt, caused by Fusarium oxysporum, can cause yellowing/browning on flax seedlings and ashy-grey discoloration of roots. Seedling blight or damping-off, which can be caused by a number of plant pathogens, will cause seedlings to turn yellow, wilt, and die; this might be seen in individual plants, rows, or entire patches. The base of the stem may have the most obvious symptoms. Early infection can may cause red-brown root lesions. Conditions that slow plant growth and favour pathogen infection will increase the risk of these diseases. If the disease is unknown, send a plant sample to the Crop Protection Lab for diagnosis.

Yellow plants may also be caused by fertility or herbicide residue issues, so consider the pattern of damage and review past fertilizer and herbicide application records, as well.

Diverse crop rotations are important in minimizing disease risk and pathogen pressure within a field. Continuous cropping of one crop may result in higher disease incidence and severity levels, which can also cause poor emergence.

Insects

The lower the plant stand density, the more severe the impact of insect feeding. A population of insects will have fewer plants available for feeding, therefore the damage will be relatively higher than in a field with higher plant stand densities. Emergence might be patchy. Fields expected to have higher insect populations can be seeded at a higher rate, but crops with lower stand densities should be scouted more frequently and intensively to ensure action thresholds for control are caught at the right time. Flea beetles are more active in dry, warmer conditions, and seedlings are at the most risk in the first two weeks after emergence. If defoliation is not obvious due to lower insect populations or cooler, moist conditions, still check the underside of cotyledons or near the soil surface. Cutworms may fully or partially sever the plant near the soil surface, and damage may be more pronounced near hilltops.

Insect scouting charts and other information such as action thresholds to help guide insecticide application decisions can be found on the Prairie Pest Monitoring Network Blog. If you are unsure if above-ground symptoms are caused by insect or disease pests, also examine roots for additional symptoms or consider submitting a sample to the Crop Protection Lab.

Weeds

Poor weed control in the spring will stunt the crop. Poor sprayer tank clean-out or incorrect order of mixing products can negatively affect their efficacies or cause crop damage. Improper timing of broadleaf herbicides will also damage the crop; be sure to check product labels. Information on timing and the weeds controlled by a herbicides can also be found in the 2019 Guide to Crop Protection.

Hail

Damage resulting from hail will likely be patchy throughout the field. Seedlings that do not survive are those with both cotyledons broken off, or where the stems have been snapped. Damage at the growing point will be exceptionally damaging. Damage can also result in opportunistic infection by plant pathogen, so note if there is higher incidence or severity of disease in patches that have been damaged by hail.

Wind

Strong winds can move seeds and even seedlings out of the ground; symptoms can also appear as sandblasting damage and can tip or break the plant. Damage will be more evident on hill slopes facing the wind, or hill tops, and may be patchy. Consider the entire field before reseeding decisions are made. Adopting zero-tillage can alleviate concerns of soil erosion by wind under most conditions.

Should you reseed?

Reseeding will not necessarily improve your yield potential or profitability. Aside from the added cost of seed, late-seeded crops typically have lower yield potential because the early-season advantage has been lost and maturity may be late enough to put the crop at risk of first fall frost. Due to the "plastic" physiology of canola, mustard, and flax, they can be forgiving crops, as they will increase branching to take advantage of lower competition from fewer neighbouring plants of a reduced plant stand. This can differ between varieties even within a crop type. In cases where an insurance claim is submitted with Saskatchewan Crop Insurance Corporation, make sure you are aware of their deadlines.

Poor emergence could be due to many causes. It is important to know the numbers in the field, to consider the net profitability of taking a hit with the current crop, or accepting the risks and costs of re-seeding.

For additional information, sign up for the Canola Council of Canada's CanolaWatch newsletter or try their plant survival calculator, see SaskFlax's producer resources or sign up to receive the "Flax on the Farm" newsletter, see SaskMustard's Production Manual, or contact the Ministry's Crop Protection Lab at 306-787-8130 for information on diagnostics.

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