Many canola fields are still a week away from 50% to 60% seed colour change — the ideal swathing stage — but damage from sclerotinia, hail or frost has some growers eager to get he crop cut. Early action is not usually the best for overall crop yield and quality. Step 1 before taking any action is to assess the level of damage.
Sclerotinia
In the case of sclerotinia stem rot, 2% of plants damaged can look like a lot from the road. But damage at these low levels is not enough to alter harvest plans. Growers are encouraged to swath as they normally would.
The key with sclerotinia infection is to determine where most of the yield will come from. If healthy plants will provide the most yield, then make harvest decisions that favour those healthy plants. Swathing at 50% to 60% seed-colour change is optimum for yield and quality.
If infected plants account for most of the yield potential, then swathing at 30% seed colour change may be more appropriate — as long as seeds in pods on branches and upper main stems are firm. Swathing early limits shelling of pods that are diseased but contain healthy seed. Sclerotinia fungus may continue to grow on swathed canola if conditions are wet, but seeds that have reached the firm green stage or later should still mature.
When swath-rolling diseased canola, do it lightly to tuck swath edges into the stubble. Light rolling limits pod shatter and reduces the spread of sclerotinia, which tends to move faster in compact swaths. If crop is heavily diseased, it may be preferable to avoid swath rolling altogether, particularly if the swath is heavy enough to settle into the stubble and does not appear at high risk for wind damage. Swath in the direction of prevailing winds.
Hail
Hail tends to damage top pods more than bottom pods. Since top pods are at later stages and typically have lower yield potential, don’t cut early to save these pods if seeds in these pods are still watery. Do what’s best for undamaged lower pods. That means swathing at 50% to 60% seed colour change.
An exception: If seeds in bruised top pods are firm when rolled between thumb and forefinger, then swathing immediately may save these top pods from shelling out. Seeds lower in the plant should be more advanced and also suitable for swathing.
Frost
Growers have two swath decision scenarios when it comes to frost: 1) Should a grower swath canola when frost is in the forecast? 2) Should a grower swath immediately after a frost?
Swathing canola in anticipation of frost only works when seeds are firm not watery, and when the swath has 3 good drying days before frost hits. This gives seed time to dry to below 20% moisture. At that point, frost damage to the seed will be minimal.
If light frost occurs before 3 good drying days, there is not enough time to have seed colour change. Frost can stop de-greening enzymes and lock in high green counts. Green seed levels can still be reduced after the frost event — but only if there’s enough time before the next frost and if there’s adequate moisture to rehydrate the seed.
When canola is fairly green and frost risk is only slight, there is more upside to leaving the crop standing. Swathing too early to avoid the risk of frost can often translate into yield and quality losses.
When to swath after a frost is more complicated. Assess fields one to 3 days after the frost, then make the harvest decision. Here are 4 situations and decision-making tips for each:
50% of the field has moderate to severe damage. Yield and quality will be significantly reduced. With severe damage, the canopy turns white, pods have a bleached and shrunken appearance, and seeds shrivel and turn white. If the remaining 50% of the field has light to minimal damage, swathing too early may further reduce yield and grade. Swathing when plants with minimal damage reach 50% to 60% seed colour change can allow the intact seed to continue to change colour and fill, improving both grade and yield. Anything severely damaged will likely shell out or be separated with the chaff or dockage.
More than 50-60% severely damaged. The crop will shell so it is best to swath to protect any viable seeds. Quality is likely to be poor anyway, so it is more important to protect as much yield as possible. If the grower decides to swath right away, the field in question should be one of the last fields combined to allow as much time as possible for green seed clearing.
Light to moderate damage in portions or throughout the field. Leave this for swathing at the proper stage. With moderate damage, pods will have white speckling on the outside and some seeds will turn brown and shrivel. However, pods remain reasonably intact and pliable and some seed remains green and turgid. Swathing when healthy seed is at 50% to 60% seed colour change gives healthy seed time to reach optimum yield and quality. Damaged seed will shrivel and blow out of the combine with the chaff or ending up as dockage. With moderate frost damage, growers will want to monitor the crop more closely than with other frost damage. If pods desiccate, they are prone to shattering, so consider swathing the whole field or just the affected areas if shattering losses could exceed gains from leaving the crop an extra day or two.
Some light damage in portions or throughout the field. Leave the crop and swath at the proper stage based on seed colour change of the healthy seed. Light damage may cause some seeds to turn “shoe polish” brown, but pods and most seeds generally remain intact and turgid. Swathing immediately after a light frost may result in higher economic and yield losses than if the crop were left alone.
