Canola Watch: lygus | cost of early swathing | assess maturity in uneven fields

Plan now for pre-swath disease scouting. Prior to swathing is the ideal time to assess the crop for numerous plant diseases. In this very short video, CCC agronomy director Clinton Jurke explains how to complete a canola disease survey and what questions the results help answer.

What is the yield cost of swathing too early? 

Except in cases where the crop is very thin, swathing is often recommended as the best and least risky option for uneven stands. Avoid losing yield or quality by swathing too early: once canola is swathed, the seed does not continue to fill. Swathing should only be done when as much of the crop as possible is a minimum of 60% seed colour change (SCC) on the main stem (or highest yielding portion of the plant in cases where the main stem was compromised due to early season stress). A Canola Council of Canada study from the early 2000s showed canola swathed at 30-40% SCC on the main stem yielded at least 8% lower than canola swathed at 50-60% SCC, and 11% less than canola swathed at 60-70% SCC. With shatter resistant cultivars, the need to swath early is reduced (click here for shatter ratings on current cultivars).

Swathed canola under cool, wet conditions can be ready to combine earlier than standing canola, but only in some cases. Canola swathed green cures much more slowly than canola swathed at 60-70% SCC or later. Keep in mind that there is a big difference between maturity and dry-down.  (What to do with uneven fields) (Scenarios: swath or straight combine) (Swath later for higher yield) (Swathing guide) (Harvest guide)
 

Are you double checking the spray to swath interval every time?

Before applying any pesticide post-flowering, check that product’s spray to swath interval, which is the number of days that must pass between a pesticide application and swathing or straight cutting. The required pre-harvest interval is critical to maintaining canola’s marketability. Keep it Clean’s Pre-Harvest Interval Calculator makes this process easy: simply enter the intended product and crop type into the calculator to calculate when the crop can be cut or desiccated after spraying, or enter the intended application timing and crop harvest date to generate a list of acceptable product options.
 

What went wrong? What went right? What can be done now?

A successful canola crop is the sum of countless environmental factors and management decisions. Be a student of the crop:

• Scout, scout, scout for developing insect issues and disease concerns. Assess: were pest management decisions to date (including sclerotinia fungicide decisions) correct?
• At flowering and early podding, ask: what could be done differently at seeding, in fertility, in early weed control, etc. to achieve a better result? Patchy, uneven stands likely highlight areas that could be improved earlier in the season (especially seeding).
• If you are tempted to try a ‘rescue product’ for drought stress or hail damage, do the TWO check strips you left show return on investment? (*Note: there is no published evidence that any ‘miracle cure’ achieves any difference in the crop, so two check strips helps decrease the likelihood of incorrectly attributed positive results).
• While a nutrient-short crop might benefit from additional nutrients, think 4R: does a foliar in-season application achieve right source, right rate, right timing, right placement? Could a mid-season soil test help? Did you leave a check strip… or two? 

 While it’s difficult, some optimism should remain for a canola crop that is currently suffering significant drought stress. Canola is highly plastic. Late moisture, especially on crops with flowers remaining, may still allow canola to produce more pods, fill more pods or simply make larger seeds in a manner that is much more flexible than most other crops.
 

How to manage lygus?

Researchers have made significant progress in understanding lygus bugs’ damage potential since the insects plugged swather screens in Albertan canola fields in the mid-1990s. Rather than the older recommendation to do 10 x 180° sweep net sweeps in each of 15 sample locations, Hector Carcamo (an entomologist at AAFC Lethbridge) says two paired locations (4 total locations) provides as accurate a population assessment more than 90% of the time.

Immature lygus – which are prone to dying in hard rains, are predated by beneficial insects, and don’t have large enough “piercing” mouthparts to damage canola pods – should not be counted in sweep counts. Only count lygus nearing or at adulthood: at minimum large enough to have black dots on their backs or wing pads starting to appear.

Scout first as the crop is coming out of flower, then re-scout regularly to confirm if you should be concerned about lygus numbers. Lygus thresholds have been increasing over time with new research. While current thresholds are 20-30 lygus per 10 sweeps, growers willing to increase protection of natural enemies can choose to take action when the population is over 30 lygus per 10 sweeps. Carcamo’s newest (unpublished) research may even suggest a higher threshold. He also notes that “15-17 lygus in 10 sweeps may have a yield-boosting effect on canola”.

Lygus populations and maturity are in a race relative to the maturity of a canola crop. As the crop matures and seeds begin to be firm to roll, the potential damage from Lygus sucking likely decreases daily. A late-maturing crop with a high lygus population has much higher potential for damage than a crop where the seeds and pods become leathery earlier in the season. Ideally, canola seeds mature quickly enough to produce large plump seeds, but early enough that lygus that are present focus on late green raceme ends that aren’t likely to produce yield. Heat will not only speed up canola maturity, it also speeds up the lygus lifecycle. (Lygus bug scouting, timing and thresholds)
 

How to assess maturity in uneven fields?

Uneven emergence and challenging growing conditions mean plant staging is patchy and/or variable in many canola fields. Maximizing profitability depends on accurately assessing maturity and determining where the bulk of the yield will be. Starting 10 days after flowering ends  in the mature patches and regularly after:

1. Segregate the field into ‘zones’ of obviously different maturity.
2. Within each maturity zone, select a 10×10 meter assessment area that appears representative of that zone’s average maturity.
3. Starting in the zone that is most mature, compare the number of pods on the main stem versus side branches.
4. Open pods from both the main stem and side branches to assess colour change and firmness. If swathing, this should wait until the majority of the crop is at 60% seed colour change or even later with pod shatter traits.Seed colour change will typically increase 10% every 2-3 days but can be faster in hot and dry conditions.
5. Note or GPS the sample location to enable repeated sampling.
6. Follow the same assessment procedure for less mature areas of the field to assess overall readiness for swathing and/or determine which portions of the field should be managed separately.

(Harvest management – timing, quality and yield)