When does a killing frost no longer damage soybean seed?
Late planted fields or double cropped soybeans may still be green in late September or October. A killing frost will not significantly harm seed quality once the field is at the R7 (beginning maturity) growth stage. R7 is reached when 50% or more of the plants in a field have a pod that has matured and turned brown or tan. Most leaves and pods will have turned yellow by this stage. When looking at seeds inside the pod, R7 occurs when the pod wall interior membrane is no longer attached to the seeds. When opening the pod this white membrane stays attached to the pod wall not the seed. See Figure 1. This indicates the seeds are physiologically mature and safe from frost injury. The pod membrane status is a visible indicator that soybean seed has reached physiological maturity (maximum dry matter achieved). This stage is equivalent to “black layer” in corn.
Purple Seed Stain
The amount of purple seed stain is increasing in some parts of southwestern Ontario. It can be a challenge for IP growers and fields grown for seed production. This fungal disease is seed-borne, so subsequent crops will be infected from diseased seed. The inoculum also overwinters in soybean residue so it’s more prevalent in fields with a poor crop rotation. Severely discoloured seed will have lower oil and protein content, but the main problem resulting from this disease are the purple stains left on the seed. See Figure 2. Planting clean seed that is treated with a fungicide will greatly reduce the problem in subsequent crops. There are also varieties with greater tolerance, so proper variety selection can be important in the management of purple seed stain.
Phomopsis Seed Decay
Phomopsis seed decay is the most widespread seed quality issue in Ontario, especially if harvest is delayed. Symptoms include seed that is cracked, shriveled, lightweight, and has a chalky white appearance. See Figure 3. This disease is favoured by warm, wet, and humid weather during the pod-fill and maturity stages.
Uneven Maturity at Harvest Time
A field that has many green patches or individual green plants can delay harvest. Sometimes the cause is obvious, such as late emerging plants. See Figure 4. This is a difficult problem to address, but planter units that place the seed at a more consistent depth will have more even emergence. Some growers report they can start harvest a few days earlier when using a planter compared to a drill because the crop matures more evenly.
When individual plants remain green, it’s usually a result of having fewer pods or pods with no seed. These abnormal pods are called parthenocarpic (seedless). Drought, herbicide drift, viruses, or in many cases a genetic mutation will cause plants to have little seed and remain green. When seedless pods are present, something has adversely affected normal pollen development. Stems and even leaves stay green because the plant has produced more carbohydrates than the number of seeds on that plant demands. This results in excess photosynthate in the fall so dry down is delayed.
If the whole plant has seedless pods from top to bottom, that usually means there was a genetic mutation in that plant (or in a previous generation that expressed itself this year). Those plants will show up as the odd plant here and there, but make up only a small percentage of the overall crop. This mutation is more prevalent in certain varieties and may only be evident in dry years.
If there are specific areas in a field that remain green, that often means an environmental factor was the cause, not a genetic mutation. Herbicide drift or drought may have impacted normal development, resulting in fewer pods per plant. Insect feeding and viral infection may also cause plants to remain green. In those cases, field edges are more impacted. Fields with low levels of P and K will not mature as quickly or evenly as fields with good fertility. This difference in senescence in the fall is also due to a lack of pods and seeds per plant in the low fertility areas. Fields with uneven maturity at harvest should be soil tested to see if additional fertilizer is needed in future years.
Weather Data – September 11 – 18, 2022
|Location||Year||Highest Temp (°C)||Lowest Temp (°C)||Rain (mm)||Rain (mm) April 1st||GDD 0C April 1st||GDD 5C April 1st||CHU May 1st|
|||10 YR Avg. (2011-20)||23.3||10.8||8.7||509||3008||2156||3424|
|||10 YR Avg. (2011-20)||23.1||8.6||8.2||462||2836||1988||3187|
|||10 YR Avg. (2011-20)||22.6||9.4||8.7||485||2808||1964||3162|
|||10 YR Avg. (2011-20)||22.9||9.3||15.1||432||2827||1982||3190|
|||10 YR Avg. (2011-20)||21.5||6.6||10.2||480||2537||1707||2821|
|||10 YR Avg. (2011-20)||21.2||7.6||8.9||490||2527||1704||2845|
|||10 YR Avg. (2011-20)||22.1||5.3||13.2||430||2567||1735||2813|
|||10 YR Avg. (2011-20)||22.4||7.4||16.7||483||2698||1869||3011|
|||10 YR Avg. (2011-20)||20.6||5.4||22.1||435||2200||1454||0|
|||10 YR Avg. (2011-20)||21.3||7.1||12.6||457||2360||1591||2680|
|||10 YR Avg. (2011-20)||19.8||4.7||16.1||422||2117||1358||2283|
|||10 YR Avg. (2011-20)||19.8||4.6||14.5||421||2242||1465||2420|