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Soybeans: Low Temperatures and Maturity

Forecasted overnight temperatures across the province are expected to dip as low as 7°C. In some areas this morning, temperatures dipped down to 4°C. When overnight temperatures dip below 5°C, there is a risk for a light frost in low-lying areas.

While there’s little risk for a killing frost (-2.2°C) with these forecasted temperatures, low overnight temperatures can impact seed fill and maturation. Cool temperatures (<10°C) during seed filling stages can result in reduced seed size and delayed maturity. Optimal temperatures for soybean ripening are 19 – 20°C and minimum temperatures for seed ripening are 8 – 9°C.

Some soybean fields are only entering R6 (full seed) stages this week. On average, soybeans will spend roughly 20 days at the R6 growth stage. With these low temperatures, we can expect this period to be extended even longer, putting soybeans at risk of experiencing a frost in September.

• Soybean Maturity and Low Temperatures →

Fall Frosts: Risk and Yield Impact

On average, the first fall frost occurs between Sept. 9 to Sept. 24, depending on region. One in four years, a frost may occur as early as Sept. 1 to Sept. 16.

Light frosts (0-1°C) may kill top leaf growth but generally won’t penetrate the crop canopy. Plants will continue to mature, but will be delayed and green seeds will occur within pods that were frosted. Hard frosts (<-1°C) will cause damage to green stems, pods and seeds, reducing yield and quality. When entire plants are killed, seed fill stops.

Factors that influence frost severity:

• Duration of cool temperatures – longer duration of freezing temperatures can cause more damage.
• Soil moisture – moisture in the soil will retain heat.
• Canopy thickness – narrow, thick rows maintain heat longer than wide rows.
• Wind speed – stronger wind can reduce the severity of frost.
• Cloud cover – more nighttime cloud cover can reduce plant damage.

R5-R6 (Worst case scenario) – Green and immature beans will shrivel and remain green. Up to 50% yield loss at R6.
R6.5 – Green-yellow beans will have a mixture of green seed that will not mature and yellow seed that will mature. Up to 30% yield loss at R6.5.
R7 – Mature, yellow beans will continue to dry down slowly with minimal yield and quality loss. Considered ‘safe’ from frost and less than 10% yield loss expected at R7.

• Impact of Early Fall Frosts on Soybeans →

Dry Bean Desiccation Timing

Dry beans are ready for desiccation once they’ve reached R9 (full maturity). At this stage, plants will have:

• 80% pod colour change,
• 80-90% leaf drop, and
• <30% seed moisture in the least mature parts of the field.
  • What does that mean? At <30% seed moisture, seeds will rattle within the lowest pods, upper pods will be yellow and the seeds within the upper pods will have lost their green colour when split.

Download the Dry Bean Desiccation and Harvest Guide →

Consult with you buyer about the best product option and potential product limitations. Many bean dealers are no longer accepting pre-harvest glyphosate on dry beans.

  

Dry Bean Harvest

Dry beans may be harvested by undercutting and windrowing, swathing or straight cutting. Undercutting is typical for row-cropped beans and vine-type varieties that pod low to the ground. Straight cutting or swathing is more common for solid-seeded beans and bush-type varieties with higher pods. Straight cutting also requires flat soil that has not been inter-row cultivated.

Dry beans are ready for harvest when:

• All leaves have dropped
• All pods have changed colour
• 75% of pods are dry and the remaining pods may be flexible and yellow with seeds that have fully changed colour (R9 stage)
• Seed moisture is 16-18%

Harvesting Tips to Improve Dry Bean Quality:

Tips to reduce soil and smearing:

• Screen kits at the header and under the feeder house will help eliminate soil.
• Windrowing beans that have been undercut will help remove any soil clinging to the roots.
• Increasing fan speed can help if the soil is dry.
• For soft, puffy immature beans, smearing can occur when the moisture combines with the soil entering the combine. Waiting until these beans dry down further in the field is one option, but if you can keep these beans from earth tagging, they will dry down normally in the bin.
• Using a conveyor mounted on the combine to unload can also help reduce smearing.

Tips to reduce splits and cracks:

• Reduce cylinder speed. Starting speeds range from 300 to 600 rpm depending on bean type and harvest conditions. Reduce speed in 100 rpm intervals until you see a noticeable difference in cracks.
• If this doesn’t work, try closing your concave to thresh beans out quicker. A more open concave will allow beans to flow further into the threshing system, interacting with the rotor and cracking beans.
• Install a wide-wire concave. This allows beans to flow through the concave faster.
• More material entering the combine through larger windrows or a wider cutting width will also help cushion beans.
• Harvest when plants are dry so pods open more easily as they move through the cylinder and concave. Harvesting damp plant material may not open as quickly, resulting in more cracks and splits.

Fall Rye Ahead of Next Year’s Dry Beans and Soybeans

• Properly managed, fall rye as a cover crop ahead of dry beans and soybeans can protect soil from erosion and minimize weed pressure while maintaining yield potential.
  • Dry beans are planted late in spring, resulting in a long window of bare soil from the time when snow melts in the spring to when the crop goes in the ground.
• Rye termination must be carefully timed in the spring to maintain yield potential during plant establishment under dry conditions.
  • Under relatively dry conditions from 2018 to 2020 at Carrington, ND pinto bean yields following a fall rye cover crop were statistically similar to the check when the rye was terminated with glyphosate 17-36 days before planting. Find the quick summary of Dr. Greg Endres, NDSU’s research on fall rye preceding soybeans and dry beans. Or, read the more detailed summary of managing rye as a cover crop.
  • Recent MB research evaluated termination timing of the fall rye cover crop in the spring. In this experiment, soybean yield was not different among the control, plots terminated 14 days before planting, terminated 4 days before planting or terminated 1 day after planting. To conserve moisture, terminate the rye earlier in the spring, to use more moisture, let it grow longer.

Soybean Aphids and Insecticide Resistance

Pyrethroid-resistant soybean aphids were confirmed in Manitoba and several northcentral U.S. states (North Dakota, South Dakota, Minnesota, Iowa) in 2017. Resistance to lambda-cyhalothrin and bifenthrin has also been documented in other regions. Soybean aphids don’t overwinter well here and populations typically blow in from more southern areas of their range. In some years, they bring with them insecticide resistance.

If fields exceed thresholds (250 aphids per plant and increasing), make sure insecticides are applied correctly. Use recommended label rates, proper nozzles, spray volume and pressure, and spray under favourable conditions.

Following applications, scout fields again after three to five days to ensure management occurred. If control did not occur, try to rule out other potential causes for insecticide failure. If another application is required, use a different insecticide group.

Once soybeans reach R6 (full seed) stage, soybean aphid damage is minimal and insecticide applications are no longer considered economical.

• Managing Insecticide-Resistant Soybean Aphids →

Waterhemp

Waterhemp that has been found in Manitoba has been resistant to combinations of groups 2, 5, 9 (glyphosate) and 14. With soybeans and dry beans, it is easy to spot pigweeds poking up above the crop canopy at this time of year.

Check up on those pigweeds you’re seeing in the field. If their stems are rough and hairy, it’s likely red root pigweed. If stems are smooth, it’s time for more testing. You can collect plant samples and have them tested at the PSI lab in Winnipeg or contact Manitoba Agriculture for next steps.

Waterhemp:

• Emerges throughout the growing season, typically after most other summer annual weeds have been sprayed.
• Flourishes in row crops due to more sunshine in the inter-row spaces.
• Produces 250,000 seeds per plant, and individual plants may produce more than one million seeds.
• Is a Tier 1 Noxious Weed, meaning all of the plants must be destroyed.
• Seeds can move with water, so scout along drains, creeks and in previously flooded areas.