Early season, The Bean Report

May 20, 2015

Seeding and weather update

Across the province, approximately 20-25% of soybean acres have been planted. The majority of these acres were in the eastern region in early May despite cool soil temperatures. Dry bean planting is yet to fully get underway in Manitoba and was 2% complete in North Dakota as of May 10. H

istorically, soybean seeding progress to the third week of May has been 68% (source: MASC 2008-2012). We are behind normal but there is still plenty of time to plant soybeans as yield potential will hold until the end of May. Soybeans planted April 27-May 2 are showing radicle growth of 2” (pictured above) but no emergence has been observed. This was good for protection from frost but the prolonged emergence period increases susceptibility to insects and disease as seed treatment wears off. Seed decay and reduced emergence in wet soil conditions is also a concern.

Soybean planting resumes this week in the northwest region, where the lowest rainfall amounts were received from May 11 to 18. The rule of thumb is to begin soybean planting when soil temperature at the desired seeding depth (0.75-1.5”) reaches 10°C at 10 am. Soil  temperatures are expected to climb rapidly in the next few days and given the calendar date, planting will resume as soon as field conditions allow in all areas.

Cool and wet describes spring weather so far (Table 2). Generally, soybeans require 150 GDD to reach 90% emergence, which has not been reached.

Yield Potential of Soybeans Planted in Late May in Manitoba

Long-term crop insurance data indicates that the yield potential for soybeans generally holds at 100% in Manitoba until the last week of May. At this point, yield
potential may decline to 85-95% for central and eastern Manitoba (Table 3).

That being said, on-farm seeding date studies at Carman have shown soybean yield to remain within 1-2 bushels of the highest yield when planted up until the end of May. Throughout May, farmers should continue with soybean seeding plans. If planting is delayed to June, consideration should be given to increased seeding rates and narrower rows in order to reach canopy closure and increase plant height.

Potential concerns for early-seeded soybeans

1. Flooding: Seed germination and seedling weight can be reduced significantly by prolonged periods of flooding. Research in controlled conditions has shown that flooding for 48h beginning 3 days after the start of water uptake reduced germination by over 40% compared to a reduction of 20-30% when flooding occurred for 12-24h.

2. Soil crusting: The recent rains may lead to soil crusting which could inhibit soybean emergence. It is possible to perform a light harrow operation to break the crust for soybeans that are still underground

3. Seed and root rots: cool, wet soils favor seed and seedling diseases. This will be particularly important in water-logged fields with a frequent history of soybean. Seed treatments provide protection for 2-3 weeks and this protection begins at seeding, not emergence. For a detailed description of early season root rot pathogens,
see below.

4. Reduced plant stands: The factors listed above (flooding, fungal pathogens) can lead to reduced emergence and plant stands. It will be important to assess the plant stand and yield potential in each field. Soybeans compensate remarkably well for lower plant stands and re-seeding is generally not required. For example, even at 80,000 plants/ac, 84% of optimum yield is expected (Figure 3) and re-seeding is rarely profitable. With reduced plant stands, timely weed control and lower pod height should be kept in mind.

5. Frost damage: Soybeans that have not emerged are protected from freezing temperatures but emerged soybeans are at risk to frost damage because the growing point is above ground. At the cotyledon stage, they are fairly tolerant to –2°C for a short period of time. Once true leaves emerge, they are more susceptible. Always wait 3-5 days after a frost event to assess damage. For soybeans, even if the main growing point is lost, re-growth generally will occur from axillary buds (Figure 3). More information on assessing frost damage to crops is available here.

6. Herbicide injury: Wet soil may increase activity of some soil applied herbicides, leading to crop injury.

Target plant stand and seeding rates for soybeans

To determine your seeding rate for soybean, you must first identify your target plant stand.

From 2011-2013, a range of soybean seeding rates from 80-200,000 seeds/ac were tested at 8 locations across Manitoba in both a wide and narrow row system. Figure 4 illustrates the final results from this study where the actual plant stand (live plants/ac) was plotted against relative yield. This relationship accounted for nearly 70% of the variability observed in yield data. The quadratic relationship reveals important information on what soybean plant stand we should be targeting:

1. The maximum yield is achieved at an established plant stand of 160,000 plants/ac

2. Plant stands higher than 160,000 plants/ac do not result in increased yield

3. Actual plant stands of 80K, 120K and 140K produced 84, 95 and 98% of optimum yield.

Here at MPSG, we combined this data with soybean price, cost of seed and expected yield to determine the economically optimum plant stand. For 2015, the target
plant stand that will deliver the highest return has been identified as 140,000 plants/ac. But remember….

Target plant stand ≠ Seeding rate

From your target plant stand, you determine your seeding rate based on “expected seed survival”. In other words, how many seeds should you plant to ensure you
reach your target? Expected seed survival takes account for % germination and other factors affecting emergence:

  • Seed quality
  • Equipment and handling
  • Seedbed conditions
  • Pest pressure

Suggested Seeding Rates for Soybeans:
Air seeders: 190-210,000 seeds/ac
(based on expected seed survival of 70-75%)
Planters: 170-180,000 seeds/ac
(based on expected seed survival of 80-85%)

Early-season Seed and Root Rot Pathogens of Soybeans

Consider a PRE herbicide for soybeans and dry beans

PRE herbicides are a good foundation for dry bean weed control and will help slow or prevent the development of glyphosate resistant weeds in Roundup Ready soybeans. Glyphosate resistant kochia was confirmed last year in two Manitoba fields and giant ragweed, common ragweed or wild oat could be next. Glyphosate resistant ragweed is present in North Dakota and wild oat is predicted by Canadian weed scientists. Richard Zollinger from North Dakota states that pre-applied soil herbicides are predicted to be used on less than 10% of acres in North Dakota (likely similar or less in Manitoba) and provides several reasons for growers to consider using one:

1. Reduce early-season weed competition and provide residual control, protecting yield.
2. Lengthen the time before the first post-emerge herbicide application is required. [This provides flexibility if post-emerge applications are weather
3. Provides an alternative mode of action and reduces the number of plants present at the time of the in-crop application, reducing the likelihood of selecting for herbicide-resistant biotypes.
4. Reduce the size of weeds present at the time of the post-emerge herbicide application [thereby increasing efficacy].

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