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Strategic Farming: Let's Talk Crops! Session Talks Pushing Soybean Maturities to the Max

By Angie Peltier, UMN Extension crops educator and Seth Naeve, UMN Extension agronomist, and Anibal Cerrudo, UMN visiting professor, Department of Agronomy and Plant Genetics

On January 31, 2024, Seth Naeve, University of Minnesota Extension soybean agronomist and Anibal Cerrudo, a visiting professor in the Naeve lab joined UMN Extension crops educator Liz Stahl for a wide-ranging discussion of pushing soybean yields to the max by adjusting maturity and planting date in Minnesota. This was the third weekly episode of the 2024 Strategic Farming: Let’s talk crops! webinars. The series runs through March.

 
Watch this episode and other recordings: z.umn.edu/SFrecordings

Planting date, relative maturity and growing season length are interrelated and all impact yield
Crop growth and development is driven by the sun’s energy. The earth’s tilt on its axis is responsible for our seasons. During a Minnesota summer, the earth’s northern hemisphere is titled toward the sun, while during the winter the earth is tilted away from the sun. Variation in solar radiation and temperature is responsible for our seasonality and the length of the Minnesota growing season. Soybean yield is maximized when one can plant as early as possible but within the window in which a spring, killing frost or a season-ending frost in fall is unlikely. Also key is planting a variety with a relative maturity long enough so that the most yield-impacting, critical reproductive portion of the soybean life cycle coincides with the year’s maximum solar radiation and temperature. The crop’s yield is maximized when it is able to take advantage of as much of the season’s light energy and temperature as possible before it reaches physiological maturity.

 
Cerrudo and Naeve are working to test the yield effects of soybean relative maturities and planting dates. This data is being used to develop a predictive model to help Minnesota soybean producers and agronomists to balance soybean yield potential and risks to the crop. Data inputted into the CROPGRO model includes solar radiation, air temperature, precipitation, soil profile and water holding capacity, planting date, plant population, soil fertility, irrigation and soybean relative maturity and after enough data is inputted, the model should be able to provide insights into crop developmental milestones, yield and water use efficiency.

How did 2023 stack up?
In 2023, the team set out to test how much yield potential would be sacrificed when the growing season is shortened. Soybeans were grown in a St. Paul field in which all other yield limiting factors were controlled; soybeans were drip irrigated, had adequate fertility, weeds were managed both with herbicides and by hand-weeding and foliar fungicides were applied. Soybean planting date treatments were approximately 10 days apart beginning on May 1 and ending on June 10.
Cerrudo and Naeve found that when planting was delayed from early May, the per-day yield penalty was 0.6 bushels per acre. The loss in yield potential when planting was delayed until June was much steeper at 1.5 bushels per acre per day. Essentially, the crops’ critical developmental milestones is compressed due less radiation and heat exposure.


Using the CROPGRO model to simulate the last 23 years of yield data given weather parameters experienced in St. Paul, the model predicted a drop in soybean yield potential similar to that observed in the field in 2023. Reduced soybean yield potential when planting is delayed through May was 0.4 bushels per acre per day and when planting was delayed until June the lost yield potential averaged 1.0 bushel per acre per day. The addition of weather and crop data from more years and locations will only strengthen the CROPGRO model’s predictive capabilities.

Planting date and yield
Delaying soybean planting date tends to reduce a crop’s inherent yield potential by shortening the time available for the crop to capture the sun’s energy and convert it to yield. Lost yield potential also results from a decrease in radiation use efficiency because of lower than optimum temperature later in the growing season. The duration of the crop’s reproductive growth stages can greatly impact yield, with the R4 (3/4 inch pod at one of the top four nodes) through R6 (a pod at one of the top four nodes that contains a seed that fills the pod cavity) most critical for yield.

Planting date and stress
Waseca, MN in 2023 experienced very dry growing season conditions, with 8.7 inches less rain falling in June through August than normal. When soybeans experience stressful conditions such as this, the yield advantage to timely planting, or rather the yield disadvantage with planting delays may disappear, explaining how soybeans planted on April 26, May 10 and May 25 saw statistically similar yields during 2023.

Relative maturity and yield
Cerrudo and Naeve used the same crop growth cycle framework to investigate the impact of soybean relative maturity and yield. As planting date is delayed, the one thing that is within a farmer’s ability to adjust and control is the crops’ relative maturity. Planting a variety with a shorter relative maturity can ensure that the crop experiences the most intense solar radiation and optimum temperature possible during those most important reproductive growth stages for yield determination (R4-R6). Conversely, the longer the relative maturity of the variety, the greater the chance that the crop can overcome stressful conditions experienced early season. While we have elite shorter maturity varieties to select from, the shorter time period that the crop has to recover from and compensate for earlier season stress is compressed when compared with a longer maturity variety.

 
Data was collected in 2023 from a research trial in St. Paul combining soybean relative maturities of 1.0, 1.5, 2.0 and 2.5 and planting dates of April 26, May 10 and 25 and June 12. In general, soybean yield was reduced as planting date was delayed. For this particular season and location, the team did not see any advantage to switching to an earlier relative maturity soybean until planting was delayed until mid-June. The team also did not see an advantage when planting early to growing a later relative maturity variety, begging the question of why one would risk planting a later maturity soybean when there is no yield advantage to doing so. In 2023, the growing season extended later than normal because temperatures were warm throughout much of the fall. More data from additional years and locations will help the team to provide reliable recommendations that are likely to apply most years.

Desiccants
To advance harvest, one can artificially force maturity through the use of desiccants, or herbicides that kill the crop. The team experimented with the impact of desiccant timing on soybean yield at two Minnesota locations in 2023, applying desiccants at R6 (when there is a pod at one of the four uppermost nodes on the main stem that has a green seed that fills the seed cavity), R6.5 and R7 (one mature-colored pod anywhere on the main stem). Using a desiccant too early can negatively impact yield; in the 2023 experiments, yield was 20 to 32% lower when a desiccant was applied at R6 than when no desiccant was used. An R6.5 desiccant application reduced yield by 5 to 8% when compared to no desiccant plots. As the number of pods and seeds was fixed at the time of desiccant application, the yield depression associated with early desiccant use was caused by a reduction in seed size.

Waiting to use a desiccant until the crop has reached the R7 growth stage did not result in a negative impact on yield. If the only time that spraying a desiccant did not cause a yield loss is at physiological maturity (R7), the growth stage immediately before harvest maturity (R8) occurs, one questions whether the desiccant costs (application + active ingredients) are worth incurring to buy a harvest advantage of a handful of days.

Fielding audience questions
Naeve and Cerrudo answered many audience questions including: 1) how did you isolate the effects of planting date on yield potential from other pressures on crop yield such as weeds, insects, fertility, soil moisture, disease?, 2) over time, could your model assumptions and accuracy change as variety genetics are improved?, 3) were all of the varieties of different maturity groups in your studies harvested on the same day, and if so, were the earlier maturities ‘short changed’ in yield due to low moisture or shattering?, 4) will a desiccant help with harvest headaches due to greenstem disorder?, 5) what is the recommended last date of planting date for double-cropping soybean after peas?, 6) is there any relation between planting date and seeding rate?, 7) based on your work, what do you think about pushing planting dates earlier?

Join us February 7!
Join us February 7 when we welcome Dr. Tommy Butts, University of Arkansas Extension weed scientist, and Tom Hoverstad, Researcher at Southern Research and Outreach Center, to discuss "Making herbicides work better." 

University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series, offered Wednesdays through March, features discussions with specialists to provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies for 2023. 

Source : umn.edu

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