• By Jeremy Ross •
With the exceptional rainfall we have had over the past two weeks, especially in southern Arkansas, I have had numerous calls asking how flooding will affect a soybean plant. The answer to this question is, it depends.
There are several factors that will determine if a soybean plant will survive flooded conditions. These factors include plant growth stage, duration of the flood, depth of the flood, clarity of the water, movement of the water, air temperature, and amount of sunlight.
There could be additional factors, but the bottom line is, the longer plants are in submerged conditions the less likely they will survive. As a rule, soybean plants in submerged conditions over 48 hours will begin to stress and die. At this point, early estimates for the southeastern portion of Arkansas that had the most flooding is 20% or greater of the soybean acreage will be affected.
Many of these acres will be replanted, and below are my recommendations to consider for late-planted soybean.
1. No-till where possible
To save time and preserve soil moisture, use no-till practices to plant soybeans if possible. Tillage and cultivation will delay planting and reduce soil moisture. Tillage may be necessary if fields are rough due to water movement, weed pressure, or bed preparation.
2. Herbicide options
Start clean and stay clean. Once conditions are good to start land preparations, for weed control, hit the weeds with herbicides quickly and often. Use multiple modes of action, and overlay residual herbicides.
Weeds will be emerging and growing very rapidly, so timely control will be very critical. Be aware of plant-back restrictions on some of the more popular soybean herbicides. Some have as much as 10-12 month plant-back to corn, grain sorghum, and rice (MP519; 2021 Row Crop Plant-Back Intervals for Common Herbicides).
Because the longest day of the year occurs on June 21, and all days get shorter after that, soybeans need as much sunlight as possible to make pods, seed, and yield. Narrowing up row-spacings with drills or narrow spaced planters will be necessary to insure canopy closure.
Recent research shows during the optimal planting window, there is a significant yield advantage having row widths less than 30 inches. Narrowing row spacing will also help decrease weed competition and increase water use efficiency by shading the soil quicker than wide row.
4. Seeding rate
Seeding rates for normal timings would be between 110,000 seed/ac to 150,000 seed/ac. Depending on your normal seeding rate, you may need to increase that by 10%-15% for drilled and planted soybeans to help achieve canopy closure and increase light interception.
5. Herbicide tolerance
Switching to the Liberty Link or Enlist system will provide growers with additional options for weed control over the Roundup, Conventional, or Xtend systems.
6. Maturity group
Recent research has shown a yield advantage by planting MG 4 soybeans late, when compared to other maturity groups. This goes against what we have recommended 10 years ago for late or double-crop soybean production. The table below shows the estimated relative yield for different soybean MG’s at different planting dates from a 3-year study conducted here in Arkansas.
We have already lost between 20-25% of our maximum yield potential due to delayed planting. Every day planting is delayed, yield is being reduced. We also need to consider the first frost/freeze date. The average first frost for Dumas, Arkansas, is Nov. 3 and the first freeze date is Nov. 14.
There is at least a 50% probability that the first frost/freeze will occur on these dates. Probabilities go up to 90% for a first frost/freeze two weeks later. When planted by the end of June, the average days to maturity for early-MG 4 is 105 days, late-MG 4 is 112 days, early-MG 5 is 117 days, and late-MG 5 is 120 days. Between July 1 to Nov. 1, there are 123 days.
7. Fungicide/insecticide seed treatments
Use seed treatments to prevent soil insect/disease damage, and reduce the likelihood of a replant situation. Data shows a yield advantage with these seed treatments on the extremes of the planting window, and we are at the end of the planting window.
Previous research has shown a significant yield increase with the use of soybean inoculants, even where soybeans have been grown in the past. Data from a multi-year evaluation of inoculants and planting dates show a soybean grain yield increase of 6 and 11 bu/ac with June and July planting dates, respectively.
Several inoculant products were tested, and all preformed equally. When treating seed with inoculants, ensure that the inoculant product has not expired, check inoculant label for recommended insecticide/fungicide seed treatments, and do no use chlorinated water for seed treatment slurry.
9. Late-Season disease/insect control
Scout field often prior to and during reproduction. Corn earworms and stinkbugs could be a huge problem on late planted soybeans. Try and plant soybean varieties with good disease packages. Strobilurin-resistant frogeye leaf spot has been reported in Arkansas, and combination products will need to be applied to control this disease.
Dr. Jeremy Ross is a University of Arkansas professor and Extension soybean agronomist. He may be reached at firstname.lastname@example.org.