Early planting (late March through mid-April) of soybeans in the Midsouth is now commonplace, and is the norm according to NASS surveys that show a large percentage of Midsouth soybeans are planted by early May. It is used as a mechanism to avoid drought and some pest problems, plus ensure early harvest.
The greatest risk with early planting is perceived to be from low air temperature that occurs after soybean emergence. There are known cases of soybean seedlings surviving frost after emergence, but the severity and duration of these cold temperatures are not documented.
Presumably, there is a difference in how 2 hours at 36 degrees that may result in a light frost and 8 hours at 33 degrees that may result in a heavy or “killing” frost will affect soybean seedlings.
Knowing the estimated last spring date of a particular low temperature is important. Estimated dates for 36, 32, and 28 degree temperatures at most Mississippi locations have been calculated. These data for locations in other states can be found on the NOAA-NCDC site.
Again, there is no documentation of just what level of frost will be detrimental to soybean seedlings. One choice to lower risk of stand loss is to time planting so that emergence will occur after the estimated 50% last spring frost date for a given location.
Seedling emergence in early plantings will generally take about 4 days longer than seedling emergence from plantings made a month later, and this should be factored into an early planting date decision.
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If any stand failure resulting from frost or freeze injury is unacceptable because of a shortage of seed available for replanting, then delaying planting to ensure that emergence occurs after the 10% last frost date may be preferred.
If this approximate 2-week delay in planting is too great for production and/or marketing goals, then planting on dates that fall between the 50% and 10% last frost dates will impart a risk of stand loss that falls between those for the two dates.
For those who choose the early planting option, here are a few important points and tips.
1. It is generally recognized that the standard germination test (used to determine seed germination potential under ideal laboratory conditions) is deficient as a measure of the potential field performance of seeds, and this is especially true for early plantings.
A seed vigor test more accurately measures seed properties that determine the potential for rapid and uniform emergence, and development of normal seedlings under a wide range of field conditions.
2. The accelerated aging test is the preferred method for evaluating the vigor of soybean seeds. This test evaluates the germination capacity of seeds that have been subjected to high temperature and humidity stresses for a defined period before the standard germination test.
Farmers who anticipate planting early should request information on seed vigor from the supplier of a seed lot, or obtain this information from an independent laboratory.
3. Preferably, lower quality seeds should not be planted in the conditions that usually occur with early planting. However, when seed lots with a lower-than-desired germination (<80%) must to be used, the vigor test is especially important. Also, these seed should be planted at an increased rate.
4. High-quality seeds that have received an appropriate fungicide seed treatment to control both seed- and soil-borne pathogens will germinate and emerge. Emergence time may be extended by cold soils, but emergence will occur as long as adequate soil moisture is available.
The most important point to remember about seed treatment use in early plantings is that a product that is effective against both seed- and soil-borne fungal pathogens should be used in all early-planting situations to ensure a stand.
5. Soybeans can be planted at varying seeding rates because different rates within an acceptable range can be used without affecting yield potential. It is generally accepted that seeding rates between 120,000 and 150,000 per acre should be used to attain a final stand of at least 100,000 plants per acre. A lower seeding rate can be used when the appropriate seed treatment fungicide is applied to seed before planting.
6. Plantback restrictions when auxin herbicides (2,4-D, dicamba) are used in a burndown application mix to control early-season weeds that have emerged before planting are especially important since the required interval between the burndown operation and planting may conflict with intended early plantings.
7. Soybean varieties grown in the Midsouth generally need 135 to 140 days from planting to maturity to reach full yield potential. MG IV varieties planted early fit this criterion. MG V varieties can also be planted early, but their 15-20 days longer growing season provides no yield advantage. Click here and here for details.