With spring around the corner, agronomists may soon be asked one of the most common questions faced in the spring: When should I begin planting? This is a simple question with a somewhat difficult answer. The answer is different for corn and soybean.
There are several things to consider:
- Calendar date: For most growers, the first consideration is to make sure it is after the federal crop insurance date.
- Soil moisture: Both corn and soybean should be planted into soils that have enough moisture for emergence, otherwise seed is likely to stay in the ground and not germinate. On the other hand, too much moisture can cause seed to rot in the ground and not emerge. Moisture levels the first couple weeks after planting should be considered.
- Temperature: Both corn and soybean seeds prefer soils temperatures above 50° F for germination.
For corn, recommendations are to plant when the ground temperature is above 50° F. In the Midwest, growers typically begin planting in mid-to-late April as field conditions allow. In my experience, this temperature recommendation for the date of planting including the 48 hours after planting is more important than the calendar. What I mean by this is we can see negative implications of soil temperatures below 50° F whether crops are planted in mid-April or late May. The best advice I can give is to keep an eye on the weather forecast. If soil temperatures are 50° F or below OR a cold rain is forecast, even if it is a nice sunny day, it is best to keep the planter in the shed.
The biggest complication from planting into cold soils or with cold rain is imbibitional chilling injury. After seed is planted in the soil, it begins to absorb (imbibe) water from the soil. When this water is cold (less than 50° F), it can cause the cells in the seed to rupture. The most common symptom is swollen seed without germination. This usually results in reduced plant population in the field, and with the direct relationship between corn population and yield, decreased population often results in decreased yield.
In cases of moderate exposure to cold temperatures, plants may experience leaf twisting underground but not at a level high enough to prevent emergence. In this case, the plant may emerge and try growing normally even with some leaves twisted (Figure 1). Depending on the level of leaf twisting after emergence, the plant may grow out of the injury or it may disrupt plant growth and result in an ineffective plant.
Moisture levels also can cause issues with emergence. If the ground is too saturated or if rainfall shortly after planting is excessive, even with warm temperatures seed sitting in prolonged saturated soils can be negatively impacted. Excessive moisture prevents the seed from getting the required oxygen level it needs to live. This can lead to slower growth, less nutrient uptake and even death. If the seed has not germinated, it can rot underground before emergence. Sprouting seedlings can become twisted as they try to emerge (Figures 2 and 3). For recently emerged plants, excess moisture can lead to death of emerged plants. Young corn can survive roughly four days of saturation.
Another potential complication of excess moisture is soil crusting or compaction. Hard-pounding rains on recently tilled ground can result in a hard crust that plants cannot penetrate.
Estimated time from corn planting to emergence varies based on growing degree units (GDU). Corn requires around 130 GDU to emerge. In the Midwest with temperatures above 80° F, this may occur in as few as five days. With cooler weather, fewer GDUs per day accumulate; therefore, plants may take as long as three weeks to emerge. The longer it takes to emerge, the greater the chance of stand loss from any of the reasons discussed above. Agronomists usually recommend seed treatments on corn for this reason as various diseases thrive in cool, wet conditions and the longer the plants take to emerge, the longer they are exposed to these various diseases.
Effects on soybean from cold chilling are different from corn. While we still recommend planting soybean seeds into soil temperatures of 50° F or warmer, soybean seeds can typically handle planting into to cool soils better than corn. While the growing point of corn remains below ground until the V5 growth stage and is protected from frost injury, the soybean growing point at the hypocotylenedary arch is the first structure to emerge from the ground (Figure 4). Therefore, while soybean seeds have less germination sensitivity than corn seeds, a hard frost after emergence can kill soybean seedlings. Damage is determined by how cold it gets and for how long it remains cold. Growth stage also impacts survivability. A soybean stand can be evaluated for frost damage by inspecting how far down the plant the tissue has been killed. If frozen above the growing point, the plant may recover.
One advantage of soybean plants is the ability to better compensate for stand loss. Unlike corn, which is very population dependent for yield, soybean plants can often branch out and compensate for reduced stands.
Nielsen, R. L. 2019. Emergence failure of corn. Corny News Network. Purdue University.
Web source verified 1/27/20/
Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
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