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Issue No. 10, Article 5/May 29, 2009

Soybean Development and Considerations After Planting

According to the Illinois Weather and Crops Report, Illinois soybean acres planted increased from 1% to 12% between May 17 and May 24, and corn acres planted increased from 20% to 62%. This still left us 57% behind the 5-year average for planted soybean acres by May 24, but it was the most progress of any week so far this spring. Most of us have spent recent weeks worrying about when conditions will allow planting, but a new set of worries presents itself once the crop is in the ground. Here are some things to expect and look for once your soybeans have been planted, whether under ideal or not-so-ideal conditions.

Under ideal conditions, soybean seeds rapidly imbibe water after planting; the radical, or primary root, elongates out of the seed coat in 24 to 48 hours. The cotyledons appear after 3 or 4 days. The primary root grows rapidly, and secondary roots develop in 4 to 5 days. The upper region of the hypocotyl also grows rapidly and straightens the "crook," pulling the cotyledons above the soil surface in approximately 6 to 7 days. Emergence can easily take as long as 14 days depending on soil compaction, soil moisture, and air and soil temperatures. The growing degree-day (GDD) requirements are not as well refined for soybean as they are for corn, but preliminary work done by Shawn Conley in Wisconsin suggests about 100 GDDs are needed to achieve emergence. That is a little less than the GDD requirements for corn emergence. The energy that's needed for seedling development as just outlined comes from lipids stored in the seed and cotyledons. The stored energy is a finite amount, so once seeds start germinating, it is a race to get the cotyledons above the soil surface. They turn green as they emerge above the surface and photosynthesize to help supply energy for continued growth. The cotyledons still supply some energy for the seedling for several more days after emergence, but the green vegetative tissue slowly begins to take over.

Under not-so-ideal conditions, a number of adverse things can occur that slow growth and development, or even stop them altogether. One problem multiple producers have faced this year is heavy rainfall that leads to saturated and possibly compacted soils. I have witnessed several farmers avoiding the "wet holes" and several rotary hoes in action this spring. The germinating soybean seed needs oxygen to live and can perish after anaerobic conditions of 2 to 4 days, depending on soil and air temperatures. You might want to start scouting to assess how much of the wet holes you will need to replant. Dig up the seeds and determine if they are rotting or just developing slowly.

If the soil surface became compacted by the combination of hard rainfall and fast surface drying, you might dig to look at the diameter of the hypocotyl. If hypocotyls do not have the strength to lift the cotyledons through compacted soils, they swell from continued growth, which makes them increasingly brittle. With subsequent rainfall to "soften" the soil surface again, these seedlings may still emerge until they run out of energy reserve or the hypocotyls break under pressure. A rotary hoe may also help; however, rotary hoes can damage soybean seedlings more easily than emerging corn seedlings, so they can do significant damage. This is primarily because soybean seedlings are a much larger target than corn seedlings, particularly if they are brittle because they are actively trying to pull the cotyledon above the surface.

Soybean seedlings 11 days after planting with swollen hypocotyls due to a crusted soil surface. Seeds were planted May 12 and the field received 6.5 inches of hard rainfall on May 14.

Other adverse conditions are biotic ones, such as seedling diseases, and bird or insect feeding. Within the first week or two is a good time to scout to determine whether your seed treatment helped protect you from these pests, or if their presence means you might want to consider seed treatments next year.

Damping-off of soybean seedlings caused by Rhizoctonia.

Seedling diseases generally are considered more of a problem when planting earlier than now into cooler soils (see "Considerations for Using Fungicide Seed Treatments on Soybean" in issue 3 of the Bulletin, April 10). The four main pathogens often involved in the soybean seedling disease complex (Fusarium, Phytophthora, Pythium, and Rhizoctonia) have different temperature requirements, and some are more active at higher soil temperatures. In general, seedling diseases caused by Pythium in Illinois tend to occur when soil temperatures are less than 60°F. When soil temperatures are higher, soybean seedlings emerge much more quickly and Pythium is less active, which helps seedlings avoid infection by Pythium. Pathogens such as Fusarium, Phytophthora, and Rhizoctonia become more active when soil temperatures are above 60°F. In fact, some strains of Rhizoctonia present in Illinois become even more active as soil temperatures climb. So even when soil temperatures are warmer, seedling diseases can still be a problem when soil conditions are soggy. For seedling diseases caused by Rhizoctonia, additional stresses can predispose seedlings to infection by this fungus. Under certain conditions, some soil-applied herbicides can increase severity of seedling diseases caused by Rhizoctonia (Table 2). Resistant varieties are available that protect soybean seedlings against Phytophthora infection, but varieties resistant to Rhizoctonia and Fusarium are not available. For help in diagnosing if a seedling emergence problem is due to a biotic pathogen, send samples of affected seedlings to the University of Illinois Plant Clinic.--Vince M. Davis and Carl A. Bradley

Vince Davis
Carl A. Bradley

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