Issue No. 7, Article 5/May 21, 2010
Assess Soybean Emergence for Those Early-Planted Fields
The story this spring has ranged from "off to the races" to "another time of worry" for some growers. According to the May 16 crop report, soybeans are 42% planted and 12% emerged. This is slightly above the 5-year average, but little planting progress has occurred much of this week. Soybeans planted in April should be emerged to VC or have expanded unifoliate leaves. Those planted in the first week of May should also be emerged or close to it. There have been high amounts of rainfall and unseasonably cool temperatures in much of Illinois. In the northern half, particularly the northwestern regions, temperatures have been 5 to 7°F below normal and rainfall has been 2 to 4 inches above normal in the last week. Flooding and drowned-out areas are a major concern that will need attention by many growers.
Some growers are concerned about soybean emergence and the number of acres that will need replanting. There is no doubt there will be some acres with a clear need to replant, but some acres will be difficult to assess. Two important, albeit basic, things to keep in mind: give the seed enough time to germinate and emerge, and assess the whole field by dissecting the field for different soil types, conditions, and elevations.
Give it enough time. There is always an element of emotion in the time between burying your seed investment in the soil and being able to observe the plants. However, it takes heat units for those seeds to germinate and emerge, and when heat is slow to accumulate, seedling development is also slow. Soybean can emerge with as few as 100 (base 50) growing degree-days, but it can take as many as 150. In Illinois few locations north of Springfield, if any, have received more than 150 GDD since May 4 (two weeks ago). Some worries are that the seed may run out of stored energy to emerge and that the longer it lays in cool, wet soil, the more susceptible it is to attack by seedling diseases, particularly pythium. An advantage, however, from these overcast days and moist soils is that soil crusting should be less of a concern, even after hard rains, because the soil surface has not been baked dry very fast.
Scout accurately and precisely. In some cases, wet holes are obvious, and if soils have been standing in water or completely saturated, causing anaerobic conditions for longer than 48 hours, there is a good chance that area will need replanting, should it be large enough to warrant the trip. However, slight changes in elevation, soil type, or even residue density could have enough affect on soil temperature to alter seedling survival from low temperatures or alter seedling disease development. Slight changes in elevation or soil types in the field can also cause big differences in soil moisture levels and/or the amount of time some areas were saturated. The only way to assess these areas independently is to dig in many different areas of the field and look at many different seedlings.
Make two scouting trips. I know time is valuable, but if you're deciding whether to replant a whole field because of no or extremely poor emergence, you will likely make the first scouting trip when it is still too wet to actually till or replant the field. I would suggest always scouting again one or two days after the first trip to observe the change in seedling development, particularly if you get a warm sunny day or two, when growth should clearly occur.
What about partial stands? I have written in several previous articles that 100,000 uniformily spaced plants can maximize yield. However, I suggest that the plant population number to warrant tilling and replanting a field is much lower than that. Data would suggest that in many cases full yield potential can still be achieved with as few as 50,000 plants per acre. Keep in mind that there are real costs associated with tilling and replanting with new seed, even if the seed is free, and the new planting does still come with a second round of risks. One option is planting directly into thin or partial stands. I will discuss this situation more in next week's article, with data from replanting experiments conducted several years ago.--Vince M. Davis