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Issue No. 10, Article 7/June 11, 2010

Rapid Early-Season Soybean Growth

According to the June 6 crop report, soybeans were 82% planted and 69% emerged. This compares to 81% planted and 63% emerged for the five-year average, so statewide we are right on a "normal" pace for planting and early-season development. However, field conditions range widely, from excellent down to the other end of the scale. In areas where heavy rainfall and severe storms have been the story, scouting for appropriate stands and areas that may need replanting due to flooding is critical.

Growth stages are likely anywhere from VE to V2 for most fields. During recent travels I have noticed many "ugly" fields, where plants are not uniform in color, and in some cases not uniform in development. Most of this variability relates to excessive soil moisture. Hopefully, sunshine and drier weather will bring many of these areas back to better health. I would scout these areas for nutrient deficiency symptoms and root rots, but there is probably little that can be done in most cases.

The biggest problem I have noticed is early-season weeds. Keeping up with nitrogen and herbicide applications in corn between rain events has no doubt hampered activities for early-season weed control in soybean. There is a visible difference between areas where preemergence herbicides were and were not used. Scouting these fields and making timely postemergence herbicide applications are critical to protecting yield potential, and don't forget to pay close attention to the pressure of volunteer corn (see Aaron Hager's article in this issue for more on that topic). Also, be looking for Japanese beetles and bean leaf beetles.

There are some soybean fields planted in mid-April that are between the V2 and V5 stages. I have had questions about why some of these early-planted, larger beans are flowering and whether this is considered R1. Soybean initiates flowering based on plant size and the length of darkness. Soybean plants usually need at least three fully expanded trifoliate leaves to flower. If soybean is planted early enough, subsequently getting large enough by early June, the plants may flower for a brief period.

Explaining the physiology of this could get complicated because it deals with interactions of light, heat, and plant growth stages. Simply put, the longest day of the year (the summer solstice) is June 21. Soybeans usually flower 10 days to 2 weeks after the time when the right ratio of day length and night length coincide. So 10 days to 2 weeks before then, the "magical" day length and night length ratio will also be encountered. The difference is that plants should stop flowering because the nights will get shorter and not longer. Whether this plant "confusion" affects yield potential is not well known, but it is little cause for concern. However, with the increased popularity of later-season foliar soybean applications, understanding these growth stages is important.

In a strict sense of reading the common definitions used for soybean growth stages, if you see a flower, then the plant is in the first reproductive growth stage (R1). However, if you are considering foliar nutrient or fungicide applications and R1 is a growth stage to "trigger" those applications, I would wait, either until you see the flowering cease and start again in a couple of weeks or until a overwhelming majority of the canopy is flowering. I would expect both cases to be in early July.--Vince M. Davis

Vince Davis

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