ISSUE 9   July 8, 2010


Soybean varieties in general are classified as indeterminate, semi-determinate or determinate in growth habit. Indeterminate varieties develop leaves and flowers simultaneously throughout the first part of their reproductive period. Varieties grown in our region are mainly indeterminate. Soybean development is driven by photoperiod, temperature (heat units), and the interaction between these two major growth and development drivers. The limiting factor in the growth of varieties is the length of the growing season which ends at the first killing frost.

Soybean maturity groups are based on adaptation within certain latitudes. These maturity belts run east to west in the United States with only about 100 to 150 miles from the north to the south of each belt. Maturity groups range from 000 in the extreme northern regions to group I in the southeastern part of North Dakota. Shortening of the day length (or in other words, increasing of the night length) and warm temperatures control soybean flowering. Soybeans must reach at least the first trifoliolate in growth before they can be induced to flower. Soybean in the northern United States has shorter minimum night length requirements for the onset of flowering. However, even within a variety, variations in time of flowering may occur from year to year with the same day length, this is closely associated with temperature conditions.

Each variety depends on a critical period of darkness to change from the vegetative to the reproductive state. An early maturity group variety is sensitive to a shorter night and therefore, requires fewer hours of darkness to initiate flowering than does a later maturity group variety. A late maturity group variety requires longer hours of darkness to initiate flowering, which allows a longer period of vegetative growth, and thus it matures later in the season. For example, when a variety adapted to a southern latitude (shorter days and longer nights) is grown further north, the longer day lengths and cooler night temperatures cause it to remain growing vegetatively longer, resulting in and tall plant, delayed flowering and delayed maturity. A variety adapted to a northern latitude, when grown further south under shorter day lengths and warmer nights, will cease vegetative growth earlier than normal, resulting in smaller plants, earlier maturity and a reduced yield potential. The actual calendar date that a variety matures is highly influenced by the latitude location where the crop is grown.

Soybean flower colors are usually white or purple;
however, pink flowers are also possible.  A pure
variety will have flowers of the same color.

Some of the soybean varieties in North Dakota are starting to flower. The reproductive phase in soybean starts when at least one flower is located on the plant at any node on the main stem. This stage is called R1. Flowers are mostly self-pollinated. About 50 to 80% of the total flowers on the plant actually produce pods. Soybean flowering will start on the third to sixth node on the main stem depending on vegetative stage when flowering begins. Flowering will progress up and down the plant. Branches will eventually also flower. Within each raceme (cluster of flowers along the main stem) the flowering will occur from the base to the tip, therefore basal pods are always more mature. In Carrington, N.D., it took 52 and 54 days after planting to reach the R1 growth stage for soybean varieties with 0.0 and 0.4 maturity ratings respectively. The R2 growth stage is reached when an open flower is seen at one of the two top nodes of the main stem.


Hans Kandel
Extension Agronomist broadleaf crops



The weather this past week has been favorable for corn growth and many corn fields look impressive as inter-rows and gaps associated with poor emergence are hidden by the rapidly expanding leaf area. Recently I received a corn plant where the upper-most leaf was curled so tightly that it looked like a buggy whip.

Twisted whorls observed in eastern ND in 2010.

Buggy whip or twisted whorl syndrome are used to describe this phenomenon in corn. Twisted whorls are most commonly a problem of young corn plants (five to six leaf stage), but can occur much later. In 2008, for example, we observed plants that had twisted leaves restricting the emergence of the tassel.

Example of twisted whorls occurring late
in the season, 2008.

The cause of the twisted whorl syndrome is not well understood. Occasionally it can be traced to a misapplied herbicide or a severe stress. Though it is generally difficult to identify a clear causal factor, frequently this problem is observed when conditions favor rapid growth after a period of slow growth. The good news is that these tightly curled leaves will generally unfurl with little impact on subsequent development and yield. For additional information about twisted whorls in corn visit

Joel Ransom
Extension Agronomist - Cereal Crops

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