By David Nicolai, University of Minnesota Extension

Tracking soybean development

As of Sunday, July 13, the average height of soybeans in Minnesota was 13 inches compared to 19 inches last year and 16 inches for the 5-year average. Twenty-eight percent of soybeans were blooming compared to 59 percent last year and 45 percent average. The rate of plant development is related to temperature, thus with recent warm temperatures and “timely” rainfall this summer we will continue to gain ground toward normal crop development in spite of the late planting start in May.

Growers and ag professionals may track the reproductive stages of soybean development for the balance of this growing season by dividing soybean growth into eight stages of development. Herbicide and fungicide labels may refer to these growth stage descriptions when providing timing instructions and crop-harvest intervals:

R1: Beginning bloom or one open flower at any node on the main stem.
R2: Full bloom or open flower at one of the two uppermost nodes on the main stem with a fully developed trifoliate node.
R3: Beginning pod or a pod at least three-sixteenth inch long at one of the four uppermost nodes on the main stem.
R4: Full pod or a pod is three-fourth inch long at one of the four uppermost nodes on the main stem.
R5: Beginning seed or seed is one-eighth inch long in the pod at one of the four uppermost nodes on the main stem.
R6: Full seed or pods contain a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem.
R7: Beginning maturity or one normal pod on the main stem that has reached its mature color, normally brown or tan, depending on the variety.
R8: Full maturity or when ninety-five percent of the pods have reached their mature pod color.

In many fields soybean plants are now at R1 (15 to 18 inches) tall and have 7 to 10 trifoliate leaves. Flowering of these plants begins on the third to sixth nodes of the main stem and progresses upward and downward. The flowers (normally self-pollinated) will flower over a period of three to four weeks for the indeterminate varieties we grow in Minnesota. Three to four days after an individual flower opens, the flower petals open, the flower petals dry out and the pod begins to elongate. Two and-a-half weeks after an individual flower opens, a full-length pod has formed.

In the Midwest, 60 to 75 percent of all soybean flowers produced abort and never contribute to yield. The overproduction of flowers and pods seems desirable because they offer a degree of escape from short periods of stress. Stressful conditions in stages R1 through R3 generally do not significantly reduce yields because some some flowers and pods can still be produced until R5 to compensate.

Soybean aphid update

A few scattered fields have reached economic threshold levels (an average of 250 aphids per plant with 80 percent of plants infested and populations increasing) for soybean aphid. However, fields with these levels are not typical and populations remain low. This being said, soybean aphid population can rapidly increase so scouting soybean fields to determine if the threshold of 250 aphids per plant, when 80 percent of plants have aphids and populations are increasing is recommended. If over 70 percent of the plants are infested, use speed scouting found at http://www.soybeans.umn.edu/home.htm to rapidly assess whether the field should be treated.

David Nicolai is a crops educator with University of Minnesota Regional Extension in Hutchinson.