Soybean Growth and Management Quick Guide

A-1174, June 1999
Reviewed and Reprinted August 2004

D.A. McWilliams, D.R. Berglund, G.J. Endres
North Dakota State University and University of Minnesota

Click here for an Adobe Acrobat PDF file suitable for printing. (141KB)

Introduction
Vegetative Growth Stages
Reproductive Stages

Growth, development and yield of soybeans are a result of a variety's genetic potential interacting with environment and farming practices. Correct production decisions using plant growth staging and timing are important for successful soybean production. Minimizing environmental stress will optimize seed yield. Farmers who understand how a soybean plant grows and develops can establish their field practices to maximize the genetic potential of the varieties grown. Management practices that may influence crop growth include seedbed preparation, variety selection, planting rate, planting depth, row width, pest management (diseases, insects and weeds), fertilization and harvesting.

Soybeans are classified as indeterminate, semi-determinate or determinate in growth in the United States. Many southern varieties are determinate in growth and cease vegetative growth when the main stem terminates in a cluster of mature pods. Most northern varieties are indeterminate in growth habit. Indeterminate varieties develop leaves and flowers simultaneously throughout a portion of their reproductive period, with one to three pods at the terminal apex. With soybean development being driven by photoperiod, northern varieties have vegetative growth limited by the season length. Semi-dwarfs, determinate varieties that are usually only 40-50% as tall as indeterminate varieties, are commonly grown in the Midwest. They were developed for their better standability (they are less susceptible to lodging) and are very appropriate for solid-seeding or drilled fields.

Figure 1. Soybean plant. (32KB color photo)

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 U.S. to VIII in the southern Gulf Coast states and most of Florida.

Short daylength and warm temperatures control soybean flowering. Soybeans must reach at least the first trifoliolate in growth before they can be induced to flower. Soybeans in the northern U.S. have longer minimum daylength requirements for the onset of flowering (often greater than 14 hours of daylight). However, even within a variety, variations in time of flowering may occur from year to year with the same day length closely associated with temperature conditions. Planting a specific variety farther north than its adapted maturity range will extend the period of vegetative growth, delay flowering and delay maturity due to the extended summer daylength and cooler temperatures. Likewise, planting a variety farther south than its adapted range will shorten the vegetative growth period, cause earlier flowering and result in an earlier maturity due to shorter summer daylength and warmer temperatures.

A fully developed leaf node for the vegetative stages has a leaf above it with unrolled or unfolded leaflets. These unfolded leaflets have their edges no longer touching. Stages are counted from the unifoliolate leaf node and upward. All other stages have true leaves that are trifoliolate and produced singularly on different nodes with these leaves alternating on the stem. The reproductive stages are divided into 4 parts: R1 and R2 describe flowering; R3 and R4 describe pod development; R5 and R6 describe seed development; and R7 and R8 describe plant maturation.

Vegetative Growth Stages

1. Emergence (VE)

Soybean seed begins germination when water is absorbed equal to about 50% of the seed's weight. The radical, or primary root, is first to emerge from the seed. Shortly afterward, the hypocotyl (stem) emerges and begins growing toward the soil surface pulling the cotyledons (seed leaves) with it. This hook-shaped hypocotyl straightens out once emerged and as the cotyledons unfold. Emergence (VE) normally takes five to ten days depending on temperature, moisture conditions, variety and planting depth. During this time, lateral roots are also beginning to grow from the primary root. Root hairs can be visible within five days of planting and provide the key nutrient and water absorbing functions of the plant in this early stage. The taproot will also continue growing and branching so that lateral roots can reach the center of a 30-inch row within five to six weeks. Eventually the soybean root will reach a depth of 4 to 8 feet with most of the roots in the upper 6 to 12 inches of soil. Soybeans should be planted 1 to 1� inch deep but not deeper than 2 inches. Because the soybean must often push through crusted soil, deeper planting can limit viability of seed and final stand number. Rotary hoeing can help seed push through crusted soil as well as help in early weed control. Very small amounts of fertilizer (P or K if needed) in a band to the side and slightly below the seed may help early plant growth, especially if soils are still cool. Do not place fertilizer too near the seed or directly in the furrow as salt injury from the fertilizer can result. Soybeans are very salt sensitive (about twice as sensitive as corn). If fields have not been in soybeans in the last four years or the field has been flooded, seed should be inoculated with Rhizobium japonicum bacteria (such as Brady) to form nodules on the soybean roots that will later provide much of the plant's nitrogen supply. As the hypocotyl arch is exposed to light and straightens to pull the cotyledons out of the ground, growth called epicotyl growth begins with expansion and unfolding of unifoliolate leaves.

Figure 2. Germination. (28KB color photo)

2. Cotyledon Stage (VC)

The VC stage is begun when these unifoliolate leaves are fully expanded. During the VC stage, the cotyledons supply the nutrient needs of the young plant (for about seven to 10 days). The cotyledons will lose about 70% of their dry weight to this nutrient reallocation. If one cotyledon is lost during this time, there is little affect on the plant's growth rate. However, loss of both cotyledons at or soon after VE will reduce yields 8-9%. Later, after V1, photosynthesis in the developing leaves allows the plant to sustain itself. New V stages will now begin appearing around every three to five days through V5 and then every two to three days from V5 to shortly after R5 when node number usually reaches a maximum.

3. First trifoliolate (V1)

This stage is achieved when the first trifoliolate is fully emerged and opened. The V stages after VC are defined and numbered by the upper, fully-developed leaf node on the main stem (the stage is numbered by fully-developed trifoliolates). Trifoliolate leaves on branches are not counted when determining V stages, only the trifoliolates off the main stem are used in the count.

4. Second node (V2)

Plants are 6-8 inches tall and have three nodes with two unfolded leaflets. Active nitrogen fixation from the bacteria is just beginning to occur. Most of these root nodules are within 10 inches of the soil surface with millions of bacteria in each nodule. Nodules that are pink or red inside are active in nitrogen fixation. White, brown or green nodules are not efficiently fixing nitrogen and are probably parasitic on the plant. A minimal amount (50 pounds of nitrogen in North Dakota and the Red River Valley of Minnesota, no addition in the rest of Minnesota) of nitrogen fertilizer can be supplied to soybeans as a starter fertilizer. The plant will use bacteria-produced nitrogen and the applied or intrinsic soil nitrogen. Too much nitrogen fertilizer will cause the plant to use this supply rather than develop nodules and fix nitrogen. The top 6 inches of soil have soybean lateral roots developing rapidly now. Any cultivation for weed control should be no deeper than absolutely necessary to minimize root pruning.

Figure 3. V2 soybean. (38KB color photo)

5. Third to Fifth nodes (V3-V5)

Soybean plants are about 7-9 inches tall with four nodes (three unfolded leaflets) at V3 and will be about 10-12 inches tall with six nodes of unfolded leaflets at V5. The number of branches seen on the plant may increase at this point in wider row spacings and under lower plant densities, depending upon variety grown. Up to six branches are normally developed under field conditions in branched (rather than upright) varieties with the largest branch being the main stem. At V5, the plant normally has axillary buds in the top stem that will develop into flower clusters (racemes). V5 is about one week from R1 or first flower. At V5, the total number of nodes that the plant can produce is established. With indeterminate soybeans, this total is higher than the number of nodes that will actually fully develop. However, these extra axillary buds allow the plant to recuperate from any hail or wind that may damage some of the buds. Although the stem apex (main growing point) is dominant, damage to this growing point will allow axillary buds lower on the plant to suddenly branch and grow profusely. Thus, soybeans are capable of producing new branches and leaves after hail destroys almost all the above ground foliage as long as some (at least one) axillary buds remain intact. If the plant is broken off below the cotyledonary node, however, the plant is killed as no axillary buds are below this node.

Figure 4. V5 soybean. (45KB color photo)

6. Sixth node (V6)

Soybeans are often 12 to 14 inches tall at this stage with seven nodes with unfolded leaflets. The unifoliolate and cotyledons may have senesced from the plant. New stages are quickly unfolding every two to three days. Lateral roots have crossed over the row underground in any rows 30 inches or less. The plant is still capable of recovering from damage; a 50% leaf loss at this stage will only affect yield about 3%.

Figure 5. V6 soybean. (34KB color photo)

For more information on this and other topics, see: www.ag.ndsu.nodak.edu

[ More ] [ Reproductive Stages ]

A-1174, June 1999
Reviewed and Reprinted August 2004