Soybean Hail Decision Guide

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Field Facts written by Pioneer Agronomy Sciences

Assessing Your Stands

Count the number of plants in 10 feet of row. Divide by 10 to obtain plants per foot of row. For drilled soybeans, 30 plants in a 3x3 foot square equal 150,000 plants per acre. A hula hoop is a good tool. Randomly roll the hoop, and count the plants inside the hoop after it settles. Plants per hoop can be converted to plants per acre using the multiplication factor below. Always check several areas of a field, as hail damage can vary greatly throughout a field.

Table 1: Stand count calculation factors by row width.

Table - Soybean stand count calculation factors by row width.

Unlike corn plants, which have one growing point, soybean plants have many growing points located at the base of each leaf. As long as these growing points are not injured, new growth can occur from any point.

There are no growing points below the cotyledons. A plant cut or seriously injured below this point will not re-grow. Leaf tissue is important for regrowth, as leaves produce the food needed for growth. Injured plants with the equivalent of one cotyledon or one leaf have a good chance of recovery. If no leaves remain, regrowth will be very slow, even if growing points remain intact.

It is often difficult to determine damage until a few days after a storm. Soybeans add a new trifoliate every three to five days under normal conditions. When determining stands, count questionable plants as one-half. After a potential surviving stand level is determined, estimate its yield potential.

Soybeans in the vegetative stage can compensate well for thin stands. In general, one-half of a stand may produce close to 90 percent of a normal yield; one-fourth of a stand may produce 75 percent of a normal yield. In 30-inch rows, 3-4 plants per foot is an acceptable stand if there are no large gaps and weeds are controlled. In 7.5-inch rows, 1.5-2 plants per foot of row are acceptable. Estimate yields accordingly, or use the chart below.

Table 2: Stand reduction loss chart.

Chart - Soybean stand reduction yield loss.

Plant Damage

Damage to leaves and stems is more difficult to judge in soybeans than corn because soybeans have more flexibility for regrowth. In very young soybean plants, some leaf area is critical for recovery. Young soybean plants can tolerate a fair amount of leaf loss without significant yield loss. Before flowering, losses are determined by cut-off or bent-over (broken) branches. First, estimate the number of original nodes located above cotyledons per plant. Then, count the number of nodes above stems that have been cut or broken, and calculate the percentage. Refer to Table 3 to estimate yield loss. For example, in a 20-plant sample of plants with four original nodes, 4 of the plants are cut above the second node, and 4 plants are broken above the second node. Of the 20-plant sample, 16 of the original 80 nodes are lost or broken. (16/80 = 20% = 3% yield loss)

Table 3: Estimated soybean yield loss from cut or broken nodes.

Table - Estimated soybean yield loss from cut or broken nodes.

Source: National Hail Insurance Association

After flowering, yield losses from cut or broken plants increase dramatically, as do yield losses directly attributed to defoliation (Table 4).

Weed Competition

Timely cultivation is often a good option if possible. It is always a good option to consider your herbicide options carefully following crop injury. A balance between adequate weed control and avoiding herbicides that may cause stunting or burning of the plant can be a challenge.

Determining Replant Potential

If the condition of the existing stand is questionable, assess the potential for replanting. Always check with your insurance provider prior to replanting for options and considerations. Soybeans are largely day length sensitive and therefore latitude will affect their ability to attain enough growth between planting and when they will be forced to flower. Soybeans replanted at approximately 39 N latitude (I-70) on July 10 can still get enough growth before flowering to attain 80% of a full yield, but soybeans planted at 41 N latitude (I-80) should be planted prior to June 20 to attain the same yield. For most of the soybean growing area, soybeans can be planted up to a date of June 20th and have the potential of achieving 80% or more of the optimum yield potential.

Table 4: Estimated soybean yield loss from defoliation.

Table - Estimated soybean yield loss from defoliation.

Source: National Hail Insurance Association

% Based upon maturity zones in the Great Plains and Corn Belt states at approximately 41 N latitude. Consult with your hail adjuster, local Pioneer agronomist or sales professional for information pertaining to your agronomic area.

Maturity of a Variety for Replanting

In general, it is suggested to use an adapted, medium-full season variety for your region. Replanted soybeans will have less time to grow the plant structure before they begin flowering. As such, late planted soybeans may not attain full height and canopy. Fuller-season soybeans usually can attain more growth before flowering, which helps set more of the pods at a harvestable height. Consider increasing seeding rates 10-20% if establishing a crop canopy is a concern.

Replanting Into Existing Stands

Many times it appears as if this practice would be the best option. Consider the following prior to doing so:

Planting between rows means driving on old rows.
Will existing soybean plants outgrow and shade newly emerged plants?
Does cultivation of the crop present a problem.
Is the difference between planting dates going to be enough that the plants will mature at different times, creating a harvest problem?

There are exceptions to this comment however in most cases, it is best to either leave the existing stand or destroy it and replant rather than attempting to interseed.

Soybean Decision Worksheet

Keep Existing Stand

Potential Yield (Table 2)

_______________%

Cut/Broken Node Adj. (Table 3)

(______________)%

Expected Yield

= ______________%

Yield Goal

X_____________bu/A

Commodity Price

X_____________$/bu

Estimated Income

=______________$/A

Replant

Potential Yield (Table 4)