Soybean damage and replanting

Hail

Hail annually causes millions of dollars of soybean crop losses in Minnesota.

Producers spend around millions each year on crop insurance premiums to hedge against these losses due to hail.

Hail causes many types of crop injuries. When this happens early in the season, it may require producers to decide whether to replant the crop.

Insects, flooding and other causes

Hail is not the only soybean crop destroyer. Insect feeding, flooding, low air temperature, soil crusting or nutrient deficiencies also injure crops.

Inadequate stands may occur due to poor germination or inadequate emergence. Factors such as a cold seedbed and excess moisture can slow soybean emergence and promote seed rots and seedling diseases.

Chemical injury from fertilizers, insecticides or herbicides can also contribute to poor emergence.

Soybean growth stages are defined as vegetative (V) or reproductive (R). The vegetative stage begins when the seedling emerges (May) and continues until flowering (July), which marks the beginning of the reproductive stages.

During the vegetative stages, leaves develop and grow, the root system develops, nodules form to fix nitrogen, the main stem and branches grow and reproductive structures begin to form.

Vegetative stages normally develop to the V4 stage before flowering and the reproductive stages begin. Reproductive stages begin when the plant starts flowering (R1) and end with mature beans in early September (R8).

How to determine growth stage

Plant stages are determined by classifying leaf, flower, pod and/or seed development.

Staging requires identifying the nodes, the part of the main stem where the cotyledons and leaves are, or were, attached. Emerging seedling soybean plants are shown in Figure 1.

A leaf is considered fully developed when the leaf at the node directly above it (the next younger leaf) has expanded enough so the two lateral edges of each leaflets have partially unrolled and no longer touch. Soybean plants at V1 and V3 are shown in Figures 2 and 3, respectively.

List of growth stages

Table 1 defines both vegetative and reproductive growth stages.

Cotyledons

Food stored in the cotyledons provides energy for the young plant until it has developed enough leaves to manufacture its own food (Figure 1).

Leaves

Intercepting light (solar energy) and converting it into food (chemical energy) is one of the primary functions of leaf tissue. The young plant uses the food to develop roots, four stems and more leaves. The growth rate partially depends on the amount of leaf area.

Growing point

The terminal growing point at the top of the plant is a group of rapidly dividing cells where new leaves are forming (Figure 4). The growing point is part of the “shoot apex”.

Other similar cell groups are located in each leaf axil and may be sites for re-growth when the plant’s uppermost growing point gets damaged or removed.

Leaf axil

The leaf axil is the point where the leaf's petiole (its stem) attaches to the main stem or a branch.

Axillary buds

These buds are located inside the leaf axils and are also growing points. However, they are semi-dormant as long as the growing point at the top of the plant is alive.

When the main stem’s top is cut off, any one of these axillary buds may grow and ultimately look like a main stem. On normally growing plants, branches may grow from these axillary buds when soybean stands are sparse. Flowers and pods may also form from these buds.

How damaged soybean plants regrow

A soybean plant will usually regrow when the main stem gets cut off due to insect feeding, hail, sand blasting or other causes.

After a main stem has been cut, one or more axillary buds may develop. One usually becomes dominant because it develops to a greater degree than other branches. Later, it can easily be mistaken for the original main stem, unless you carefully inspect the lower plant section to locate the cut-off point.

Plants cut off below the cotyledons will not recover. In any plant stand evaluation, count these plants as dead. While some hail-damaged plants that look like they should recover eventually die, most regrow from either one or both of the axillary buds located at the node below where they were cut off.

Main stem cut above the cotyledons

Plants cut above the cotyledonary node will re-grow if there’s sufficient cotyledon tissue to provide the plant with energy to regrow.

If the plant doesn’t have sufficient cotyledon tissue left to sustain the plant, count this plant as dead. Plants with all of one and part of the other cotyledon, and should regrow.

The amount of remaining cotyledonary tissue influences the regrowth rate.

Main stem cut above the unifoliolate node

Plants cut off above the unifoliolate node can regrow from any of the four axillary buds located in axils at the cotyledonary node and the unifoliolate leaf node. However, they’re most likely to regrow from one or both of the upper buds at the unifoliolate node.

Green leaf tissue is the key to generating regrowth. Even if unifoliolate leaves are shredded and torn, the remaining green tissue can still generate regrowth. You should see this regrowth within three to four days if growing conditions are favorable.

Bruised stems

In addition to shredding and cutting stems, hail may bruise plant stems. Bruises usually occur on the stem’s lower portion. Intensity ranges from a mild bruise, which is a simple break in the outer stem tissue, to a severe bruise, which exposes the central stem tissue.

Plants with bruised stems that recover after a hailstorm may break at any point before harvest. Such broken-over (lodged) plants usually produce pods and seed. However, because they lie on the ground, harvesting them may not be possible.

Bruised plants that don’t break over don’t affect yield. The challenge is accurately determining which plants will break over at a later date, and which plants sustained only slight bruising. Unfortunately, it’s nearly impossible to determine this shortly after hail damage.

When hail reduces stands early in the growing season, plants produce additional branches to compensate for skips within the row. If all branches are harvested, seed yield reduces only slightly, even if more than half the plants are missing.

Branches contribute to yield and should be harvestable during normal combining with the header close to the ground.

As plant density within the row decreases (fewer plants per foot), the distribution of remaining plants within the row becomes important in producing yields comparable to those from higher-density fields.

How plant population affects soybean yield

Table 1 shows how plant population affects soybean yield. This relationship is valid for all soybean row spacings.

For example, when stands reduce to 50 percent of optimum population, yields only reduce by 10 percent, assuming weeds don’t compete with remaining plants. If weeds aren’t a problem, replanting just to re-establish a full stand can’t usually be economically justified.

How to count healthy plants

To determine the number of live plants in your field, do not try to visually estimate the remaining stand. You’d underestimate the live plant population.

Instead, follow these steps to estimate health plants and the corresponding yield:

1. Measure the distance for one-thousandth of an acre for your row spacing, using the length of row equivalents in Table 2.
2. Count the number of live plants in that row section. Don’t count badly bruised plants, or those you don’t think will recover.
3. Multiply that count by 1,000 to determine the number of healthy plants per acre.
4. Make several checks throughout the field. Ideally, scout the entire field to identify areas that don’t need replanting.
5. Use Table 1 to determine the expected yield reduction from the reduced stand.

Very narrow rows and solid-seeded soybeans: Counting healthy plants

The Drum Hoop Method is an easy way to make a stand count for very narrow rows and for solid-seeded soybeans, which are seeded with a drill or an air seeder.

Here’s how to use this method to determine plant population:

1. Take the ring that holds the lid of a 55-gallon drum, and then throw or roll the hoop on the ground at random.
2. When it falls, count the number of live plants inside the circle.
3. Do this at several locations and average the plant counts inside the hoop.
4. Determine the plant population using Table 3.

Throw ring options

A 55-gallon drum ring has a diameter of 22.75 inches. You can make other throw rings from just about anything that can be formed into a lightweight ring.

The rigidity of a hard-type anhydrous hose makes an ideal ring. You can make a 22.75-inch ring from a 71.5-inch length of tubing or hose by connecting the ends with a hose connector.

Hula hoops make ideal throw rings. Most have a diameter of 32 inches, which you can duplicate by joining the ends of a 101-inch length of hose. Table 3 includes stand counts for both 22.75- and 32-inch diameter rings.

Other ring diameters

If you have a suitable ring with a diameter other than that given above, you can still use this method.

Again, simply throw or roll the hoop at random, count the live plants, average these counts over several circles throughout the area you’re measuring, and use the following formula to determine your plant stand:

Number of plants per acre = (Number of plants within the ring x 7,990) / (ring diameter in inches x ring diameter inches) x 1,000

Leaves removed from the soybean plant during the vegetative stages (up to V4) will not affect seed yield.

Evaluate plants for bruises and stand, and do not consider removed or damaged leaves. However, leaves removed during the reproductive (R) stages (e.g., grain filling) will reduce yield.

The amount reduced depends on the amount of leaf area destroyed and the growth stage when leaves were removed.

Because the R stages occur after the last recommended replanting dates, we do not consider leaf loss as a factor in the replanting decision.

Because early-May plantings usually result in maximum yields, expect lower yields with later plantings.

Table 4 lists expected yield reductions for various late planting dates. Also consider the later maturity of replanted fields.

If you decide to replant, make sure your replanting costs can be recovered because the later-maturing crop has lower yield potential than the original seeding.

Replant costs include items like:

• Seed.
• Labor.
• Fuel.

Replant costs are extra, so reduce the yield potential to pay the replant costs.

Although these vary greatly from farm to farm and year to year, be sure to include other real costs into the costs of replanting such as:

• Interest on loans taken to replant.
• Opportunity costs due to time spent replanting that could have been used for other profitable (or profit-saving) activities.

The availability of good-yielding, earlier-variety seed is an important factor. However, when replanting soybeans, resist the temptation to switch to an earlier variety too soon.

Soybeans have an amazing capacity to catch up when planted late. Warm June weather hastens vegetative growth, while short days and cool nights in September push soybeans to mature.

Planting the same variety two to three weeks later in the spring will only delay maturity by a few days to a week.

When to opt for shorter RM ratings

Unless you were already flirting with frost by planting your original stand with a very long season variety, do not move to an earlier variety until the second week of June.

At this point, plant a soybean variety with a relative maturity (RM) rating of 0.5 units shorter than your original soybeans. For instance, if you live in Central Minnesota and originally planted an RM 1.7 variety, starting at about June 10, switch to an early group I soybean, such as an RM 1.2.

If you’re replanting in late June, you may need to switch to a variety that’s a whole maturity unit shorter than what you’d normally plant.

Sometimes the replanting decision is not difficult – for example, when the population is very low (less than 50,000 plants per acre) and the weed control is excellent. Under these conditions, you can plant alongside the original row to increase the stand and yield.

Later-planted rows will mature a few days later, but that shouldn’t be a problem for harvesting and handling the crop. Just wait until the later-planted soybeans are combine-ready before harvesting.

Replanting tips

Although early summer temperatures allow soybeans to grow rapidly, they will still tend to be shorter at flowering, because flowering date is almost entirely due to the day-length (therefore, day of the year).

For this reason, replant soybeans at a slightly higher population and in narrow rows or solid-seeded, if possible. This will allow the most efficient use of the early-season sun and its energy.

For the existing crop, consider the weed situation:

• What is the population of weeds? The species?
• Can they be controlled if the crop is left?
• If not, how will they affect regrowth and yield potential of the existing crop?
• At what extra cost?

The worksheet below will help you to decide whether it will pay to replant. Fill in the worksheet and compare the yield potential of the existing crop with that of a replanted crop.

The alternative with the higher yield potential should be the more profitable option. You’ll also need to consider the availability of seed, and replant costs for seed, labor and fuel.

You may receive partial or complete compensation for replant costs if you have crop hail insurance that carries a replant clause. If you have insurance, notify your agent about the loss and ask about replant cost-sharing.