In many of the soybean fields in Northwest Minnesota the soybeans plants have emerged. With frost damage in some areas and excess water in other areas of the region it is time to evaluate stands and determine whether you need to (partially) replant. Even in the absence of either frost or drown-outs it is not a bad idea to take some time and evaluate the stands and determine whether you attained the targeted initial population and learn from your mistakes, if any.

Replant decisions can only be made after taking stand counts to determine plant populations as visual observations tend to underestimate the plant population. If a good ground cover does not exist, or gaps occur within the row or areas of the field, the conclusion may be to replant the soybean crop or parts of the field. Narrow row width may exaggerate the impression of low stands because there are larger spaces between plants within rows. Replanting is not recommended unless the cause of the previous poor stand has been corrected. Factors to consider when evaluating a stand include: why was the stand reduced (drown out areas, shallow seeding); how uniform is the remaining stand; what is it’s yield potential; what is the weather forecast for the coming period; is there still enough time to replant (consider maturity of the soybean); and what is the cost of replanting (seed and labor)?

Usually the stand reduction is not uniform throughout the field. Loss of stand is frequently irregular. Gaps may occur in rows, but if the gap is less than 2 feet in diameter, the adjacent soybean plants are capable of developing branches to occupy the space. Soybean stands adjust to low populations by producing more branches per plant and by increasing the number of pods on both the main stem and branches. Open spaces more than 2 feet in diameter may lead to loss in yield. Reseeding into the existing stand is possible, but will results in non-uniform plant size. This forces smaller plants to compete with larger, existing, ones.

A study at the University of Minnesota evaluated the effect of stand reduction at different growth stages on yield. Yield losses were 3 percent or less when stands were thinned to 75,000 plants (even with 1-ft gaps) at the VC and V3 stages. Two-ft gaps at 75,000 plants resulted in yield losses of 8 percent.

If the surviving stand is uniform and 75,000 plants or more, the producer must evaluate the economics of replanting by estimating the yield potential due to late planting (see Table 1) and other associated costs.

To determine plant stand, select the area of the field with lowest plant stand, and make at least 10 random stand counts in an area where the stand is reasonably uniform. Also take plant stands at locations with normal stands. Use a tape measure and mark off 1/1,000 of an acre for each count. The length of row equal to 1/1,000 of an acre with different row widths is summarized in Table 2. Multiply stand count from appropriate length of row by 1,000 to obtain plants per acre.

Soybean populations can vary perhaps as much as 50 percent from recommended levels without affecting yields, as long as missing gaps are not too large and weeds are controlled. In summary: before even considering re-seeding count the stand and calculate the cost.

Additional Comments on Plant Populations
and Damage to Regions Crops

There are the same plant population concerns for other crops. Unfortunately, the flooding in many areas will have the greatest impact on plant populations, concentrating stand loss to significant sized areas of fields. Table 3 summarizes the plant population of local crops that should be considered minimum stands when deciding whether to replant and assuming the plant population is relatively uniform in distribution.

Table 3. The minimum stand of several crops to avoid major yield reductions when making decisions on crop damage and replanting.

Table 4. Field crop replanting suggestions for region’s crops when soil moisture is not limiting. (Source: Peel and Endres. 1997)

Corn is very sensitive to flooding in the early vegetative stages, especially before the fifth or sixth leaf stage. In early growth stages, corn and soybeans can survive for only two to four days under water in anaerobic conditions. Moderate water movement along the soil surface can reduce flood damage by allowing some oxygen to get to the plants, keeping them respiring and alive. Drainage within one to two days increases the chance of survival. Cooler temperatures may prolong plant survival.

By mid-June, only a few crops should be considered for replanting. Early maturing sunflower, buckwheat, proso millet and very early group 00 soybeans may be options (Table 4). Before replanting, make sure herbicides used on the prior planting will not injure the replanted crop. Also, don’t seed into a wet seed bed as crops "mudded in" will result in poor stands and will have limited yield potential. Remember that favorable weather for plants after flooding is important. Cultivation, once soils are dry enough, will help aerate surface soil and promote root growth.

References:
Hicks, D, and S. Naeve. The Soybean Growers Field Guide for Evaluating Crop Damage and Replant Options. http://www.soybeans.umn.edu/pdfs/SoybeanCropDamage.pdf  

Peel, M, and G. Endres. 1997. Replanting after Early Season Crop Injury NDSU Extension Circular A-934. http://www.ext.nodak.edu/extpubs/plantsci/crops/a934w.htm

Hans Kandel Regional Extension Educator
Seth Naeve Soybean Extension Agronomist
Phillip Glogoza, Regional Extension Educator