Plant Pathology
Factsheet

PPFS-AG-S-10

USE OF SOYBEAN BLENDS IN SOYBEAN CYST NEMATODE
MANAGEMENT PROGRAMS

Soybean blends comprised of SCN-resistant and SCN-susceptible varieties, in various ratios, are used by some producers in SCN management programs. The general idea is that where SCN is at yield-limiting levels, the SCN-resistant component of the blend will compensate for any damage done to the SCN-susceptible component. Also, by using an SCN-susceptible component in the blend, less selection pressure will be placed on the SCN population; consequently, there will be a reduced likelihood that a race shift will occur in the nematode population.

Some producers use blends in the fourth year of a corn/bean rotation (i.e.,corn/resistant soybean/corn/soybean blend), to protect themselves against any SCN "hot spots" might still exist by year four.

The above reasons for growing SCN resistant/susceptible soybean blends might have some merit under some situations, but at a possible cost to the producer. One scenario where blends definitely should not be used is as a replacement for resistant varieties when SCN levels are high. In this situation, the limited damage to the resistant component of the blend will not compensate for the extreme damage done to the susceptible component. Thus, overall crop yield will be reduced compared to the yield that would have been attained if a resistant variety had been grown.

The situation where blends may be of benefit is when a few hot spots exist in a field, but most of the field has low SCN levels. If a susceptible variety were grown in this situation, overall crop yield would be reduced somewhat because of lower yields in the hot spots. Use of a blend, on the other hand, would add additional yield to those areas. Consequently, overall crop yield should be higher.

The issue of blends as they pertain to crop yield potential is not directly related to the effect of blends on SCN race shift potentials. First you must be aware, that anytime a resistant variety is grown there will selection pressure placed on the nematode population. This selection pressure is the driving force behind SCN race shifts. Susceptible varieties do not place selection pressure on the SCN population. Instead, they allow for unrestricted SCN reproduction which, in turn, stabilizes the existing SCN population. As a result, race shifts do not occur. Since blends contain resistant and susceptible components, they place more selection pressure on SCN than a susceptible variety, but less selection pressure than a resistant variety.

Based on the above, blends can either enhance or reduce the potential for a race shift to occur. The potential is enhanced if blends are grown in place of a susceptible variety. The potential is reduced if blends are grown in place of a resistant variety. The question concerning the value of blends, thus, has to be placed with the context of what would have been done if the blend was not used. If a blend is used in place of a susceptible variety, yields might be higher, but a price might be paid in the form of a race shift. If the blend is used in place of a resistant variety, the price might be lower yield, but with reduced chances for a race shift. Perhaps the best place for resistant/susceptible blends is where SCN levels are generally low, but the producer is not willing to consider a susceptible variety. In this case, the yield of the blend should be acceptable and the chances for a race shift are certainly lower than if a resistant variety had been grown.

Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!

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