Tank Mixes and Double Cropping: Do They Increase Yields?

In the quest for high soybean yields, it may be tempting to tank mix an insecticide with a planned fungicide treatment for a single-pass application. After all, insecticide doesn’t cost much and mixing with another treatment could save a lot of time and wear and tear on the field.

“Insecticide is like a cheap insurance,” says Laura Lindsey, director of the Soybean and Small Grain Crop Agronomy Program at The Ohio State University. “But we don’t like blanket application because of resistance issues.”

Lindsey, who is often asked by growers about this dual application, has been working on a checkoff-funded project to investigate the potential of this tank-mixed insecticide to increase soybean yields.

Her two-year project revealed that fungicide application at the R3 stage in late July did increase yields significantly during the 2015 season. This effect, however, was most likely due to higher disease incidence that year because “2015 was really, really wet,” she explains. Addition of an insecticide to the fungicide treatment, however, had no effect on soybean yields.

Lindsey also found that her soybean fields were still prone to damage from insects like bean leaf beetles a month or two after application. Without long-lasting insect protection and no observed yield benefit, Lindsey determined that growers can enjoy a small cost savings by eliminating insecticide from their mid-season fungicide regimen.

Despite the negative results on traditionally grown soybeans, Lindsey suspects tank mixed insecticide and fungicide may prove beneficial for double-crop systems. Double-crop soybeans, sometimes referred to as second-season soybeans, are planted immediately after a winter wheat or other small grain harvest in late spring.

These soybeans, while planted late compared to traditional methods, still mature in time for the autumn harvest. However, the developmental differences in double-cropped soybeans, compared to the traditionally planted ones, can cause issues for growers.

“When you have green plants and everything else around them is dead, this attracts insects,” Lindsey notes.

Double cropping is not especially common in Ohio.

“Ohio is marginal. Growers south of I-70 usually can do a double crop, but above I-70 is more variable,” she explains.

It can be a profitable endeavor, and Lindsey wants to help farmers establish conditions needed for high double-crop soybean yields. To do this, Lindsey’s checkoff-funded project examines the effects of planting date, soybean relative maturity, row spacing and seeding rate.

Planting early is the most important factor for profitability, she determined, because “the later you plant, the higher seeding rates need to be.” In this multiyear study, a seeding rate of 250,000 seeds/acre was necessary to achieve the highest partial economic return. Soybean relative maturity groups with maximal vegetative growing time prior to the reproductive stage are also key for high yields.

Is double cropping soybeans or tank mixing insecticides right for your farm? Here are a few considerations to help you decide:

• Planting date: The earlier the better. Later plantings require higher seeding rates, which diminishes the crop’s economic return. Planting early also gives greater assurance the soybeans will be fully developed by harvest time.
• Relative maturity groups: Choose wisely. The optimal RM group will have the longest vegetative stage possible to provide more branches for the flowering stage, while still reaching full maturity prior to the autumn frosts.
• Row spacing: Narrow is best. In Ohio fields, narrow row spacing i.e., 7.5 and 15-inches, resulted in higher yields compared to 30-inch rows commonly used in other states.
• Mode of action: Repeated use of a single chemical or mode of action is discouraged when insect populations are below threshold levels due to risk of insecticide resistance.
• Tank mixing: Treatments of fungicide + insecticide, fungicide + insecticide applied separately, fungicide + crop oil, and fungicide + insecticide + crop oil resulted in yields not statistically different than fungicide treatments alone.