Background

For years, iron deficiency chlorosis (IDC) has robbed Minnesota soybean growers of potential profit. During this time, various research projects have focused on developing management practices that might be used to reduce the severity of the problem. There has been progress; but, progress has been slow.

Producers and ag professionals have observed in recent years that the problem seems to be getting more severe and affecting more acres. While the exact cause is not known, it is obvious that the IDC is a consequence of complex interactions among:

• soybean genetics
• soybean physiology
• soil chemistry
• environment

This complexity explains why there is no easy answer to the problem.

Research conducted in recent years has produced the following conclusions:

• development of tolerant varieties has been a major improvement in reducing lost yield
• severity of IDC is closely related to stress on the soybean plant
• there are several sources of stress
  • soil properties (carbonates, soluble salts)
  • cold, wet soils
  • some herbicides
  • seedling diseases

For the past two or three years there has been an active research program funded by the Minnesota Soybean Research and Promotion Council which is designed to develop and evaluate management practices that can be used to reduce the severity of IDC. Some of the results of those research efforts will be described in the sections that follow.

Seeding Rate

Some promising management practices have been discovered by accident. Mistakes in seeding in the past produced a double population that was not affected by IDC. So, it seemed logical to evaluate the effect of seeding rate on the severity of IDC.

Seeding rate studies were conducted at two sites in 2001. Considering the two sites, chlorosis was minimal at the Redwood County location (Table 1). In this situation, yields increased up to a planted population of 224,000 seeds per acre, then decreased. However, yield continued to increase as planted population increased at the Swift County site. IDC was severe at this site.

These results suggest that higher seeding rates may be needed in situations where IDC is a serious problem. The data collected do not identify a specific recommended planting rate. It would appear that a rate in the range of 200,000 to 225,000 seeds per acre would be appropriate.

Table 1. Soybean yield at two locations as affected by planted population.

Seed Coating and Foliar Iron

Management practices that might be effective include coating the seed with iron and the foliar application of this micronutrient. The combination of these two practices would supply iron at more than one point during the growing season.
Research trials designed to evaluate the use of iron on the seed with or without foliar applications were initiated in 2001. The positive result of this approach is illustrated by the following data.

The seed was coated with the EDDHA chelate at a rate to supply 0.06 lb. actual iron per acre. Iron sulfate was used for the foliar application. Two applications of 0.25 lb. iron per acre each were used. The results were positive. A combination of coating iron on the seed and foliar iron was needed to provide for optimum yield.

The concept of applying iron during a large part of the growing was evaluated in more detail in Swift County in 2002 (Table 2). Chlorosis was severe and yields were about 75% of what was expected. However, the concept of applying iron more than once during the growing season proved to be effective in limiting yield reduction caused by IDC.

Table 2. Soybean yield as affected by frequency of foliar application of iron.

* seed coated with EDDHA to supply 0.06 lb. Fe/acre
** foliar applied iron = 0.5 lb. per acre in one or two applications

In 2003, the concept of coating the seed with iron was expanded to evaluate the rate of iron supplied in this way. The seed coating was again combined with foliar iron (Table 3). Two sources of iron (iron sulfate, Ruff-n-Ready) were evaluated for use in a foliar application. The rate of iron was held constant at 0.5 lb. iron per acre for the foliar application. Two equal applications were used. The 1st application was at the 3rd trifoliate while the 2nd application followed two weeks later.

Table 3. Soybean yield as affected by amount of iron applied on the seed in combination with foliar iron. Chippewa County, 2003.

* seed was coated with EDDHA chelate

Compared to the control, the application of some iron increased yield. Increases, however, were not as large as in past years. This observation is currently attributed to the dry weather in August. In general, dry soil conditions in the latter part of the growing season tend to reduce the severity of IDC. Considering the product used for the foliar application, both appeared to have an equal effect on yield.

The results of the studies conducted from 2001 through 2003 were consistent. Optimum yield was produced by the combination of seed coating and foliar applications.

Use of seed coating and foliar iron is expensive. The challenge in the immediate future is to refine these management practices so that they are economical.

Competition Crop Effects

In recent years, several soybean growers have attempted to evaluate the effect of a competition crop on the severity of IDC. More detailed trials were conducted in Swift County in 2002 (Table 4) and Grant County and Yellow Medicine Counties in 2003.

In 2002, the oat crop was seeded in the row with the soybean seed at a rate of 1 bushel per acre. After reaching a height of 10 to 14 inches, the competitive oats were killed with a leading herbicide used for grass control.

Table 4. Soybean yield as affected by using oats as a competition crop.
Oats Used Yield

Yield at this site was not affected by the use of the competition crop. The rainfall during the early part of the growing season was less than adequate. With oats as a competition crop, the soybean plants showed moisture stress early in the growing season.

This study also provided a good opportunity to measure any potential effect of the oat crop on soil properties. Soil samples (0 to 6 inches) were collected from either directly under the soybean row or in the middle of two rows. The samples were analyzed for carbonates and soluble salts (Table 5).

Table 5. Impact of oats as a competition crop on two major soil properties that are related to IDC.

Results of the analysis of the soil samples show that the oat crop used for competition had no effect on the measurement of soluble salts or carbonates.

The concept of the competition crop was expanded to two sites in 2003 (Table 6). Two crops (oats, wheat) were seeded with soybeans at time of planting so that there was adequate competition. The seeding rate for the small grain crops was ½ and 1 bushel per acre. Because of differences in bushel weight, there were differences in seeding rate measured as pounds per acre.

Table 6. The effect of two competition crops seeded at two rates on soybean yield.

At both sites, the seed was coated with EDDHA to supply 0.06 lb. iron per acre. A foliar application of iron was also used to supply 0.5 lb. iron per acre.

Yield at the Grant County site was reduced by hail. Dry weather reduced yield at the Yellow Medicine County site. In general, the competition crop had no effect on yield if seed coating of iron and foliar iron was used. Visual observations early in the growing season suggested that the soybean plants suffered moisture stress as a result of planting with the competition crop.

The detailed evaluation of the effect of competition crops shows no positive effects on yield in 2003. The results are not consistent with observations of growers who have tried this practice in previous years. The weather during the growing season (cold, wet vs. warm, dry) may have an effect on the value of this management practice.

The use of the competition crop is not without risk. If moisture is limited, the use of this practice may have a negative effect on yield.

Summary

Iron deficiency chlorosis (IDC) continues to have a major impact on soybean yields in Minnesota. However, research projects conducted for the past three years have identified management practices that might be used to increase yield where IDC is a problem.

Variety selection remains a major factor to consider. However, IDC cannot be eliminated by using just this one management practice. The practices of seeding rate, seed coating with iron, foliar application of iron and use of a competition crop can also increase soybean yield.

Thanks to the support provided by the Minnesota Soybean Research and Promotion Council and the seed industry, there has been progress. Additional research is planned and additional positive results are anticipated.

Presented at the Minnesota Fertilizer and Ag Chemicals Shortcourse. December 9, 2003.