By Daniel Kaiser
Extension Soil Fertility Specialist
Management of Iron (Fe) deficiency chlorosis (IDC) in soybean is seemingly and endless topic of research in soybean growing areas with high pH, calcareous, parent materials. We are just finishing a three-year summary of a series of IDC management strip trials that began in 2010. Our main focus for this work was to study the variability in response for a tolerant and susceptible variety to an oat companion crop and a 6% EDDHA-Fe treatment applied in-furrow (we used Soygreen at a rate of 3 lbs of product per acre). The field areas were selected to have some variation in the severity of IDC.
We have had mixed results with the oat companion crop the past three years. Our greatest risk with this strategy was reductions in yield if the oat crop was not terminated at the correct time. Our treatment was a 2 bu/ac rate broadcast seeded and dragged in immediately before planting. Our targeted time of termination was when the oat crop was 10 inches tall, which was challenging to say the least. With unpredictability in rainfall events the past three years we were only able to successfully terminate the oat crop at the correct stage about 50% of the time. The oat crop did increase yield, but the increase was seen only for the susceptible variety. Yield without oat for the tolerant variety was similar to the susceptible with oat. Thus, in terms of risk, the oat companion will work but is riskier than other management strategies.
The in-furrow iron treatment gave us the best chance for return with the lowest risk. No yield reductions occurred from the in-furrow application of the EDDHA-Fe fertilizer source. Yield responses were greatest in areas severely affected with IDC (soybean plants would generally die off during the season) and generally were still positive in areas that would show significant yellowing of leaves with some stunting of plants. There was no yield response in areas with little to no IDC pressure. The data we have strongly supports to only apply these products in field areas where moderate to severe IDC is present. Even with the in-furrow product, the susceptible variety could not out yield the tolerant. Again, selecting a tolerant variety is the best course of action.
There are a few unknowns with this work. First, we only tested at 1 rate of the product. Similar strip trials with multiple rates would be beneficial to determine variable rate applications. Second, predicting where the moderate and severe areas are can be difficult. When we speak to most growers, salts and carbonates are what is generally used to judge IDC severity in fields. However, these factors did not have much bearing on the severity of IDC in our studies. It generally followed that the field sites were we saw the best response to in-furrow Fe where higher in carbonates and had some higher EC values (electrical conductivity is used to assess the salt content). However, within the sites there was no clear correlation between the severity of IDC and these factors. What seemed to factor in greater than soil tests was precipitation in June. If we focus on past research the major driving force is soil moisture content. If you take soil from a field area affected by IDC, the severity of the problem has been shown to increase if with increasing level of water saturation of the soil. Thus, for our work in West-central Minnesota, soil moisture should be a major driving force where IDC will occur, but it is rarely if ever measured.
One thing we are doing for 2013 is trying to establish a set of IDC sentinel trials around the state. We currently are looking for 6 field locations where IDC has been present in the past. What we would prefer is:
1) Field site where IDC has been previously shown
2) Field area where only a tolerant variety will be planted and no in-furrow or foliar treatments will be applied
3) Close access to the road
4) Field that can be harvested with a combine equipped with a GPS and yield monitor
The idea behind this work is to look more closely at the causative factors, and if the funding is available, look across multiple years. In order to better establish variable rate guidelines we need to have a better understanding on why field areas vary in terms of IDC severity on a year to year basis.
We currently have one year of funding from the Minnesota Soybean Research and Promotion Council to launch this project. These studies are meant to be as non-invasive as possible. There will be no specific treatments to apply by the grower, the field areas need only to fit the criteria above.
We will install a time lapse camera to monitor the severity of IDC in the margins of the field and a data logger equipped with a probe to measure soil temperature, soil moisture, and soil electrical conductivity throughout the growing season. Everything will be installed after planting and prior to harvest.
A more intensive sampling will be conducted early in the season (mid-June is the target for sampling). An intensive grid sampling program will be used in order to take routine 0-6" soil samples as well as plant tissue. In addition the severity of the IDC and the soil moisture will be intensively mapped to look at spatial variation in these factors. This data can then be overlaid with per-existing soil maps and the yield data to look at correlating response.
We are hopeful that some insight on how to map fields for variable rate application can be achieved with this project. While we understand some of the causative factors there still are many questions in order to best be able to target moderate and sever areas for either targeted planting of differing varieties or for in-furrow application of Fe.
Interested parties can contact:
439 Borlaug Hall
1991 Upper Buford Circle
Saint Paul, MN 55108