Kristen P. MacMillan, MSc, PAg, Research Agronomist, University of Manitoba
The yellowing over of soybean fields, caused by iron deficiency chlorosis, during June in Manitoba is a mystery that continues to be investigated. It hits close to home for me – over half the soil tests on our farm come back at a “high” risk for IDC. We choose varieties carefully and continue to grow great soybeans, but a look into the literature describing this unique soil-plant interaction offers more insight into how we could manage it in the future.
UNDERSTANDING THE SOIL-PLANT – WATER INTERACTION
To manage a problem, you must first understand the system.
Iron deficiency chlorosis (IDC) is a challenge unique to high pH soils (often called calcareous due to the presence of calcium carbonates), which is why we don’t hear about it from all soybean growing regions. Manitoba soils are calcareous by nature; calcium carbonates in our soil are derived from the weathering of limestone parent material, particularly in the Interlake and Red River Valley.
In wet soil, carbon dioxide builds up and reacts with these carbonates, leading to bicarbonate which impedes iron uptake in soybeans. Despite iron being abundant in most of our soils, soybeans need to convert it to an available form for uptake by acidifying the area around their roots. Bicarbonate neutralizes the acidification process, reducing the availability of iron, leading to IDC.
In addition to wet, calcareous soils, high nitrate levels are also thought to be involved with bicarbonate presence in the soil and salinity and is another soil factor that contributes to IDC.
Good news though – the ability of soybean to acidify their root zone and take up iron differs among cultivars, which is why variety selection is the best management tool for IDC prone environments.
HOW ARE VARIETIES EVALUATED FOR IDC AND HOW MUCH DOES IT IMPACT YIELD?
The susceptibility of soybean varieties to IDC is tested annually at an IDC prone site near Winnipeg (Figure 1). Each variety is grown in single rows over three replicates and a visual rating from 1 to 5 is assigned based on its reaction, with 1 being tolerant and 5 being highly susceptible. This information is then used to choose varieties when growing soybeans in fields prone to IDC.
The IDC test site was taken to yield in 2017 for the first time in order to demonstrate the effect of IDC rating on yield. In Figure 2, soybean yield decrease in response to IDC rating is shown through regression using the 2017 data.
The results may surprise you. Based on last year’s trial, soybean yield was reduced by 20 bu/ac with each 1-unit increase in IDC rating at V5/R1.
For example, varieties with an IDC rating of 1.7 produced an average soybean yield of 43 bu/ac compared to 23 bu/ac for soybean varieties with an IDC rating of 2.7, in an IDC prone environment. These results may represent the extremity of yield impact due to IDC as symptoms in 2017 persisted for several weeks; in other years, chlorosis comes and goes within a week and may have less of an impact on yield. However, these results are in line with previous data from North Dakota, where Goos (1998–2000) reported 9–19 bu/ac yield decrease per chlorosis unit at V5–6.
So if you were skeptical of variety selection as a management for iron chlorosis, I hope this convinces you otherwise.
BUT WHAT IF WE ARE DEALING WITH IDC PRONE AREAS, NOT ENTIRE FIELDS?
This past winter, I spoke of this topic to farmer audiences in Brandon and Clandeboye – and I surveyed the groups on their experiences with iron chlorosis. The majority indicated that IDC occurs every year, and that when IDC occurs, 10–25% of their acres are affected.
The reason I asked these questions is because Helms et al. (2010) found that in North Dakota and South Dakota, varieties suited for IDC affected areas did not maximize yield in non-IDC parts of the field, although in Kansas they did.
In other words, varieties can perform differently depending on the environment, but also potentially by site within environment or field. To optimize yield across the whole field, we could be planting multiple varieties or planting the overall best variety. But what varieties and where? Does this yield drag with IDC score exist in Manitoba? What is the overall best variety? The answers are not readily available.
MOVING TOWARDS SITE-SPECIFIC MANAGEMENT OF IDC
Currently, soil test values for calcium carbonates and soluble salts are used as predictors to evaluate field risk for IDC; this index was developed by AgVise and was able to predict IDC occurrence 73–81% of the time. The best management strategy begins with soil testing, using this index to assess field risk, and then choosing varieties based on field risk in order to prevent the yield loss previously discussed.
Another approach is using those soil layers for site-specific management – however, temporal and spatial variability in the occurrence of IDC across years and within fields remains a challenge.
This is likely due to the interaction of soil factors with moisture and the heterogeneity of soil properties at a fine scale. Mapping the occurrence of IDC when it’s actually happening, and letting the plants tell the story, is something I encourage you to start doing – it may provide the foundation for future site- specific management.
To move forward and address some of these questions, future work in Manitoba aims to evaluate soybean yield performance on both IDC and non-IDC sites within the same field, potentially expand to multiple fields and attempt to characterize where IDC occurs in fields by building on previous literature. Stay tuned as more clues are unveiled.