Climate Change and Aflatoxin in Corn

By Jina Yu, David A. Hennessy, Jesse Tack, and Felicia Wu

The possible impacts of climate change on field crop production are many; however, most attention to date has been paid to projecting locational effects on yield and commercial viability (e.g., Zhao et al. 2017). But an altered climate will also have more nuanced effects through impacts on grain composition, safety, and quality. Our interest here is in how changing summertime weather patterns in the US central Corn Belt can provide an opening for increased aflatoxin damage in corn.

When conditions are warm and dry in mid-summer and the crop is stressed, the fungus Aspergillus flavus (A. flavus) can colonize the corn ear, feeding on kernels and producing aflatoxins. The fungus must breach the plant’s defense mechanisms to colonize, which it typically does through the silks or through insect-damaged kernels. A. flavus can also colonize grain stored in humid conditions.

The resulting grain is not just waste—the toxins will cause morbidity and even mortality at high doses when fed to almost any species. Hence, the US Food and Drug Administration (FDA) regulates “total aflatoxins” in human food, pet food, and livestock and poultry feed through action levels. These toxicity routes are subject to the strictest FDA aflatoxin action level: 20 parts per billion (ppb). No more than 300 ppb may be included in feed for beef cattle, 200 ppb in feed for finishing hogs, 100 ppb for poultry, and less than one ppb for milk. Consequently, affected grains are discounted at point of sale and, if contamination is severe, may have no market.

The market for biofuels is also compromised because the toxins persist after distillation and are found in concentrated amounts in the dried distillers grains co-product that is marketed as animal feed (Wu and Munkvold 2008). Because aflatoxin is a more common problem for corn produced in interior eastern Texas, Oklahoma, Kansas, and the Mississippi Alluvial Plain, the problem constrains corn production in these areas. As some of the corn produced in this region is unavailable for feed in Southern Plains cattle feedlots and southern broiler poultry farms, feed needs are met through additional shipments from the Midwest.

Others, such as Battilani et al. (2016), note that aflatoxin-caused damage to the European corn crop will likely increase as a warming climate generates conditions favorable to the fungus A. flavus in more northerly parts of the continent. In what follows, we review and discuss recent findings by Yu et al. (2022) on how changing weather patterns may induce a similar northerly migration of occurrence here in the United States.

Historical patterns in insurance claims

As far as we know, in no grain-growing region of the world do grain intake facilities report aflatoxin test results to entities that make summary data available. Thus, evidence on the regional and temporal extent of aflatoxin incidence in commercial markets is indirect. In the United States, data are available from insurance claims made to the USDA Risk Management Agency for quality loss due to mycotoxins, where aflatoxin is the primary mycotoxin afflicting corn in the US South and in much of the Corn Belt. These data, while far from perfect, have broad coverage both temporally and spatially. For all the crops affected by aflatoxin, these insurance claims data are available at county level for each year at https://www.rma.usda.gov/SummaryOfBusiness/CauseOfLoss.

For more than 25 years, most corn acres have been enrolled in the federal crop insurance program. However, a claim is only made when the loss exceeds the deductible, a choice variable made by the farmer. In large part because insurance subsidies have become more generous over time and newer contract forms have become available, the deductible taken out by farmers has generally fallen over time such that the number of recorded claims has likely increased over time. Figure 1 shows the number of acres for which mycotoxin claims were made in Iowa, Illinois, and Indiana each year between 1989 and 2021. This figure also includes acreage claims made for mycotoxin in corn over the Central and Southern Great Plains states of Kansas, Nebraska, Oklahoma, and Texas. In the Corn Belt, the drought year of 2012 saw the most acres claimed, but claims have declined in recent years. In the Central and Southern Great Plains, 2017 saw a large number of claims but otherwise claims activity has declined since about 2014 in comparison with the 2004–2022 period. Not coincidentally, a new plant-incorporated toxin, Bacillus thuringiensis (Bt) became available around 2010. This Bt trait—Vegetative Insecticidal Protein (Vip)—proved very effective in preventing the sort of insect damage to the corn plant that allows for colonization by A. flavus. Unfortunately, evidence has emerged that resistance to these proteins is occurring in the field (Yang et al. 2021).

Weather/climate projections

The relationship between a year’s weather patterns and aflatoxin prevalence in that year is involved and somewhat challenging to identify because plant growth-stage during weather events is more relevant than calendar date. However, insights that have been well-established from laboratory and experimental trial work, as well as from the one existing study on commercial data (Yu et al. 2020) are that drought conditions in midsummer dispose a crop to colonization and that high temperatures in July are particularly problematic.

Figure 2 provides historical mean hours per day exposure to temperatures in the 30°–40° C interval over five-year historical periods, which we obtain for each county and then average across Iowa, Indiana, and Illinois. We also obtain projected future values of these variables from climate forecasting models. The represented values are from the MPI-ESM-LR climate model under the highest greenhouse gas (GHG) emission scenario, which Yu et al. (2022) use as a baseline model. The key point is that values are projected to rise (i.e., hours in which corn is exposed to hot temperatures that will render it more susceptible to A. flavus infection and subsequent aflatoxin contamination are expected to increase).

More generally, according to the Fourth National Climate Assessment, Midwest summers are expected to become hotter, wetter, and more humid, with more drought and flood extremes (Reidmiller et al. 2018), ideal conditions for the spread of A. flavus. One should bear in mind that adaptation in crop production will take place, and that some of these adaptations may reduce the impact of aflatoxin incidence. For example, the Midwestern corn growing season has already been shifting earlier because of climate change as well as altered tillage practices. In addition, irrigation is known to mitigate the incidence of aflatoxin and, water availability allowing, this may become a more common practice in Corn Belt corn production.

Policy issues and discussions

A long list of inquiries has been made into how climate change will affect corn production levels in the United States. Food quality consequences of climate change have received less attention than have quantity consequences, perhaps in part because quality measurements are harder to assess. It may also be true that in the United States, corn quality is not as climate-sensitive as quantity produced. Furthermore, the large majority of US corn production is destined for livestock feed and biofuel production, and not directly for human consumption in either domestic or international markets. In addition, the United States has a strong science and regulatory infrastructure to call upon to detect and regulate issues as they emerge. Nonetheless, our view is that more attention should be paid to quality and safety implications for grain production as these relate to near-term climate change.

Yu et al. (2022) use crop insurance claims data to place a monetary value on losses due to aflatoxin arising from climate change. For 15 states in the US South, Southern and Central Great Plains, Iowa, and Illinois, Yu et al. (2022) regress indemnity per premium dollar in a county on weather variables for that year as well as on a variety of controls. The approach uses Tobit analysis to account for the fact that most counties in most years saw no acres reported for mycotoxin loss. Yu et al. (2022) then insert climate projections as weather variables so as to establish how, all else fixed, changing weather patterns would affect mycotoxin claims. One adaptation allowed for was a shift in the corn growing season. Adjustments, through a markup, were then made for the fact that a claim will not be made when losses exist but are small. Table 1 reports estimates for states whose proximate locations allow for comparison with the Corn Belt.

The model identifies a monetary loss of about $225,000 per year for Iowa over the 2012–2021 period. In constant dollar terms, this amount is projected to increase tenfold from 2031 to 2040. Illinois—the majority of which is further south than Iowa—has always had greater risk of aflatoxin in its corn crop. We also expect that Illinois will see a large increase in damage done, as will Missouri, Nebraska, and Kansas. States further south that currently have major aflatoxin problems may not see an increase in damage done. In our analysis, this is because temperatures above 40° C are not favorable for aflatoxin production. In addition, but not modeled in Yu et al. (2022), corn production becomes increasingly problematic at higher temperatures so that corn acres in the South may decline for reasons unrelated to aflatoxin.

WOur analysis does not address technological adaptation. It is possible that varieties or pesticides will be developed that either directly address the fungus colonization issue or remove stressors that allow for the responsible fungi to establish. Bt toxins have provided protection since their commercialization in 1996 (Yu et al. 2020). However, because of high levels of use, insects have become less susceptible to these toxins. Of technologies on the horizon, new Bt and Vip proteins and their combinations may emerge to promote continued efficacy of Bt corn against insect pest damage and subsequent aflatoxin contamination. Additionally, biotech corn varieties may become available in which aflatoxin cannot thrive or becomes degraded (Wu 2022).

References

Battilani, P., P. Toscano, H.J. Van der Fels-Klerx, A. Moretti, M. Camardo Leggieri, C. Brera, A. Rortais, T. Goumperis, and T. Robinson. 2016. "Aflatoxin B1 Contamination in Maize in Europe Increases due to Climate Change." Scientific Reports 6: 24328.

Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.). 2018. Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II (pp. 987–1035). US Global Change Research Program. doi.org/ 10.7930/NCA4.2018.CH23.

Wu, F. 2022. "Mycotoxin Risks are Lower in Biotech Corn." Current Opinion in Biotechnology 78: 102792.

Wu, F. and G.P. Munkvold. 2008. "Mycotoxins in Ethanol Co-Products: Modeling Economic Impacts on the Livestock Industry." Journal of Agricultural & Food Chemistry 56: 3900–11.

Yang, F., D.L. Kerns, N.S. Little, J.C. Santiago González, B.E. Tabashnik. 2021. "Early Warning of Resistance to Bt Toxin Vip3Aa in Helicoverpa zea." Toxins 13: 618. https://doi.org/10.3390/toxins13090618.

Yu, J., D.A. Hennessy, F. Wu, and J. Tack. 2022. “Climate Change will Increase Aflatoxin Presence in US Corn.” Environmental Research Letters 17(5): 054017.

Yu, J., D.A. Hennessy, and F. Wu. 2020. “The Impact of Bt Corn on Aflatoxin-related Insurance Claims in the United States.” Scientific Reports 10(1): 10046.

Zhao, C., B. Liu, S. Piao, X. Wang, D.B. Lobell, Y. Huang, M. Huang,…, and S. Asseng. 2017. “Temperature Increase Reduces Global Yields of Major Crops in Four Independent Estimates.” Proceedings of the National Academy of Sciences 114(35): 9326–9331.

Suggested citation:

Yu, J., D. Hennessy, J. Tack, and F. Wu. 2023. "Climate Change and Aflatoxin in Corn." Agricultural Policy Review, Winter 2023. Center for Agricultural and Rural Development, Iowa State University. Available at www.card.iastate.edu/ag_policy_review/article/?a=151.