Exploring County Level Corn Yield Trends

As corn genetics and management have advanced and improved over time, yields have increased and become less variable. Advances in corn breeding and testing have allowed more genetic combinations to be tested each year in a diverse range of growing environments. Only those genetic combinations that can exceed the current class of hybrids in yield and yield stability are advanced into the marketplace. Technological advances such as biotech trait integration and double haploid inbred development have reduced the susceptibility of corn hybrids to pest pressures that historically caused high year-to-year yield swings, as well as increased the throughput of the hybrid product development pipeline.

Over the course of the hybrid corn era, corn yield has steadily increased at a rate of 1.9 bu/acre/year and year-to-year variability has decreased. However, these trends have not been uniform across the entire corn production area of the Midwest, nor over time periods. In this Crop Insights we explore how the adoption of improved genetics and traits has translated into real world results at the county level in the Midwestern United States over a key time period of seed corn development.

County-level yield estimates are developed each year by the USDA National Agricultural Statistics Service through surveys with farmers. Since 2020, model-based county-level estimates have been incorporated into the process for row crop county estimates. Inputs include National Commodity Crop Productivity Index (NCCPI) values for the county, current survey ratios, and survey standard errors. These estimates are benchmarked against previously released county estimates.

For a county’s estimate to be published, data must meet certain criteria, including a minimum number of reports where the respondent reported both harvested acreage and yield. Estimates that don’t meet these standards are combined with other counties of the state and published as “other counties” totals and averages. Yield estimates for a county used here are from all production methods, including irrigated and non-irrigated, in a weighted average.

County-level yield estimates were extracted from the NASS Quick Stats 2.0 database, selecting years 1946 to 2023, for twelve Midwestern U.S. states (North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, Illinois, Indiana, Michigan, and Ohio). The starting year of 1946 was selected to represent the beginning of the hybrid corn era. Hybrid corn was rapidly adopted by farmers as it became widely available in the late 1930s and early 1940s and, by 1946,  accounted for the majority of U.S. corn acreage. These data were analyzed for four time periods, the full hybrid corn era from 1946 to 2023 and three subperiods that correspond to significant advances in corn breeding:

• Hybrid Corn Era: 1946 to 2023
• Double Cross Era: 1946 to 1962, when double cross hybrids were most common. (Note: Kansas started reporting county-level corn yields in 1958).
• Single Cross Era: 1963 to 1995, starting when single cross hybrids were introduced.
• Biotech Era: 1996 to 2023, during which corn hybrids with biotech traits were widely adopted.

For each period and county, a line was fitted to the data points of year and county yield. The slope of the line indicates the year to-year increase in yield during the period. The relative yield variability of each period and county was also determined by taking the mean absolute error of county yearly yield vs the fitted line and dividing by the average yield for each period and county. Relative yield variability provides a measure of how much variability exists in yields year-over-year for a period and county.

Corn yield data from two counties (Washington County, in southeastern Iowa and Phelps County, in south central Nebraska) provide examples of the output in Figure 1 and Figure 2. Each black dot represents the yield reported for that year. The blue line is the yield increase fit line for all available years for the county, and red, yellow, and green lines are fitted for 1946 to 1962, 1963 to 1995, and 1995 to 2023, respectively. Each period’s yield increase and relative yield variability is also included on the graph.

In these examples, both counties appear to have relatively low levels of variability, with Phelps, Nebraska exhibiting very low variability. This may be in part due to the benefits of irrigation in moderating weather-based yield swings. Corn production in Phelps County, NE is largely under irrigation, while Washington County, IA is mostly rainfed.

Over the full time period of 1946 to 2023, the greatest year-over year yield increases occurred in central Illinois and the upper west parts of the Corn Belt (northwest half of Iowa, southern Minnesota, and eastern South and North Dakota), along with irrigated areas of Nebraska and southwestern Kansas (Figure 3).

During the double cross era of 1946 to 1962, yield growth was very slow in the Dakotas due to poorly matched or adapted genetics. Pockets of Iowa, Illinois, and Indiana saw large yield jumps in this period. Kansas is not included due to lack of complete county-level data. Central and Northwestern Nebraska also saw major gains due to growth in acreage under irrigation.

The single cross era of 1963 to 1995 saw major yield increases in Nebraska, southwestern Kansas, and the Missouri bootheel, presumably due to the introduction of large-scale irrigation management. Yield growth languished in southern and eastern Iowa and eastern Illinois. Upon deeper examination this was due to highly variable yields during this period for those counties.  For example, Washington County, Iowa (eastern part of the state) had a yield increase of 0.84 bushels per acre per year during this period with a coefficient of variation of 16.6%. Drought years of 1983 and 1988 pulled down the generally increasing yield trend. See the Washington County, Iowa graph above for more details.

In the biotech era of 1996 to 2023, major yield growth occurred throughout the Midwest, primarily in Illinois but also in Indiana, Ohio, Michigan, Nebraska, and the Dakotas.  Despite the benefit of irrigation, Kansas yield growth stagnated during this time to less than 1 bu/acre/year. This may be due to more acres of lower yielding dryland corn coming into production during this time, utilizing newly developed drought-tolerant genetics. Curiously, muted yield growth was exhibited in a band of counties running north-south through central Iowa. The drought of 2012 and wet conditions in 2013 played a part in this reduction of yield growth for that area.

The relative yield variability is a useful statistic to illustrate the relative variability of yield that a period and county has compared to its yield levels during that period.

Any reduction in relative yield variability can be due to improvements in genetics, both increasing overall yields and reducing losses during stressful environments. This also may be due to technological improvements, such as irrigation, pest control (weeds, insects and disease), and operations timeliness due to larger and faster planting and harvesting equipment.

The relative yield variability map for 1946-2023 provides guidance to the overall variation of corn production across the Midwest. The irrigated areas of Nebraska and southwestern Kansas, northern Iowa, southern Minnesota and Wisconsin, and northern Illinois have lower relative yield variability than other parts of the Midwest, such as the Dakotas, Kansas, and Missouri.

During the double cross era of 1946-1962, parts of Illinois, Indiana, Wisconsin, and southern Minnesota exhibited lower relative yield variation. However, this variation increased into western Iowa, Nebraska, Missouri, and the Dakotas, likely due to genetics that were not as tolerant of the variable environments found in those regions. The benefits of irrigation are visible in Central Nebraska, reflected in reduced relative yield variability compared to Eastern Nebraska where irrigation was not in place at the time.

Variation is reduced during the single cross era of 1963-1995 in the central Nebraska and southwestern Kansas regions, again due to the implementation of irrigation during that time. The Dakotas, eastern Kansas, Missouri, and southern Iowa still exhibit higher variation due to the environment and genetics matched in those areas.

However, during the biotech era of 1996-2023 there was a great reduction in variation across large areas of the Midwest, including previously variable regions, like the eastern Dakotas.  Improvements in genetics and traits, including weed and insect control, are likely contributors to the reduction in variation. A few areas of southern Iowa, Missouri, eastern and northern Kansas, and west of the Missouri River in the Dakotas still show relatively high levels of variation compared to the main Corn Belt region. Improvements in genetics helped to bring down the overall variation in many areas, but environmental conditions still have greater influence in others.

1983, 1988, and 2012 were years in which widespread severe drought sharply reduced corn yields and in 1993, extensive flooding and below average temperatures reduced yields and delayed maturity (Figure 5). These four years are selected to illustrate the effects that localized droughts and flooding have on yield performance in particular regions. These depressed yields generated higher variability in yield and reduced the year over-year yield growth during those periods for those affected counties. The yield loss was due in part to the environment (weather and soils), but also due to the way the genetics at that time responded to the environment. Advances in genetics – particularly, improvements in drought tolerance – have reduced the impact of environmental extremes over time. Corn yields in 1983, 1998, and 2012 were all sharply below the trendline average for their respective time periods; however, yields were considerably higher in 2012 compared to 1983 and 1988 in most areas.

Focusing on the compiled results from the 12 states, there was a 1.88 bu/acre/year yield increase from 1946 to 2023 (Figure 6). During the double cross era of 1946 to 1962, the yield increase was 1.66 bu/acre/year. Interestingly, the yield increase was about the same during the single cross era of 1963 to 1995 with 1.59 bu/acre/year, along with an increase of relative yield variability from 7.5 to 9.4%. However, during the biotech era of 1996 to 2023, both the yearly yield rate of increase improved to 1.90 bu/acre/year and a reduction of variation occurred, with the relative yield variability dropping to 3.8%. The one year with a great deviation from the trend line was the drought year of 2012 with a mean yield of 122.5 bu/acre for the region.

Corn yields in the Midwestern United States have increased over time, primarily due to improved genetics, traits, and management. However, the improvement has not been uniform spatially or temporally. Some periods show more improvement in yield growth rate than others. Some periods show a greater reduction in variation than others. Areas like Illinois and central Nebraska show strong yield growth and reduced variation, while “fringe” areas like the Dakotas, Missouri and southern Iowa still have relatively high levels of variation and less yield growth.

These maps and graphs help to show where growth has occurred and where it can still take place with focused genetics and management. While the soils and weather in the “fringe” areas cannot be easily changed, the selection of proper genetics, along with improved management (variable rate seeding, cover crops, and trait selection) may provide an opportunity to drive greater yield gains and reduce year-to-year variation.

These maps and graphs can also provide useful context as to how far the seed corn industry has come over 78 years. Improvements in breeding, testing, and trait integration are showing their effects, especially in recent years. Environments that caused large yield reductions in the past are now increasingly muted due to the improvements made in the industry.

• National Agricultural Statistics Service, USDA.
• Butzen, S. and S. Smith. 2014. Corn Yield Gains Due to Genetic and Management Improvements. Pioneer Crop Insights Vol. 24 No. 12. Corteva Agriscience. Johnston IA.
• Nielsen, R. 2023. Historical Corn Grain Yields in the U.S. Corny News Network. News Network. http://www.kingcorn.org/news/timeless/YieldTrends.html
• Nielsen, R. 2024. Grain Yield Trend Lines: Don’t Be Fooled. Corny News Network. http://www.kingcorn.org/news/timeless/YieldTrend Estimation.html