Variability Of Soybean Yields In Kentucky

DR. DENNIS B. ELGI

LEXINGTON, KENTUCKY

When it’s soybean harvest time we start thinking about yields. This year’s yield in Kentucky is estimated at 52.2 bushels per acre from 2.04 million acres (September 1 estimate by the National Agricultural Statistics Service). That is a good yield, but it doesn’t beat the 56.6 bushels per acre in 2021. 

The average yield provides an indication of the statewide productivity, but what about yields at smaller scales? How much yield variation is there among the counties where soybean is a major crop?

I looked at the average county yields for the last 10 years (2014 – 2023) for 30 counties in Kentucky that grow most of our soybean crop. The average yields, arranged from the lowest (Marshall) to the highest yielding county (Union), are shown in the bar graph in Fig. 1. Union County produced an average yield that was 14.5 bushels per acre higher than Marshall County, but it was only 1.4 bushels higher than the next highest county (Hancock).  The big question is – what causes this variability?

The availability of technology (high-yielding, disease-resistant varieties, herbicides, pesticides to control diseases and insects, fertilizers and the industries that supply these inputs) is a vital component of any high-yielding soybean production system. This technology should be equally available to Kentucky producers in all counties, so I don’t think the availability of the latest technology caused the yield variation shown in Fig. 1.

Managing the technology is another key to high soybean yields. The availability of technology is one thing; selecting the appropriate technology and  applying it correctly is another. Management skills no doubt account for some of the yield variation among producers, but it’s hard to argue that farmers in some counties are better managers than in other counties. Especially in this information age when a blizzard of facts and figures is as close as your cell phone. I don’t think that populating Marshall County with farmers from Union County would increase their average yields by14.5 bushels per acre.

High yields are very much dependent on the weather, especially summer rainfall. The summer rainfall in any given year can favor one county over another depending upon the vagaries of the summer thunderstorms. This variation is random in the relatively small area where Kentucky grows its soybeans, but it could have some influence on average yields calculated over only 10 years. However, the variation among counties was similar when yields were averaged over 40 years (1972 – 2011), so it is unlikely, in my opinion, that weather conditions would consistently favor one county over another year after year (after all, Marshall and Union counties are roughly only 60 miles or so apart).

The final item on my list is the soil – a fundamental component of any production system. Could variation in soil characteristics account for some of the differences among counties? 

My soil science friends tell me that Union County is blessed with deep soils that store lots of water which could provide the basis for its high yields. The availability of water in the summer often limits soybean yields in Kentucky, so having a larger storage reservoir would be a distinct advantage. 

Many counties in Western Kentucky are cursed with soils that contain a hardpan (fragipans) that roots can’t penetrate, limiting the amount of water they can store and making them much more dependent on timely summer rains for high yields. Its likely that the variation in soil quality contributes to the variation in county yields (Fig. 1).

Average yields are influenced by year-to-year variation in yield (feast or famine). The yields in the lower yielding counties (e.g., Calloway County in Fig. 2) are really low in bad years (presumably dry years), while the yields in bad years in Union County are lower but not drastically so (Fig. 2). If rainfall is short, a crop on a soil with a smaller reservoir to store water will be affected more than a soil with a large reservoir.   

Several years ago, I worked with an ex-student of mine (Dr. Jerry Hatfield, recently retired as Director of the USDA – ARS National Laboratory for Agriculture and the Environment at Ames, IA) to evaluate the variation in potential yields (yield with no stress) among counties. We used a special regression technique to fit a line to the top 5% of the yields in a county over a 40-year period (dotted line in Fig. 2). We took this line, which represented the yields in the most favorable environments over the 40 years, as an estimate of the potential yield. The potential of Union County was always higher than the potential yield of Calloway County (county next to Marshall County in Fig. 1) (Fig. 2). The potential yield decreased in step with the average yields from Union County down to Marshall County. 

The low-yield counties couldn’t produce ‘Union County’ yields even with the most favorable weather conditions. The favorable weather conditions could not overcome the yield limitations (presumably soil based) in the lower-yielding counties. The lower yielding counties produced really low yields in bad years and were not able to produce really high yields in good years – a real double whammy!

We often think that yield in any particular soybean field is a result of management and weather, but there is a third factor lurking in the background – the yield expectations for that field (i.e., soil characteristics). The bottom  line is that yield expectations vary among counties (and, for sure, within counties); managing for super-high yields may not be realistic (and uneconomic to boot) in some counties in Kentucky. Don’t forget, “its not the strongest, or the fittest, but the one willing to change that survives” (Often attributed to Charles Darwin, English Biologist, 1809-1882).   ∆

DR. DENNIS B. ELGI: University of Kentucky