Kentucky Corn and Soybean Yields in 2023 Compared to Past Years

Kentucky corn and soybean yields this year were better than many expected. They didn’t set records, but they were close. November estimates put corn yield at 183 bu./acre (record is 192 bu./acre in 2021), while soybean came in at 55 bu. /acre (record is 56 bu./acre in 2021) according to the National Agricultural Statistics Service. The abnormally dry and drought conditions that developed in Kentucky in June and early July (according to the Drought Monitor, published weekly at https:// droughtmonitor.unl.edu/CurrentMap/aspx) fueled concerns that yields might be low this year. There was, however, no indication of any lack of moisture from mid-July to mid-September according to the Drought Monitor. Rainfall in the soybean growing areas of Kentucky was mostly above normal during this period and temperatures tended to be slightly below normal. The relatively cool and wet weather during the critical part of the growing season no doubt contributed to the good yields.

Corn and soybean yields in Kentucky increased steadily from the approximate beginning of the highinput era of agriculture in 1950 (Fig. 1). Linear regression curves described the trends in the data in Fig. 1 [The regression analyses were statistically significant and the r2’s were high for corn (0.93) and soybean (0.89)], and they provide no evidence that yields are starting to plateau. Apparent plateaus have occurred in the past (see, for example, corn from 1976 through 1988 or soybean from 1971 through 1976), but they were probably weather related and yield growth continued when favorable weather returned. The linear curves also suggest that climate change has not yet reduced corn and soybean yields; that reduction will probably become apparent in the future as climate change intensifies.

The variation of yield from year-to-year is primarily a result of variation in weather conditions, with rainfall being the most important. The water available to a crop depends on rainfall and the water stored in the soil. Unfortunately, Kentucky has substantial areas of soil that are shallow (often as a result of hardpans) and don’t hold much water. Year-to-year variation of yield on these soils is much greater than on soils with better water holding characteristics. Frequent rainfall is required for high yields of crops growing on shallow soils. Soils that can store more water can withstand longer dry periods without losing yield. The larger deviations for Calloway County (low yield) compared with Union County (high yield) provide a vivid example of the greater variability of yield associated with soils with lower water holding capacity (Fig. 2).

Interestingly, weather does not always affect the two crops in the same way in the same year (Fig. 1). The year 2012 is an obvious example of this differential effect – corn yields were drastically reduced (54% below the trend line) while soybean yields were just 10 % below the trend line. In 1983, both crops showed large reductions in yield (~48% below the trend line) (Fig. 1).

The key to the effect of drought stress on yield is the growth stage of the crop when the stress occurs. Both corn and soybean can tolerate drought stress during vegetative growth much better than during reproductive growth. Stress when the crop is deciding how many seeds (kernels) to produce (silking plus or minus 20 days in corn, growth stage R1 to R5 in soybean) will reduce the number of seeds (kernels) and yield. If the stress is relieved during seed filling, the crop may not be able to recover all of the lost yield by producing larger seeds, making the flowering/seed set period a very critical period. Stress during seed filling (after seed number is determined) will reduce yield by reducing seed size (weight per seed).

The importance of growth stage in determining the effect of drought stress on yield provides a mechanism to explain differential effects of stress on corn and soybean yields. For example, a lack of rainfall could result in stress around silking, reducing corn yield, but if the stress misses the soybean flowering period, yield would not be affected. The timing of stress and critical growth stages probably explains most of the differential year-to-year variation of yield in Fig. 1.

Increasing yields (Fig. 1) are usually attributed to improved varieties or hybrids (genetics) or better management. There is considerable disagreement over the relative importance of these two sources of yield improvement, not surprisingly, plant breeders tend to favor the first explanation, while agronomists and crop physiologists favor the second. In reality, I think it is impossible to separate the two sources. I don’t think it would be possible to produce bragging yields by applying modern management techniques to a 1960’s variety (hybrid). Conversely, the same can be said for growing 2020’s varieties (hybrids) with 1960’s management. Improved management and improved varieties (hybrids) go together, and it is a waste of time to argue about which is more important.

Improvements in weather could also increase yields. Interestingly, one important aspect of the environment is the steady increase in CO2 concentrations in the atmosphere which could contribute to higher soybean yields. Photosynthesis in soybean (C3 type photosynthesis) increases as the CO2 concentration increases which should result in higher yield. Higher CO2 concentrations do not directly affect photosynthesis in corn (C4 type photosynthesis). Ironically, the same gas (CO2) that is causing climate change that is expected to decrease yield is also increasing soybean yield.

This year was a good year for Kentucky corn and soybean producers as yields were near record levels. Good weather conditions allowed the high yield potential created by modern high yielding varieties (hybrids) and up-to-date management practices to be expressed. Producers can ensure high yield potential by selecting high yielding varieties (hybrids) and utilizing the best management practices, but they are still at the mercy of the weather.

Dr. Dennis Egli

UK Professor Emeritus

(859)218-0753

degli@uly.edu