Are Corn Yields Increasing Faster Than Soybean Yields?
DR. D.B. Egli
LEXINGTON, KENTUCKY
Corn and soybean yields increased steadily since the beginning of the high-input era of agriculture in the 1940s and ‘50s. Improved varieties (hybrids) and better management practices drove these increases. Many producers believe that corn has benefited more from improved technology than soybean. Are corn yields increasing faster than soybean yields?
The fact that these two crops are very different lends credence to this observation. Corn is a grass of tropical origin with C4-type photosynthesis that produces a high starch seed. Soybean originated in northern China; it is a legume with C3-type photosynthesis that produces a seed containing high levels of oil and protein. Corn responds readily to N fertilizer; soybean, as a legume, makes is own N. The C3-type photosynthesis in soybean responds to higher CO2concentrations in the air, corn does not. Historically more breeding effort has been allocated to corn than to soybean. Given these differences, it would not be surprising if there were differences in the rate of yield improvement. The evidence, however, suggests that the rates are about the same for both crops.
One way to look at this question is to evaluate changes in the ratio of corn yield to soybean yield over time. The ratio will increase if corn yield is increasing faster than soybean yield. This ratio, calculated from average U.S. yields (after conversion from bushels/acre to pounds/acre), did not change from 1980 through 2019 (Figure 1). Yields of both crops increased steadily during this period, but the relative rate of increase was the same. Dr. Jim Specht at the University of Nebraska also found no difference in relative growth rates between the two crops.
The ratio in Figure 1 fluctuated from year to year as the weather and growing conditions affected the yield of the two crops differently. In some years, corn was favored (higher ratio), in other years, soybean was favored (lower ratio), but, on the average, the ratio did not change.
If we go farther back in time, back to the beginning of the high-input era of agriculture, the ratios (using average state yields from Indiana, Illinois, Iowa, Kentucky, Tennessee, and Missouri) increased steadily from 1950 (when the ratio was approximately 2.0) through the late 1970s (high-yield states) or 1980s (low-yield states) before they plateaued at approximately 3.0. During this early period, corn yields were increasing faster than soybean yields, so the ratios increased, but after they plateaued, there was no further change in the ratio through 2019, except for Iowa, where it increased very slowly (but significantly).
Yield ratios did not change from 1972 through 2015 in most of the counties in Kentucky and Nebraska (irrigated only). However, 30 of 47 counties in Iowa showed small, but significant, increases in the ratio, but they were usually less than 0.002 ratio units per decade. Four of the six counties in Kentucky with most of the soybeans double cropped after wheat also showed significant increases in the ratio over time, reflecting the lower rate of yield growth of the late planted double-cropped soybeans.
Why do so many producers have the perception that corn yields are rising faster than soybean yields? First, I think comparing absolute growth rates (bushels/acre/year) confuses us. Corn has a much higher absolute growth rate than soybean, by virtue of its higher yield. The correct comparison is the relative growth rate (percent per year) which is evaluated by the ratio. Secondly, a 10% yield increase for 250-bushel corn (25 bushels) is much larger and more obvious than the same increase for 60-bushel beans (6 bushels). These illusions incorrectly suggest that corn yields are more responsive to new technologies than soybean yields.
Perceptions can be misleading and the perception that corn yields are increasing faster than soybean yields is not correct. Careful evaluation of the numbers at national, state and county levels shows us that, by and large, yields are increasing at the same relative rate for both crops with only a few minor exceptions. The formula for increasing yields for either crop is the same – select the best varieties (hybrids), use good management practices that include providing adequate soil fertility, good weed, disease and insect control, and pray for rain. ∆
Dr. D.B. Egli: Dept. of Plant and Soil Sciences, University of Kentucky