A fundamental component of agricultural productivity is genetic gain in crop yields. Improving the genetic potential of crop yields has been central to most of the world escaping the recurring cycle of famine and starvation. But how exactly can we measure the contribution of genetic improvement to crop yields over time? It is straightforward to estimate statistical trends in crop yields, but the resulting trends confound genetic gain, management improvements like increasing rates of fertilization, and climatic change. Crop scientists have developed a clever approach to disentangling these different factors in “heritage” or “era” studies. In simplest terms, seed is collected from crop varieties developed over many decades (one of the global storage facilities is located here at the University of Illinois), all the varieties are planted under the same experimental conditions, and yields are collected. One can then plot yield versus year of variety release as a measure of genetic gain. An example of a heritage study for soybeans is Rinker et al. (2014).
Heritage studies are the gold standard for measuring the contribution of genetic improvement in crop yields, and dozens have been published. However, these studies are expensive to conduct and are limited to the most popular varieties that have been saved in germplasm banks. An alternative approach is the use of “check” varieties in ongoing crop yield trials, which is precisely the subject of a study by Jared Hutchins and me that was recently published in the American Journal of Agricultural Economics (AJAE). The article is entitled, “Productivity Growth from Genetic Improvement: Evidence from Illinois Soybean Trial Data.” In particular, we estimate yield growth due to genetic change in soybeans using Illinois variety trial data and public check varieties that are planted consistently from 1997 to 2020. The public varieties serve as a control variable to identify the gain in yield across years using only yield variation within the plot and year. The public check varieties that appear in the Illinois soybean yield trials were originally developed by the Crop Sciences Department at the University of Illinois. The following figure shows where and in what years each of the three public varieties was included in the trials.
The main results of the study are illustrated in the next figure. Based on this data, we calculate the linear rate of annual genetic improvement to be 25–26 kg/ha (0.34–0.4 bushels per acre) or roughly 0.7% per year, an estimate that is a little lower than previous estimates of soybean yield growth from genetic improvement. Without controlling for weather, the soybean yield trend in Illinois yield trials over the same period was 47–54 kg/ha or 1.2%–1.4% per year. This suggests that genetic improvement explains roughly half of the yield improvements in these trials, independent of management. We also produce the first estimates of genetic gain in private soybean varieties post-2010 and find that yield gain from genetics grew at a roughly linear rate from 1997 to 2020. To the degree that the results can be generalized to other important soybean growing areas in the U.S., our findings suggest there has been no slowing in the rate of genetic improvement in U.S. soybean yields.
There is an interesting backstory to how this study came about in the first place. Soybean yields began jumping notably around 2014, and the argument in the grain trade was that “new seeds” were responsible. So, in the Fall of 2017, I decided to ask the best-known soybean breeder at the University of Illinois, Brian Diers, whether he thought there was any foundation to the argument. Brian was very generous with his time, and I learned a lot about the basics of soybean breeding from him in our conversations. After one meeting, he thought I might be interested in some soybean yield trial data that he had collected over the years, which included public varieties that could serve as a genetic improvement control variable. Brian kindly shared this data, and I used it in a presentation I prepared for the Illinois Farm Economic Summit Meetings held in December 2017.
I thought that the data Brian shared was quite novel and might make the basis of an interesting article for the agricultural economics literature. But I knew I would need to work with someone with the requisite technical skills in order to get an article published on the topic. Fortune shined on me in 2020 when Jared Hutchins joined our Department after finishing his PhD from the University of Wisconsin. I approached him shortly after he arrived to see if he was interested in working on a paper using the data. Luckily for me, he was interested. We proceeded to collect a more comprehensive version of the variety yield trial data and then wrote the paper that was ultimately published in the AJAE. One of the real pleasures of academic research is the opportunity to work with incredibly talented young scholars such as Jared.
I would be remiss if I did not give a big thank you and shout out to Brian Diers for sharing the original version of the data and providing valuable comments on earlier drafts of the article. We are also very grateful to Jesse Tack, the editor in charge of the paper at the AJAE, for patiently helping us improve the paper through the review rounds. The peer review process takes a lot of heat these days, but this is an example where the process worked as intended.
Looking to the future, now that we have a good handle on the rate of genetic improvement in Illinois soybean yields, I would like to work on a more complete explanation of soybean trend yield. As I noted above, genetics explains about half of the yield trend in Illinois since the late 1990s. What explains the other half? Is it climate changing for the better (not as crazy as it sounds)? Is it something called CO2 fertilization? Is it due to earlier planting dates? Or, is it something else? There is still much work to be done.
Laurence J. Norton Chair of Agricultural Marketing
University of Illinois at Urbana-Champaign