Corn Grain Yields, 1930 to Today

The national average corn grain yield in the United States began to increase steadily in the 1940s (Figure 11). In the most recent decade, the average yield was 125 bushels per acre, nearly five times greater than 70 years before. Several studies conducted by universities have indicated that much of this improved yield was the result of improved genetics; that is, it occurred because farmers were planting improved varieties of corn developed through plant breeding. Greater use of fertilizer, more and better herbicides, improved soil tillage, and other altered production practices also contributed to the increased yields. 

The beginning of the yield increase coincides with the beginning of the transition by farmers from planting open-pollinated varieties to planting hybrids. However, not all of the yield increase that occurred during the past 70 years can be explained by hybrid vigor.

The yield advantage of a single-cross hybrid produced by crossing two inbreds developed from two different open-pollinated varieties over the average of the two open-pollinated varieties varies greatly depending upon the open-pollinated varieties that are chosen. It may be as great as 100%, but in many instances will be less. But improvement in average yields from 1930 to 2002 was 400%. In addition to hybrid vigor, genetic improvements were made. Today’s single-cross hybrids yield more than the single-cross hybrids of 70 years ago. Also, public corn breeders have developed many varieties (often called populations or synthetics) that are superior to the open-pollinated varieties that were popular before the introduction of hybrids. 

Why were corn breeders in the mid- and late-20th century able to make such substantial genetic improvements for grain yield, whereas no increase in yields was realized from 1870 to 1930? The development of single-cross hybrids was partly the answer. But two other factors contributed.

• New testing methods: By the 1920s, many people were beginning to question the validity of the corn shows. One of the outspoken leaders of this anti-corn show group was Henry Wallace, an eventual founder of the Pioneer Hi-Bred Seed Company. Not only did some seed companies and state universities begin conducting yield tests to compare various strains of open-pollinated varieties, but these tests began to be designed using new statistical methods that produced more meaningful comparisons between different entries. This new reliance on yield testing helped to usher in the quick acceptance by farmers of the new higher yielding hybrids when they became available.
• New breeding methods: For centuries, farmers had made selections based on the performance of individual plants. The seed for the following year’s crop was taken from the most desirable ears from the most desirable open-pollinated plants. This type of selection is known as mass selection. The new professional breeders were using various methods of what is known as family selection. Mass selection and the new selection methods are topics of other lessons in this series.