Nitrogenous fertilizers are not solely responsible for increases in grain yields of cereals. Consider yields of corn in the U.S. In the early 1900's it was discovered that inbreeding and hybridization offered an alternative route for improving corn yields over mass selection, which had been deemed to be ineffective. However, the marked loss of vigor due to inbreeding of parents tended to preclude the commercial use of hybrid vigor. Double cross hybrids were introduced in the 1930's. In double cross production, commercial seed is produced on high yielding single cross parents, so that limitations imposed by low-yielding inbred parent lines are removed. It should be noted that nearly all of the commercial hybrids used in the U.S. today are single crosses.

This change from double crosses to single crosses occurred in the early 1960's. Single crosses have produced a higher level of grain yield than double crosses. Modern inbred lines have been exposed to several cycles of inbreeding, with selection for increased seed production on the lines per se, and now produce adequate yields for production of commercial single-cross hybrid seed.

Approximately one-half of the gain in corn yields since 1950 can be attibuted to use of nitrogenous fertilizers and other improved management practices, and the other half of this gain can be attibuted to plant breeding efforts to develop improved hybrids. There is no indication that the yield gains per year since 1960 are flattening -- we have not yet reached the yield potential of corn [record corn yields occurred in 1994 in the U.S.]. Continued corn yield gains can be expected throughout the next 20 years as biotechnology is applied to maize.

See also: U.S. Corn Yield (1970 - present), and Corn Yield Trends for Indiana (1930 - 1999).