BOSTON (TheStreet) -- We have all heard of peak oil, but peak grains? A study released by the Institute of Agriculture and Natural Resources at the University of Nebraska-Lincoln suggests we may be heading in that direction -- if we're not already there.

The UNL study indicates that about 30% of major global cereal crops -- including rice, wheat and corn -- may have already reached their maximum yields. In fact, yields of these crops seem to have already hit a plateau and some are already decreasing, especially in eastern Asia, Europe and the United States.

"We found widespread deceleration in the relative rate of increase of average yields of the major cereal crops during the 1990-2010 period in countries with greatest production of these crops," says an article based on the study in Nature Communications. The article notes that there was a noticeable plateau or drop in crop yields in 44% of examined cases, which together accounted for 31% of total global rice, wheat and corn production.

In the past, previous projections of future food production trends were based largely on positive assumptions about yields: that food production would grow in tandem with the human population, as it has historically. Past trends were often dictated by the development of technologies or breakthroughs that enabled increases in crop yields, such as specialized varieties of wheat and rice, irrigation infrastructure and commercial fertilizers and pesticides.

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Even with continued use of these technologies, the UNL researchers found crop yields were still stabilizing or decreasing, even in areas that have been prolifically productive in the past. At the same time, the human population is expected to add another 2 billion over the next few decades -- reaching 9 billion by midcentury.

Considering that the global population gets 75% of its dietary nourishment from four crops that include corn, wheat and rice, the study could have serious implications for food security. This is not even considering climate change, which might further compromise yields.

"On a global scale, we can see pretty clearly significant changes in the weather for most places where we grow crops," agricultural scientist David Lobell of Stanford University's Woods Institute for the Environment told Scientific American in June 2011. "Those changes are big enough to sum up to pretty big losses for wheat and corn."

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UNL researchers calculated that the stagnation and decline of cereal crops could affect 33% of major rice-producing countries and 27% of major wheat-producing countries.

China experienced a 64% decline in the rate of corn yield increases between 2010 and 2011, and its wheat yields have remained relatively static. The United States has also maintained a linear yield of corn in recent years despite a 58% increase in research and development funding for the crop.

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The United States is the largest producer of corn in the world, making up 32% of the global crop as of 2010. Corn also accounts for nearly a quarter of all of the harvested crop acreage in the country. According to the National Corn Growers Association, about 80% of U.S.-produced corn is used as animal feed either domestically or overseas, while only 12% goes to food intended for human consumption. For wheat, the U.S. produces about 10% of the global supply and exports a quarter of it, with about 22% used for livestock feed. And even though U.S. rice production accounts for just under 2% of the global share, we are the second-leading rice exporter, making up 10% of the world market.

According to the UNL study, sustaining yield gains for these crops would likely require a lot of adjustments in how the crops are grown, including planting crop rows closer together and exercising better weed and pest control. Such adjustments might be costly and risky for the environment, though while offering only modest yield increases ranging from 10% to 20%.

"Today, with farmers achieving much higher yields than at the beginning of the green revolution, there are no silver bullet technologies that can give substantial increase in yield," says Kenneth Cassman, professor of Agronomy and Horticulture at UNL and co-author of the study.

The study concluded that by overestimating how much food we will be able to grow in the coming decades, we are compromising our ability to plan properly for future food security.

"Accurate estimates of rate of future crop yield gains over the next decades is essential to inform agricultural policies and for prioritization of research investments in the public sector to ensure global food security," Cassman says.