For the last decade and a half, a mysterious and worrisome trend has emerged in the farming world that has sent farmers, scientists and policy makers looking for answers.

Crop yields – how much of a crop is harvested per hectare – for some of the world’s major grains like rice, wheat and corn have gone from increasing year after year to plateauing in many of the world’s biggest grain producers.

Until about the middle of the 20th century, the only way farmers knew to increase production was to increase the area of land they farmed. But around that time, science and farming came together, and the advent of artificial fertilisers, advanced irrigation systems and a surge in selective breeding changed the way farmers grew their crops.

Suddenly, farmers could increase production without expanding farmland, and the global grain harvest exploded. From 1950 to 2011, the global grain harvest tripled, growing twice as much in those 61 years as it had in the history of agriculture before 1950, according to Lester Brown, founder of the Earth Policy Institute (EPI) and a food security expert.

But Brown, once a farmer himself, said farmers knew it would not last forever.

“The plateauing of grain yields is something we knew would come,” he said. “Farmers realize that there is a glass ceiling at some point that they can’t go beyond.”

With increases in population adding to the demand for food but eating up the land on which to grow it, that could pose an increasingly serious problem.

GROWING DEMAND

The U.N. Food and Agriculture Organisation (FAO) predicted in 2009 that a growing and increasingly affluent population will require 70 percent more food by 2050, and experts say we cannot count on creating new farmland to fill that gap.

“If we meet demand by expanding agriculture, that’s the worst thing you can do for climate change and biodiversity,” said Ken Cassman, Robert B. Daughtery professor of agronomy at the University of Nebraska-Lincoln.

He said we’re already seeing the effects of upping grain production by expansion in rainforests and on prairies, which has left us with only “remnants of ecosystems.” That leaves it to up to better yields to ensure we have enough grain to feed the world, he said.

Beginning in the 1980s, however, that has become more difficult. Around that time, rice yields in the Republic of Korea stopped increasing, and in the United States, some of the best maize farmers were increasingly hitting a wall.

Every year, farmers from all over the United States enter state and national farming contests to see who has the highest yield. The winner gets free seeds and tractors, and is paid to tour the country and give talks about how he or she did it.

“Thousands of farmers are motivated to win that contest,” Cassman said.

Yet, by the late 1990s, Cassman noticed the winning yields in those competitions had not changed significantly for several years. He gathered that if the most motivated farmers in the country failed to increase their yields, it was likely because they could not.

He hypothesised they had hit the crop’s yield potential, or the highest yield attainable using the best seeds and optimal levels of nutrients and water.

REACHING THE CEILING

Since then, he and other scientists have developed models, verified by crops grown in controlled lab settings, to estimate maize’s yield potential in different parts of the United States. They found those farmers had indeed reached it.

In December of 1998, Cassman presented a paper to the National Academy of Sciences that hypothesised national yields would plateau at about 70-80 percent of a crop’s yield potential.

“What we found initially is that all farmers can’t be operating at 100 percent of production,” he said. “You plateau because it’s no longer possible to do every little thing possible to push yields to the ceiling.”

He explained that, unlike farmers who enter yield contests, it does not make economic sense for most farmers to pour time and money into exacting applications of fertiliser and other inputs to increase yields by an extra 20 or 30 percent.

Using the models he and other scientists developed, the agronomist found countries were plateauing at just below their crops’ yield potential all over the world. Countries that have hit this point make up a sizeable chunk of the world grain market, he and other experts say.

“When you add up all the countries where yields are plateauing, they account for about 40 percent of world grain production,” Brown agreed.

Brown also thinks crops have hit a biological limit. Like an Olympic runner pushing the limits of the human body, some crops are hitting the limits of what they can produce, after being pushed as far as they can go through the use of fertilisers, better irrigation and selective breeding, he said.

“The ultimate limit is the process of photosynthesis itself, and that’s why we see gain yields plateau in country after country,” Brown said.

WATER LIMITS

But Tom Sinclair, an adjunct professor of crop science at North Carolina State University, with years of experience researching crop physiology, thinks water constraints will limit food production long before the limit of photosynthesis are reached.

“In the U.S. we’re starting to approach as much as you can get with the water we have,” he said – and the water is running out.

In some parts of the U.S., irrigation is what ensures crops prosper, and, in some cases, that they can survive at all. But nearly everywhere that crops are irrigated, water levels are dropping.

That leaves some experts concerned that if farmers continue to use as much water as they do now – 70 percent of the fresh water used each year around the world goes to grow crops – there won’t be enough left to continue irrigating in the future.

“If the irrigation level is not there then (farmers are) definitely bound by the amount of rainfall,” which a changing climate is making increasingly erratic and unpredictable, Sinclair said.

Not everyone agrees farmers are nearing yield limits, however. In the UK, many people say figuring out the cause of stagnating yields is not so simple.

“There is no one specific factor,” said James Mills, a policy advisor for the British National Farmers Union who represents the interests of wheat farmers. “If you talked to a hundred farmers you might get a hundred different answers.”

A report produced for the UK’s Department of Environment, Food and Rural

Affairs (DEFRA) in 2012 investigated apparent yield plateaus for wheat and oilseed rape (used to produce vegetable oil) and found a variety of factors at play, including individual farming practices and regulatory constraints on fertilisers.

“It’s not a black and white picture,” said Simon Kightley of the UK-based National Institute for Agricultural Botany and an author of the report.

Still, most people have a hunch what the major forces are holding back growth of yields. Kightley said he thinks on-the-farm practices, like how much or how little farmers plow, play a significant role in why yields are plateauing.

THE ROLE OF CLIMATE CHANGE

Mills said many farmers feel government regulations on fertiliser use and protective seed coating have kept them from getting the boost in yields they are hoping for. But increasingly extreme weather also has become a serious contributing factor, especially in the last year and a half, he said.

That was also the conclusion of a study published in 2010 in Fields Crop Research that asserted climate change, and the increases in drought and extreme heat that result from it, is the primary driver of Europe’s stagnation in wheat yields.

But Brown said it is difficult, if not impossible, to measure the impact of climate change.

Focusing on the places where there is still room to increase yields is key to producing enough food without further damaging the environment and its ecosystems, said Cassman, the Nebraska agronomist.

The first step, he said, is to identify those places. His work now focuses on cataloguing the “yield gap” of crops in countries all over the world, looking at how much room there is to increase yields before they reach their limits.

“I’m completely optimistic we can meet food demand, but you have to start making the right choices and investing in the right research,” he said. “Global agriculture by intelligent design. That’s what we need.”