Last winter, the United States emerged from the fourth-warmest winter on record. Farmers across the Midwest and elsewhere readied their tractors, equipment, and seeds for planting in an unusually early spring. With less snowmelt, the soil was drier than usual, and a long-term drought in the southern states of New Mexico, Texas, and Louisiana expanded to cover much of the rest of the country. By June, the drought covered 56% of the continental United States, when a heat wave hit and promised to dry the soil even further.
The drought had a significant impact on agriculture and continued to worsen into July and August—critical times for crop development. By September, the USDA had designated more than 2,000 counties as disaster areas, with more than 80% of farmland impacted by the drought. It was more extensive than any drought recorded since the 1950s.
If you drove through the Corn Belt in July, you may have thought you were in Hawaii gazing at plantations of spiny pineapples rather than a field of Minnesota or Iowa corn. Instead of being greeted by walls of the usual tall, green plants, you would have surveyed short, browning plants with curled leaves—symptoms of the persistent lack of water.
At the epicenter of the drought, corn and soybeans in the Midwest were hit the hardest. At the end of the season, corn yields were 122 bu/ac—the lowest since 1995, while soybeans reached only 39 bu—a level not seen since 2003. Commodity prices rose, as did the costs to consumers for many foods, especially meat, which is very dependent on grains for feed. Exports decreased as the nation prepared to import more food to meet the demand.
In recent history, a number of extreme weather events have placed pressure on food supplies around the world. Australia saw a 12-year drought from 1995–2007, and in 2009, India experienced its worst drought since the 1970s. Disasters in the form of droughts and cyclones led in part to the world food crisis in 2008, which saw food shortages, price hikes, and even riots in several countries.
In March last year, the Intergovernmental Panel on Climate Change (IPCC) issued a report on the effects of human-driven climate change on the frequency of extreme weather events, saying that it is likely the intensity of droughts will increase in the 21st century. The group predicts that there will be decreased precipitation and increased temperatures in many parts of the world such as Europe, the Mediterranean, North and Central America, and Africa.
The 2012 drought, while stressful for both plants and people alike, has reignited a discussion about how to adapt to extreme conditions, injecting itself into debates about climate change, food security, and plant genetics. The development of several drought-tolerant varieties of corn with breeding and genetic engineering has captured much attention, as have the benefits of improved farm management. What has been missing from this discussion is how genetics and farm management interact not only with environmental conditions, but also with each other. To beat the heat, it will take scientists from many disciplines working together.
The Contributions of Genetics
To grow, all plants require water, which is absorbed by the roots and evaporates from the leaves through the stomata. When a plant becomes stressed by a lack of water, it responds by closing the stomata, wilting, losing leaves (resulting in reduced leaf area), and stimulating the growth of roots to seek out more water. Not all plants respond to drought in the same way—plants that are more efficient at this and can continue to grow with limited water are said to be drought tolerant. Increasing the drought tolerance of crops through breeding and genetic engineering has been an important goal for plant breeders, and they say these efforts are beginning to pay off.
DuPont Pioneer is one company that has been working on breeding for drought tolerance since the 1950s. In 2011, it released Optimum AQUAmax corn hybrids, which it says deliver a yield advantage in water-limited environments. Breeding with drought tolerance in mind is a carefully controlled process, explains ASA, CSSA, and SSSA member Jeremy Groeteke, a Pioneer agronomy research manager in the West and member of the Optimum AQUAmax go-to-market team. Pioneer grows its corn varieties at dry locations such as Woodland, CA and Plainview, TX and controls how much water the plants receive with drip tape. Combined with whole-genome selection and 50 years of pedigree knowledge, they are able to make advances on what is a very complex trait.
“You need to make sure that when you are selecting whole genomes, they interact with the key environment,” Groeteke says. Growers may encounter drought stress at critical times during development such as flowering and the grain-filling period, so they use the drip tape to time the watering to mimic those different environments. They select the best performing plants under these different conditions, while also ensuring that they perform well under normal conditions.
The result, Groeteke reports, is that Optimum AQUAmax hybrids can yield up to 9% more under water-limited conditions compared with competitive hybrids. For some of the targeted farmlands, he says, this can mean 9 bu/ac more. “When we think about trying to solve global problems, adding 9 bu/ac can be pretty significant.”
Monsanto has its own drought tolerance breeding program and has developed a transgenic approach that augments the results from breeding. The company has been working on an RNA chaperone discovered in Bacillus bacteria that were exposed to cold temperatures, called cspB (cold shock protein B). This protein helps the cells correct misshapen RNA molecules affected by adverse conditions and restores them to their proper functions. When tested in plants, it increased their ability to withstand stressful environmental conditions such as cold, heat, and drought.
Monsanto transferred the gene that produces this protein into corn, and the company reports that its DroughtGard hybrids that incorporate this gene can yield as much as 10 bu/ac more than competing varieties under moderate drought, according to a trial conducted in 2012 across 250 farms. Monsanto expects to release these varieties this year.
The Impact of Management
Genetics provide one way to improve the performance of crops during a drought, but plants still need water to grow. Changes in farm management practices can help increase the amount of water available to the plants and represent another important piece of the puzzle. Certain management practices can increase the water-holding capacity of the soil, while others can lower how much water evaporates into the air—leaving more to be transported into the plant.
Jerry Hatfield is a Fellow of ASA, CSSA, and SSSA; lab director of the National Laboratory for Agriculture and the Environment with the USDA; and a collaborating professor at Iowa State University. He is also a member of the committee to evaluate the impacts of extreme climate events for the IPCC as well as a convening lead author for a report on the effects of climate change on agriculture for the U.S. Climate Change Science Program. One of the issues that he focuses on is how soil management practices can affect the water use efficiency of crops.
“In the Great Plains, you may only get 15 to 20 inches of rainfall in a season, so getting an inch or two more stored in the soil can make a great difference,” Hatfield says.
To get the soil to absorb more water, you can increase the carbon content and the biological activity in the soil. Adding manure or rotating with cover crops can increase the carbon, which also supports biological activity. This activity produces substances that affect the way the soil particles aggregate, or stick together. “If you want to start building organic matter in soil,” he says, “get continuous cover on the soil throughout the year.” Tilling also disturbs the soil, reducing carbon levels as well as biological activity, so farmers who adopt no-till practices can increase these beneficial soil components as well.
Preventing evaporation from the soil can be very important when drought conditions hit. Hatfield points out that the low humidity and high temperatures during the 2012 drought made the water situation worse. “The atmosphere was pulling on the water more than it typically does.”
A solution to this problem is leaving part of the previous crop on the ground. “Leaving crop residues on the soil can protect the soil surface,” Hatfield says. The residues lower the temperature of the soil surface and slow the evaporation rate. It can also help with the absorption of water.
With the potential for more variation in weather patterns from year to year due to climate change, it makes management decisions more difficult. What works for a crop in a wet year such as 2010 may not work in a dry year such as 2012. But some management decisions can help mitigate both kinds of extremes. “In dry years, no-till can help the soil absorb and retain more moisture, while in wet years, it helps maintain more stable soil aggregates and protects against the harmful effects of water logging,” Hatfield says.
There are trade-offs with each management strategy. While no-till and residue management can be good for drought conditions, they can also have drawbacks. Lowering the soil temperature in the spring can slow the growth of seedlings, negating the beneficial effects. “Neither full tillage or no-tillage can be pinned down as a best management practice in all environments,” Groeteke says. “Growers must manage for their local environments.”
The Importance of Interactions
The performance of a plant depends on its genetic potential and the environment that it grows in. It would be an easy task to improve agricultural production if you only had to add the effects of these factors together, but nature does not work that way. Often, one variety will grow well in one environment, but relatively poorly in another. This interaction between the genotype and the environment is known as G × E (“G by E”).
Matthew Reynolds, an ASA Fellow and CSSA member, is a principal scientist and head of wheat physiology at the International Center for Maize and Wheat Improvement (CIMMYT) in El Batan, Mexico. A main focus of his research is on adapting wheat to heat and drought stress, and he has edited a book entitled Climate Change and Crop Production. His work, like others at CIMMYT, has a global focus, breeding varieties that will grow well when the weather is good but still grow relatively well when conditions are bad. That means studying G × E.
Reynolds compares the complexities of these interacting variables to designing a general-purpose car for all conditions. “In the United States, you have really good highways, some side roads, and some farmers who are driving around on dirt, which can be muddy. You can’t have the perfect system for all environments, so the challenge for the breeder is to find those characteristics which are good for all—and there are a few—and to take the average bet for the rest of them. We call that broad adaptation.”
Similar to how some vehicles can be switched between two- and four-wheel drive, Reynolds says “the dream is to find traits that are adaptive and switch themselves on or off when needed.” A plant that can grow deeper roots when water is scarce but not invest the extra energy when water is plentiful he says is one kind of adaptive trait that would be ideal.
Reynolds and his colleagues at CIMMYT not only have to consider how different genotypes will grow in favorable and stressed environmental conditions, but also the different ways the crops will be grown. Just as individual varieties can be affected by the weather, they can also fare unequally under different management schemes.
For instance, if you have a deep-rooted genotype that performs well under drought, it may not be able to grow through the hard layer formed in soils under constant cultivation. And while some crop rotations can break up disease cycles, other diseases like Fusarium can transfer from corn to wheat in the following season. A variety that is not bred specifically for a certain farming system may not realize its full potential when it is put to use.
Both environmental conditions and management decisions are considered part of the environment that a plant interacts with. But for scientists like Hatfield and Reynolds, it can be useful to separate the outside environmental factors and the management decisions of farmers into two separate parts, E and M.
“The reason you would separate management is because it is the part you control. Temperature and weather and rainfall are things that we don’t control,” Reynolds explains. This approach can lead to considering how all three variables interact with each other, dubbed the “G × E × M.”
Groeteke, for instance, has observed interactions between genotypes and management practices while comparing Optimum AQUAmax hybrids with competitive varieties in an extensive series of Grower Systems Trials. He shared this information at the 2012 ASA, CSSA, and SSSA Annual Meetings in Cincinnati, OH, noting that there were many management factors that can affect performance under drought for different genotypes. In particular, his team found that growers received the most yield benefit when they optimized their seeds per acre for their environment.
Both Pioneer and Monsanto recommend a set of best growing practices for their drought-tolerant varieties. But since best growing practices are traditionally determined later in the breeding process, it may be possible that some ideal combinations are missed or achieved more slowly. In the book Crop Physiology: Applications for Genetic Improvement and Agronomy, Carlos Messina et al. write, “An open question is whether enhanced rates of crop improvement can be realized by a more integrated approach.”
By bringing the expected environments and important future management options back to the breeding stage, genotypes that are better suited to evolving future needs like drought, heat, and diseases, can be found.
“I don’t know if we’ve spent enough time studying whether the genotypes do better under different systems,” Hatfield admits. “We’ve really not screened genetic material for its response to management practices. If we really want to crack this puzzle, I think we need to start thinking about what the genotypic response is to some of these factors.”
Testing every genotype with every possible management scheme in every environment would be an insurmountable task. But Hatfield suggests that crop scientists and breeders can start by looking at the different components of yield.
In corn, he says, “the number of rows, kernels per row, kernels per ear, and weight per kernel each represent the different stages of development. We need to start dissecting the plant to find out which are affected most by the management practices that they are subjected to.”
Reynolds adds, “It’s the subtlety of it. You’re getting down to small effects that are hard to quantify. It is hard to come up with a genetic basis.” Quantifying effects in varied environments and systems, he says, will require an improvement in how we look at phenotypes on a mass scale—an issue he has been vocal about.
“We need to be thinking about the fact that if we are to make use of the gene revolution, we are going to need much more precise phenotypic data. To capture that, we need to be able to turn those environments into a laboratory. It’s a compromise—going that extra distance without losing scientific rigor, while embracing the diversity that we’re trying to adapt to.”