Pioneer Research to Develop Drought-tolerant Corn Hybrids

• Drought tolerance is a complex trait, much like yield. For this reason, a multi-dimensional research strategy is needed to ensure continual improvement of hybrids.
• Pioneer's research strategy includes traditional germplasm improvement, use of Pioneer's proprietary breeding tools for native genes, as well as transgenic approaches.
• Pioneer's germplasm improvement program is building on decades of drought research that has led to products with a distinct performance advantage under drought stress and the ability to yield competitively under better growing conditions.
• Pioneer's experienced researchers and network of "managed stress environments" and other drought research centers give them an advantage in both traditional and transgenic breeding approaches.
• Pioneer's drought improvement strategy fully leverages their proven marker-assisted selection capabilities and trademarked AYT™ System for advancing key traits into elite genetics.
• Pioneer has identified a number of transgenes that improve the corn plant significantly for resource capture and utilization. Transformed hybrids have more prolific silking, higher retention of ear-tip kernels, and improved yields.
• Pioneer's industry-leading agronomy research has helped growers improve yields under drought by using Early Corn Early or Ecofallow production systems, optimizing plant population, and planting the right product on the right acre.

Drought stress is the primary challenge to high corn yields in North America and most other corn-producing regions of the world. This is because even temporary moisture deficits reduce corn yields, and such deficits occur in most non-irrigated production fields every year. With multiple events or extended periods of drought, the accumulated stresses on the corn plant can result in significant yield loss, even though today's hybrids are much more tolerant than those of the past.

For over half a century, Pioneer has continually improved its hybrids for performance under drought. Today, modern genetic tools and breeding techniques are allowing Pioneer researchers to rapidly accelerate these gains. In the near future, the culmination of current research with transgenic sources of drought tolerance will provide growers with hybrids demonstrating even better yield stability under drought conditions.

Pioneer drought-tolerant hybrid (left) vs. susceptible hybrid (right) grown in drought environment.

This Crop Insights will discuss Pioneer's current strategy for improving drought tolerance of corn hybrids by integrating these three approaches:

• Continual improvement of Pioneer's germplasm base and hybrids through conventional breeding, selection and testing,
• Use of new genetic tools and breeding techniques applied to native corn genes ("Drought I" program), and
• Identification and incorporation of transgenic sources of drought tolerance ("Drought II" program).

This article will also describe hybrid traits that contribute to drought improvement, and Pioneer agronomy research to help growers improve hybrid performance under drought stress.

Past Progress
Pioneer scientists have bred and selected for drought tolerance in corn hybrids for decades. In drought-breeding nurseries and yield test locations centered in the Great Plains states, researchers have continually selected for improved performance under drought stress in both parent lines and hybrids. By accumulating genes for superior drought tolerance in generation after generation of breeding, researchers have steadily improved hybrids and the germplasm base from which future hybrids will be developed. Pioneer's emphasis on superior performance under both drought and well-watered conditions has resulted in hybrids that can withstand drought but still yield competitively when more rainfall occurs.

To gauge progress in improving hybrids for drought tolerance, Pioneer conducted studies comparing historical and modern hybrids for performance under drought conditions. In this study, the top three Pioneer® brand hybrids from each decade over the last 80 years were grown in a managed stress environment (Woodland, CA) in which they received only 12 inches of irrigation during the growing season (full irrigation would be about 30 inches). As Figure 1 indicates, the yield produced per inch of water has improved dramatically over the years, from about 3 bu/acre per inch of water in the 1930's and 1940's, to nearly 10 bu/acre per inch of water in the 1990's. This quantifies the success that Pioneer plant breeders have achieved thus far in improving drought tolerance of Pioneer hybrids.

Figure 1. Hybrids from eight decades (1920's to 1990's) grown under drought demonstrate significant improvement for drought tolerance, especially in the last 20 to 30 years.

Current Drought Program Using Conventional Breeding
Pioneer has been focusing research efforts around drought tolerance since the opening of its York, Nebraska, research center in 1958. Breeding efforts in York and locations such as Garden City, Kansas, LaSalle, Colorado, Brookings, South Dakota, Manhattan, Kansas, and Plainview, Texas, have enabled Pioneer to establish market leadership in dryland and limited-irrigation growing areas of the western Corn Belt.

In addition, establishment of managed stress environments in Woodland, California, and Viluco, Chile have allowed for precise water-deficit regulation, as well as year-round testing. Because these locations receive little or no rainfall during the growing season, corn receives almost all its water through irrigation. This gives researchers precise control over the amount of moisture supplied to the plant. The ability to "dial in" the level of moisture stress allows breeders to evaluate corn germplasm across the entire spectrum of moisture scenarios from severe drought to well-watered conditions.

This picture of the Woodland, CA research site shows just a fraction of the drought research effort at this site every year. Pioneer leads the industry in establishing extensive field testing systems to evaluate drought on this large a scale.

Pioneer has been actively engaged in applying new genetic tools in pursuit of more drought-tolerant hybrids for many years. Products developed from this "Drought I" program are now in final stages of testing and could be available to customers as early as 2010, pending their performance in on-farm drought-stressed trials in 2009. Because these hybrids are being developed using native drought-tolerance traits, they will not require regulatory approvals for commercialization or export.

Drought I hybrids contain native corn drought tolerance genes that have been identified through marker-assisted selection and advanced into elite genetics using Pioneer's Accelerated Yield Technology (AYT™ System) tools. The AYT system is the novel integration of a proprietary matrix of molecular breeding technologies into the product development process, doubling the rate of genetic gain for yield, reducing yield variability and improving performance predictability.

Drought I hybrids will be marketed in dryland and limited-irrigation growing environments of the western Corn Belt where yield expectations typically are lower (<150 bu/acre) due to lack of adequate rainfall and available water. Yield improvement targets for Drought I corn hybrids are 5 to 10 percent above leader hybrids currently available in these limited-water environments. Hybrids with improved drought-stress tolerance also may enable farmers to expand their corn acres by planting corn in more arid conditions that typically only support the production of wheat, cotton or sorghum.

In Pioneer's Drought II program, researchers are combining native tolerance with transgenes with the goal of delivering higher yields across all environments. Pioneer began field testing of transgenes in 2000, and has tested hundreds of genes in replicated field trials. This research has identified and advanced to multi-location field trials numerous genes in different metabolic pathways that have shown promise in meeting or exceeding program targets. Yield results from replicated field trials of two lead events are shown in Figure 2.

Figure 2. Yield advantage for two Drought II traits across stress levels, 2006 – 2008. Total of 225 reps.

Extensive testing over three years demonstrated that yield increases were significant for both events in drought-stressed and higher-yielding environments.

Transgenic Evaluation Process

• Identify genes from other species that are likely to confer drought tolerance in corn hybrids.
• Insert genes in test hybrids.
• Evaluate gene activity in model plant systems such as Arabidopsis and FAST corn.
• Evaluate drought tolerance in elite corn genetics in managed stress environments.
• Confirm activity in additional genetic backgrounds and stress environments in the Midwest and High Plains.
• Pursue regulatory approval for commercial use.

Yield increases are shown on a percentage basis in Figure 3. This graph confirms the increasing value of both events as stress continues to worsen.

Figure 3. Percent yield advantage for two Drought II traits across stress levels, 2006 – 2008. Total of 225 reps.

Other research results have shown that:

• Averaged across all hybrids and all yield environments, the transgenic Drought II hybrids expressed an 8% yield increase in three years of trials.
• Averaged across all hybrids, the 2008 research results demonstrated a 16% advantage for transgenic Drought II hybrids compared to their conventional counterparts in drought-stress environments.

These encouraging yield results in many diverse environments over three years demonstrate that Pioneer is well on their way to meeting their goal of delivering a Drought II product to the marketplace within five to seven years (2014 to 2016).

In general, Pioneer researchers must improve a corn plant's ability to capture and utilize available resources - water, sunlight and nutrients to increase its yield under drought. Many aspects of plant development are investigated in this process. Some specific trait goals for Pioneer researchers include improving root systems (Figure 4) and the plant's ability for silk emergence during drought stress (Figure 5).

Figure 4. Extensive root system of drought-tolerant hybrid.

Some trade-offs are inevitable in this process; for example, developing an improved root system is very important for drought tolerance, but the energy required for root development could be used directly on above-ground yield.

Extensive research is the key to striking the right balance of competing traits.

Figure 5. Silking is more prolific in an experimental hybrid (R) than in the control hybrid (L). Improving silk emergence under drought stress is one goal of Pioneer researchers.

Pioneer has identified a number of genes that improve the plant significantly for resource capture and utilization. The ultimate trait of interest in hybrid improvement under drought is kernel number (and size) at harvest. Reducing abortion of tip kernels is an obvious goal for researchers. Pioneer breeders have developed hybrids that retain several more rows of tip kernels as drought intensifies (Figure 6).

Figure 6. Less abortion of tip kernels in hybrid on right resulted in a 30 bu/acre yield advantage in this location.

In addition to developing hybrids for drought environments, Pioneer scientists and agronomists have developed, refined, and promoted alternative corn production systems for drought areas. One example is the Early Corn Early system, which originated in southeast Kansas in the early 1980's. This production method is designed to help growers achieve higher yields by growing very early hybrids that pollinate and fill ears ahead of the most severe drought conditions of late summer. Early hybrids are thoroughly tested for their adaptability to this production system before being offered to customers. Another example is the use of Ecofallow systems that enable corn growers to maximize corn yields with limited available water in arid regions or where the soil profile overlaying bedrock is very thin.

Pioneer's plant population and row spacing studies are also important to help growers choose the best production practices to maximize performance under drought stress. This information is regularly shared with growers through Pioneer publications, websites and emails. And lastly, Pioneer's strategy of field by field hybrid placement is perhaps the most important agronomic contribution to grower productivity under drought. Ensuring the right product is planted on the right acre is always key to top performance, but under extreme environmental conditions such as drought, it is crucial.

Pioneer currently provides drought tolerance ratings for all its corn hybrids. Your local Pioneer sales professional can help you select the best drought-tolerant hybrids for your farm.

Pioneer firmly believes that its multi-dimensional approach to improving drought tolerance in its corn hybrid lineup is the best drought strategy in the industry and highly weighted toward success for a number of reasons:

• Drought is a complex trait and one that is tied directly to corn yield, which itself has many contributing factors. This complexity means that over-simplified solutions (e.g., inserting a single gene) may not accomplish simultaneous goals of increasing yield under drought and high-yield conditions.
• This approach fully exploits Pioneer's current advantage in native trait drought performance. Pioneer's 50 years of drought research have led to products with a distinct performance advantage. Its current network of experienced researchers, managed stress environments and other drought research centers are unparalleled in the industry.
• This approach leverages Pioneer's proven marker-assisted selection capabilities and trademarked AYT™ System for advancing key traits into elite genetics.
• Pioneer's managed stress environments give them an advantage in the transgenic arena just like with native trait research. This is because success of transgenic drought programs is totally dependent on the ability to identify and characterize trait effects under field conditions.