Drought Tolerance in Pioneer® brand Corn Hybrids

• Drought decreases corn yields more than any other single cause. For this reason, developing hybrids with drought tolerance has been a primary goal of Pioneer corn breeders for decades.
• Pioneer has made significant progress for this trait. Emphasis on performance under both drought and well-watered conditions has resulted in hybrids that withstand drought but still yield competitively when more rainfall occurs.
• To increase the rate of gain for drought tolerance in corn hybrids, Pioneer has established "managed stress environments" in arid areas. This allows researchers to apply drought stress at exact growth stages and levels of severity.
• Pioneer researchers are also employing new technology tools to speed drought tolerance improvements, including molecular breeding, map-based cloning, and insertion of genes from other species (transgenic breeding.)
• This article will discuss Pioneer’s past progress in improving drought tolerance in corn hybrids, and current efforts that are accelerating these gains and establishing Pioneer’s leadership in drought tolerance research.
• A future Crop Insights will focus on how Pioneer establishes drought tolerance ratings for corn hybrids based on thorough testing and knowledge of hybrid characteristics that confer this trait

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Because of the impact of drought on corn yields, developing hybrids with drought tolerance has been a primary goal of Pioneer corn breeders for decades. To accomplish this goal, researchers have selected for native corn genes that confer drought tolerance by testing breeding populations, parent lines and hybrids in environments that normally experience moderate to high levels of drought. Although this has resulted in significant hybrid improvement, Pioneer researchers are now employing new technology tools that promise to increase both the rate and consistency of drought tolerance improvements.

When drought conditions develop in the non-irrigated Midwest, it is usually during August grain fill. This occurs as available soil moisture is progressively depleted from the root zone due to high summer temperatures and insufficient rainfall. In the semi-arid Great Plains, moisture limitations are also most common during grain fill, but are diminished by irrigation where available. For dryland corn in this region, however, drought conditions can also occur at other growth stages including pollination and sometimes even vegetative growth. Pioneer researchers are focusing on drought stress at pollination and drought stress during grain fill as their primary targets for development of drought tolerance traits.

Drought stress at pollination

• Kernel number is being determined
• Yield loss potential = six bushels/acre per day

Drought stress during grain fill

• Kernel size is being determined
• Yield loss potential = three bushels/acre per day

Water demands by the plant are high during pollination, especially for silk elongation, pollen germination, and pollen tube growth. Under drought conditions, silk emergence may be delayed compared to pollen shed. If this delay is several days, pollen may be limited when silks emerge, resulting in incomplete pollination and reduced kernel number. Because the last silks to appear come from the tip of the ear, barren or poorly filled ear tips can result. Drought and high temperatures can also lead to desiccation of silks, causing poor pollen germination and pollen tube growth. This may also result in reduced kernel number.

In addition to disrupting pollination, drought during the early reproductive period can result in kernel abortion. The tip kernels, the youngest and most distant from the source, are most susceptible to abortion. Kernels are most susceptible during the first two weeks following pollination. Because of the critical relationship between available moisture and successful pollination and early kernel development, yield losses may be as high as six bushels per acre per day when severe drought occurs during this period.

Drought stress during the dough and dent stages of grain fill decreases grain yield primarily due to decreased kernel size, rather than decreased kernel number. Drought reduces the rate of photosynthesis in the plant, resulting in less assimilate production. Drought may also cause premature black layer formation in the kernels, terminating starch deposition. If drought is so severe that leaf or plant death results, yield will be significantly reduced. Finally, drought often results in stalk rot development, which can reduce harvestable yield. Researchers estimate that drought stress during the grain fill stages of development can cause yield losses of up to three bushels per acre per day.

Selecting germplasm in drought environments for many generations has allowed Pioneer researchers to make significant progress for drought tolerance. Pioneer has maintained breeding nurseries and yield test locations in drought-prone environments in the western Plains states for decades. 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, top Pioneer hybrids from each decade over the last 80 years were grown in a managed stress environment in which they received only 12 inches of irrigation during the growing season (full irrigation would be about 25 to 30 inches). As Figure 1 indicates, the yield produced per inch of water has improved dramatically over the years, especially in the 1980s and 1990s. 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 (1920s to 1990s) grown under drought demonstrate significant improvement for drought tolerance, especially in the last 25 years.

The drought of 2005 in central Illinois stimulated a lot of discussion concerning the role of improved hybrids in mitigating severe drought. Even though the drought conditions reminded many growers of similar severe drought years such as 1988, yields were generally much better than expected. As a result, many extension specialists concluded that the drought tolerance of today’s hybrids is, in fact, much improved compared to hybrids of 20 years ago.

To increase the rate of gain for drought stress tolerance in corn hybrids, Pioneer has directed more resources towards this goal. One result has been the establishment of research locations where the level of drought stress can be precisely managed. These managed stress environment (MSE) locations include Woodland, California, Lasalle, Colorado and Viluco, Chile. Because these locations receive little or no rainfall during the growing season, corn receives almost all its water through irrigation. This gives researchers 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 approach is useful not only for testing hybrids, but also for identifying genetic associations that confer drought tolerance in breeding populations. It is also critical for field scale evaluations of transgenic corn traits developed for drought tolerance.

This picture of the Woodland, CA research site shows just a fraction of the drought research effort at this site every year. Pioneer is the only company with the extensive field testing systems to evaluate drought on this large a scale.

Pioneer Testing in Managed Stress Environments (MSE)

MSE is a testing location in which major environmental factors are managed to allow maximum expression of genetic variation in target stress in a repeatable manner. MSE environments share these attributes:

• Little or no rainfall during the growing season
• Precision irrigation
• Uniform temperatures, uniform soils
• Few insect and disease interactions
• High yield potential when irrigated

Managed stress environments are a critical tool for studying drought tolerance mechanisms in Pioneer germplasm.

These MSE locations allow Pioneer to conduct drought discovery and characterization research year-round. Pioneer has made significant investments in these sites in order to improve drought tolerance in its corn hybrids. Established in 2000, these resources place Pioneer far ahead of all competitors in the challenging field of drought stress research.

Pioneer also uses many other locations in Nebraska, Kansas, and South Dakota for evaluating drought stress tolerance of hybrids. While these locations do not afford complete control of moisture to the plant, they provide yearly drought stress conditions representative of dryland corn production on millions of acres. Hybrids that do well in these environments will also tolerate moderate to severe drought conditions often experienced throughout the US.

Pioneer manages the soil moisture in MSE locations by precision drip tape irrigation. Spools of drip tape are mounted on the research planter.

One drip tape line is buried next to each row of corn. Pioneer installs hundreds of miles of drip tape each year.

Drip tape allows very uniform “loading” of the soil profile with specific amounts of water prior to withholding water for drought stress studies.

Results of Hybrid Development at MSE Locations

The results achieved by testing in MSE locations have been dramatic. Precise control of moisture availability to the plant allows researchers to impose drought stress treatments at exact growth stages and levels of severity. Pioneer corn breeders can then differentiate breeding material, parent lines and hybrids with high precision (see visuals below).

Left: Ear size at 10 days after silk. Right: Kernel no. at 35 days after silk. (Top ears had full irrigation, bottom ears had severe stress)

Left: Ear phenotype of tolerant hybrid. Right: Ear phenotype of susceptible hybrid.

Figure 2 below demonstrates how Pioneer compares performance of new hybrids tested in MSE locations. The horizontal axis (x axis) represents the hybrid’s yield in non-stressed environments and the vertical axis (y axis) represents yield in drought-stressed environments. Yields are expressed as relative yields, with 100% indicating the average in each environment. Pioneer’s goal is to identify hybrids that yield greater than 100% under both the non-stress and the drought-stress environments. Such hybrids have superior top end yield potential that is protected by drought tolerance. Each point represents a hybrid. The points with green circles in the upper right-hand quadrant are exciting new hybrids now moving through Pioneer’s advancement process.

Figure 2. Hybrid yields under drought and non-stress (well-watered) conditions. The upper right hand quadrant represents hybrids that do well in both environments.

Hybrids vary dramatically for drought stress tolerance. This response can be quantified as the percent yield reduction that occurs between irrigated and drought-stressed environments. Examples of this yield response for tolerant, intermediate and susceptible hybrids are shown in Table 1.

One of Pioneer’s newer tools now being used in developing more drought tolerant hybrids is molecular breeding. Molecular breeding is used to identify corn genes associated with superior drought tolerance, and to move those genes into new germplasm to improve drought tolerance of all new hybrids. Pioneer has invested heavily in this exciting new technology over the last few years to improve hybrid performance for a number of traits including disease resistance and stalk and root strength, as well as drought tolerance. These successes have demonstrated Pioneer’s industry leadership in these technologies.

Map-based cloning is another genetic tool used in developing more drought-tolerant hybrids. This technique helps breeders optimize the use of natural variation for drought tolerance. The goal of map-based cloning is to identify the specific gene segments responsible for the phenotype (appearance, performance) of a hybrid. Using molecular breeding technology, Pioneer scientists can move the gene into elite lines by traditional plant breeding or clone the gene that is responsible for the desired phenotype and introduce it into hybrids through genetic engineering.

Both these techniques allow breeders to improve drought tolerance in new hybrids in a more predictable fashion. Pioneer researchers are hopeful that these technologies will lead to exciting new improvements in hybrid performance in the near future.

A third approach to improving drought tolerance is through the use of novel genes from other species (transgenes). Like other familiar transgenes (Bt genes, for example), drought genes are inserted into corn germplasm in the laboratory and the plants are subsequently tested in the field. Testing is first conducted in managed stress environments during the proof-of-concept stage. If the gene proves efficacious, testing then continues more broadly in environments throughout the Midwest.

Drought traits are also tested in additional hybrid platforms as they move through the advancement process. The level of drought tolerance exhibited must be significant and consistent enough to satisfy customers and justify the high cost of regulatory approval. Hybrids with drought tolerance traits must not only perform well under drought, but also under well-watered conditions. In addition, hybrids must possess all the other traits required for commercialization, including standability, disease and insect resistance, drydown, and others. It is no surprise that only a handful of genes out of hundreds tested meet these demanding criteria and advance toward commercialization each year.

Pioneer scientists have developed a state-of-the-art transgenic drought tolerance research system, and are consistently advancing new transgenes to the field for evaluation. At the same time, they are fully aware that drought tolerance is a complex trait that will almost certainly require a “stack” of several transgenes to obtain commercial levels of drought tolerance. These challenges coupled with a rigorous regulatory process mean that a transgenic drought-tolerant product is still some years away. The industry as a whole is facing these same challenges, which is why the timeline for developing transgenic drought-tolerant hybrids extends into the next decade.

Transgenic Evaluation Process

• Identify genes from other species that are likely to confer drought tolerance in corn hybrids.
• Insert genes in test hybrids.
• Evaluate drought tolerance in managed stress environments.
• Confirm activity in additional genetic backgrounds and stress environments.
• Pursue regulatory approval for commercial use.

Drought tolerance is a complex trait, because it is basically a yield trait. Pioneer has committed many scientists and testing facilities world-wide to this effort over many years. By doing so, Pioneer has taken the industry lead in understanding this trait and continually improving its corn hybrids for drought tolerance. By combining the advantages of new technology tools (including managed stress environments, molecular breeding and transgenic approaches) with established conventional breeding programs, Pioneer expects to make even more rapid improvements in drought tolerance of its corn hybrids.

Pioneer 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.

A future Crop Insights will focus on how Pioneer establishes drought tolerance ratings for corn hybrids based on thorough testing and knowledge of hybrid characteristics that confer this trait.