Characterizing Pioneer® brand Hybrids for Drought Tolerance

• Because moisture availability usually limits corn yields, selecting hybrids with drought tolerance is critical. Reviewing Pioneer drought ratings and yield information from diverse environments is recommended for this purpose.
• Hybrid traits that contribute to drought tolerance include a well-structured root system, insect and disease resistance, strong silking characteristics and yield stability across environments, including those with moisture stress.
• To rate hybrids for drought tolerance, Pioneer uses performance results from "Managed Stress Environments" and other closely monitored environments throughout the Corn Belt characterized for type and severity of drought stress.
• A hybrid's drought response is determined as the yield difference in low stress vs. drought stress conditions. Comparing this response to that of commercial hybrids with a well-known history of performance under drought allows Pioneer scientists to assign drought ratings to new hybrids.
• Using a set of check hybrids with a wide range of drought tolerance ratings but with maturities similar to the test hybrids increases the precision of Pioneer's drought ratings. Including period-of-years data also increases precision.
• The Pioneer drought rating scale is from 1 to 9 (9 = best), but hybrids below 4 are discarded during testing and 9 is reserved for future drought trait products. Thus, Pioneer's commercial hybrids have drought ratings of 4 through 8.
  

Drought is the major stress affecting most corn acres every year. For this reason, growers should select hybrids with drought tolerance for all fields that commonly face this problem. These products must also provide competitive performance if adequate or excellent soil moisture conditions develop. To help customers identify such products, Pioneer provides drought tolerance ratings for all its hybrids, as well as yield information from diverse environments.

As growers begin the hybrid selection process, they will benefit from a review of plant traits that contribute to drought tolerance, how drought ratings are assigned to Pioneer hybrids, and how these ratings can be used to help choose appropriate products for the drought risk they face in each field. This article will discuss hybrid drought tolerance characteristics and the derivation and interpretation of drought ratings for Pioneer® brand hybrids.

Figure 1. Severe drought stress during pollination and grain fill can greatly reduce yields of susceptible hybrids. This shows the importance of selecting drought-tolerant hybrids.

Hybrid characteristics that contribute to drought tolerance are complex and may not be readily apparent. For example, corn leaf rolling is often considered to be a drought tolerance mechanism to reduce moisture loss. But if a hybrid has rolled leaves when hybrids beside it do not, it is not clear if that hybrid has better drought tolerance or is showing drought stress sooner than other hybrids. Because of this uncertainty, the best indicator of drought tolerance is corn grain yield under moisture stress. Known hybrid traits that contribute to drought tolerance include a well-structured root system, insect and disease resistance traits, strong silking characteristics and yield stability across diverse environments.

Root System: A drought tolerant hybrid must have a root system that efficiently accesses all available moisture in the soil. A broad and shallow root structure may not provide adequate drought protection to the plant even though it may support the plant against lodging. A root system that penetrates deeply into the soil is preferred for reaching soil moisture as drought develops. Root systems must also be healthy to impart drought tolerance. If they are impaired by insects, diseases, or physical conditions such as compaction or cultivator pruning, the plant will be more vulnerable to drought.

• Insects: Corn roots can be attacked by soil insects from germination through grain development. Seedlings may be attacked by white grubs, wireworms, grape colaspis larvae and corn nematodes. A fast-growing plant and root system is the best genetic defense against these early feeders. In addition, Pioneer's standard seed treatment contains Poncho® 250 insecticide to help protect against several secondary soil insects.
  
  The insect most damaging to corn roots by far is corn rootworm. Pioneer® brand hybrids with the Herculex® RW (HXRW) or Herculex XTRA (HXX) traits offer the best protection available against northern, western and Mexican corn rootworm larvae. Under drought conditions, the value of in-plant genetic root protection is magnified, and consistently more effective than chemical control.
  
  Data from drought stressed regions in 2005 and 2006 repeatedly confirmed the value of transgenic CRW control for protecting yield under drought. For this reason, the HXRW and HXX traits can be accurately classified as drought tolerance traits. (For refuge acres, Poncho 1250 insecticide seed treatment is available on Pioneer seed as an option to help protect against corn rootworm.)
  
  
• Diseases: Seedling diseases can reduce root systems and drought tolerance of hybrids. Pioneer hybrids rated highly for stress emergence have the ability to establish good stand characteristics under cool soil conditions and are less affected by common seedling diseases. In addition, Pioneer offers a combination of Dynasty® and Maxim® XL fungicides plus Poncho® 250 insecticide as the standard seed treatment on all Pioneer corn hybrids. This seed treatment combination helps protect against seedling diseases like Pythium and several secondary soil insects that open the door to diseases by feeding on roots.
  

Silking: Drought stress in June and July often delays corn reproductive development. Under this stress, some hybrids delay silk emergence much more than pollen shed. When silk emergence is delayed several days, pollen shed may be mostly complete before silks finally emerge. This can result in poor pollination and dramatically reduced yield. Hybrids with strong silking characteristics under drought exhibit less yield loss by maintaining synchronization of pollen shed and silking during this critical period.

Yield Stability: Hybrids with proven yield stability across environments usually tolerate a variety of stresses, including drought. When selecting hybrids, growers should examine data from various environments, including fields that are lower yielding due to drought, to identify those that perform well under diverse conditions.

Other Considerations: Drought tolerant hybrids can adjust their plant growth to reduce excess vegetation that wastes water. Many other plant traits can contribute to drought tolerance in the plant, but because of complex interactions between these traits, they have not provided reliable predictions of drought tolerance. For that reason, growers should not depend on these secondary traits when selecting products for drought environments. Rather, they should focus on the drought ratings provided for all Pioneer corn hybrids for that express purpose -- helping growers select drought-tolerant products.

To rate hybrids for drought tolerance, Pioneer uses performance results from both research plots and on-farm trials. These tests are conducted in multiple environments that are chosen to represent the hybrid's expected area of adaptation, not all of which are necessarily drought-prone. But within the set of test environments, some locations will experience moderate to severe drought. Those locations usually provide information on hybrid performance across an array of drought regimes. In addition, Pioneer tests hybrids at one or more "Managed Stress Environments (MSEs)" - locations at which the amount of stress placed on hybrids can be carefully controlled.

In addition to testing in diverse environments, Pioneer also classifies test environments for the level of drought stress encountered. Using field attributes and weather data, crop growth models can estimate the level of drought stress on the crop at specific stages of growth. Because historical weather records show that drought stress is most common in the grain-fill period, researchers are most focused on this period for hybrid characterization. The pollination period is also important for drought characterization. Even though drought stress does not occur as frequently at this time, it can have an even greater impact on yield than grain-fill drought stress.

At closely monitored testing locations, researchers record soil moisture at the start of the season, and rainfall, temperature and other weather data during the season. With these data and information on soil type, topography, hybrid CRM, planting date and other variables, crop growth models can then estimate water used by the crop or lost by evapotranspiration. This enables the model to characterize each research location for the timing and severity of drought stress experienced by the crop.

The yield performance of each hybrid is then evaluated from locations which are characterized by the model as "low-stress", "flowering stress" and "grain fill stress". The difference in yield performance of the hybrid under stress vs. low stress is calculated as an index value. Comparing this value to that of commercial hybrids with a well-known history of performance under drought allows Pioneer scientists to assign drought tolerance ratings to new hybrids.

In this drought-rating process, it is very important that the set of commercial check hybrids includes a broad range of drought tolerance. Pioneer uses a large set of hybrids of varying, well-known drought ratings for this purpose. Drought interacts significantly with corn growth stage, so check hybrids and test hybrids must be at the same growth stage when drought stress occurs to create meaningful comparisons. For this reason, Pioneer limits the group of check hybrids and new hybrids being characterized to a narrow range of CRMs. Pioneer also requires that period-of-years data be included when assigning a drought rating to a hybrid.

Figure 2. Testing in Managed Stress Environments allows Pioneer researchers to carefully control the amount of drought stress imposed on hybrids. This helps researchers to separate and rate hybrids for drought response.

Figure 3. This image shows the same plot as in Fig. 2, two weeks later. Compare the visual difference between full and limited irrigation treatments in Fig. 2 and Fig. 3. This demonstrates how drought responses can be magnified as moisture deficits are increased over time. This response will also be reflected in grain yield.

The Pioneer scale for drought ratings is from 9 = best to 1 = worst. But because "9" is being reserved for future "drought trait" products, and hybrids rating "3" and below are discarded during testing, commercial Pioneer hybrids have drought ratings of "8" to "4". These ratings are described in detail in the sidebar on the next page. It is important to remember that ratings are relative and that hybrid performance can vary depending on the timing, duration and severity of moisture stress during the growing season.

The level of drought tolerance achieved in today's best hybrids stands in stark contrast to that of hybrids of just 20 years ago, which often protected only half of the average low-stress yields under drought. Many corn growers and researchers have estimated that if today's hybrids had been grown during the drought of 1988, corn yields could have easily been double what they were that season. This estimate is based on more recent hybrid performance measures under drought stress such as the 2005 drought in central and northern Illinois. Although 2005 drought stress levels in that area were similar to those of 1988, yields of over 100 bu/acre were common in 2005.

With continued progress in improving hybrid drought tolerance, the next 20 years may result in even more impressive gains. Corn plants will never be able to tolerate totally arid conditions and produce grain, but the ability to withstand significant periods of moisture stress can be improved. All growers, regardless of the yield level of their fields, stand to benefit from hybrid improvements and new technologies that will protect the yield potential bred into their hybrids.

Today's best drought-tolerant hybrids developed through conventional breeding often yield within 75 to 80% of their average low-stress yields under drought stress. Adding a specific "drought trait" to corn hybrids is expected to protect yields above and beyond that provided by the best conventional hybrids. In fact, researchers believe that targeted drought traits currently in testing may increase corn hybrid yields even under low moisture stress conditions, as well as providing more protection under drought stress. Yield gains of 10% over current top hybrids are within expectations for drought traits in testing today. Future drought traits could increase yields even more.

In Pioneer testing under a range of drought stress conditions from moderate to severe:

• Corn hybrids with a rating of "8" performed among the best hybrids tested and compared favorably to Pioneer's best established hybrids for drought tolerance. These hybrids are an excellent choice for fields that regularly experience drought stress due to soil type, topography and/or prevailing weather patterns.
  
  Although hybrids rated an "8" represent the best choices available for fields with the most severe risk of drought, it may not always be advisable to grow corn in those fields. Because moisture stress usually limits all corn yields to some degree, these hybrids may also be the best choice for low or moderate drought risk *.
  
  
• Hybrids with a rating of "7" had very good performance under Pioneer's drought testing system, but yields were reduced more than those of the top-tier hybrids. These hybrids are a good choice for fields that regularly experience yield-limiting drought stress, except in fields with usual extreme drought conditions that may result in crop failure.
  
  
• Hybrids with a rating of "6" had good performance under Pioneer drought testing. These hybrids showed more visual effects of drought stress than hybrids rated higher, in addition to greater yield reductions. Hybrids rated a "6" are a good choice for fields that usually experience only low or moderate drought stress with little risk of severe drought.
  
  
• Hybrids with a rating of "5" had only average performance in drought-stress locations. Visual effects of moisture stress were very evident compared to hybrids rated higher, and yield reductions under drought were significant compared to yields in low-stress environments. These hybrids are expected to perform best in fields with high water-holding capacity that usually receive timely rainfall, as well as in irrigated fields.
  
  
• Hybrids with a rating of "4" only approached their yield potential in low-stress environments. Visual symptoms were often noticeable under moderate stress and significant under severe stress. Yield losses under severe stress sometimes approached 50%. These hybrids should be reserved for fields where irrigation or natural rainfall and soil water-holding capacity are adequate to maintain good soil moisture availability throughout the season.

* Other hybrid traits should always be considered in selecting a product, regardless of the drought rating of the hybrid or the drought risk in your field. Your Pioneer sales professional can assist you in selecting appropriate hybrids for your fields.

Figure 2 and 3 images courtesy of Bill Curran, Pioneer Research Scientist.

The author would like to thank the following Research Scientists and other technical experts at Pioneer who reviewed this article and provided helpful changes and additions:

Leroy Svec - Research Scientist, York, Nebraska
Jeff Schussler - Research Scientist, Marion, Iowa
Dan Berning - Technical Services Manager, Lincoln, NE
Bill Curran - Research Scientist, Greeley, Colorado
Troy Bauer - Research Scientist, Dallas Center, Iowa