Corn Yield Gains Due to Genetic and Management Improvements
Crop Insights written by Steve Butzena and Stephen Smithb
Crop Insights written by Steve Butzena and Stephen Smithb
Ever-increasing crop yields has been one of the great success stories of modern history, helping to feed a growing global population, compensating for shrinking farm acres, and providing a source of not only food and feed, but also fuel and fiber. No crop has made more impressive gains than corn, which has more than doubled in yield in the last half century. These gains have been due to improvements in both corn genetics and crop management practices.
Whether farmers and scientists can continue to make yield gains at the same rate as in the past has been a point of recent debate. Some speculate that gains will necessarily level off as corn yields approach the "theoretical maximum" yield estimated by computer growth models. Others point to modern breeding techniques including use of transgenic traits, genetic markers, doubled haploids, improved information technology, and other advances that may actually increase the rate of genetic gain. Crop management gains are also a certainty as new technologies evolve.
Addressing the "rate of yield gain" question is important, because the future potential of crop yields has profound impacts for farmers, landowners, consumers, input suppliers, policymakers and others. For this reason, Pioneer researchers conducted studies to help determine the genetic component of corn yield increases over time. These studies were designed to measure genetic progress throughout the entire period of hybrid corn culture in the U.S. This article discusses the yield gains documented in those studies, as well as those resulting from improvements in crop management practices.
|Hybrid Corn Adoption in the United States|
By maintaining historic parent lines, hybrids from the past can be recreated and tested against modern hybrids. In controlled experiments in which all other variables are held constant, yield differences between these "era hybrids" can be attributed to genetic differences, and genetic progress achieved over time can be determined. However, plant density requires special treatment in this kind of study. Because optimal plant populations have increased as hybrid genetics have improved over time, subjecting older hybrids to currently used populations would place them at a disadvantage. Instead, several populations are tested, allowing researchers to determine the yield of each era hybrid at its most optimal population, and use this value when comparing to hybrids of different eras. This metric has been the norm for reporting genetic gain from "era" studies and can be interpreted as the rate of gain that would apply if genetic x density-based improvement was substituted for pure genetic-based improvement (Smith, et al., 2014).
Pioneer initiated studies to measure genetic gain in 1972, and has regularly updated these studies. In addition to the newest hybrids, the latest iteration has included both irrigated and drought conditions facilitated by the use of "managed stress environments." These test sites, in Woodland, Calif., and Viluco, Chile, are located in irrigated areas that receive very little rainfall, allowing researchers to impose either high yield or drought stress conditions at will. This enables the measurement of hybrid performance and genetic gain at both ends of the yield spectrum.
Genetic, physiological, and morphological data are collected in Pioneer "era" studies. This allows researchers to observe the collateral impact on various traits of selection for yield, and formulate testable hypotheses about how changes in these traits may contribute to yield improvement.
In addition to genetic gains, corn yields have benefitted from improvements in management practices. Those most beneficial and widely adopted by growers include:
These improvements, along with improved genetics, have enabled U.S. growers to improve corn yields by about 1.8 bu/acre per year since 1930 (Figure 1).
Recent Pioneer studies documented genetic gain by growing hybrids from different eras together in studies conducted over multiple environments. Hybrids chosen were the top-selling central Corn Belt hybrids of their era from 1930 to 2011. Beginning in 1997, many hybrids included in the study contained transgenic insect protection traits or trait stacks of insect and herbicide resistance. Thus, any yield gain from insertion of these genes is treated as part of the genetic gain accrued in those hybrids. Likewise, any yield "drag" associated with insertion of the genes would subtract from the overall gain. Results are shown in Figure 2. (Read about studies conducted to identify yield gains from insect resistance traits alone below in this article.)
These studies included all hybrids (single, 3-way and double crosses) predominant in their era. When a subset of only single-cross hybrids was considered (1963 to 2011 era), genetic gain estimates were slightly higher (Figure 2):
Results were similar to previous studies that tested a set of era hybrids dating from 1930 to 1980. Grown at their optimum plant density, those hybrids also demonstrated a genetic gain of 1.5 bu/acre per year (Figure 2). Another previous study, however, showed lower genetic gain of 1.2 bu/acre per year for the period of 1930 to 2001. Both these prior studies were not confined to single-cross hybrids and environments were not "grouped" by stress level (Figure 2).
Yield gains can be attributed to genetics, management and the growing environment, as well as interactions between these components. The growing environment includes weather, biotic and soil factors. If weather patterns such as prevailing temperature and rainfall, biotic factors such as insect and disease pressure, and soil characteristics such as organic matter content change directionally over time, yield trends could be impacted. For this reason, Pioneer researchers looked closely at environmental factors to evaluate their impact on yield trends.
Weather patterns, if trending in a single direction (warmer or colder, wetter or dryer) have potential to impact yields over time. Increased weather variability within single seasons could also affect yield trends. Using the EnClass® system, the Pioneer propriety software, researchers were able to evaluate historic weather patterns and model their expected impacts on yield from 1950 to 2011. This thorough analysis of weather records determined that the effects of weather on yield trends were minimal, contributing an upward bias of only 0.02 bu/acre per year during the period.
Insect and disease pressures change from year to year, depending primarily on weather conditions, but may also change depending on host susceptibility or resistance. For example, transgenic (Bt) traits have been developed that give excellent protection against European corn borer (ECB), a major pest of corn. Use of this trait in nearly all hybrids where this pest is endemic in North America has reduced natural populations to a small fraction of their previous levels. Until the advent of Bt technology, hybrids had poor resistance to ECB. Thus, the older, non-Bt hybrids in the study also benefitted from this change in the growing environment. The reduction in ECB pressure created a potential downward bias to protecting the genetic gain determined in the study. (More discussion of the genetic gain due to insect resistance traits is included in a later section.)
Theoretically, by comparing genetic yield gain to overall yield gain, % genetic gain can be computed:
Genetic yield gain / Total yield gain = % Genetic gain
In reality, apportioning gains is much more complex than the equation indicates. That is because of the interactions between genetics and management that are inherent in yield improvement. Perhaps the best example involves plant density. The main genetic component is breeding new hybrids able to withstand the stress of higher populations and so produce higher yields. However, that advantage would not be realized unless the management practice of growing the crop at higher populations is also imposed. In our calculation of yield gain, plant population is included on the genetic side of the ledger, but there is obviously a management component as well.
Another example is early planting to increase yields. Genetic components may include improved stress emergence, better resistance to seedling diseases, and full-season hybrid maturity. Management components are planter improvements, use of seed treatments, and planting earlier.
Because genetic and management gains are not completely distinct, calculating "percent genetic gain" may be somewhat misleading. Nevertheless, it is instructive to compare estimates of genetic gain to various metrics of overall yield gain to evaluate their relationship under a variety of environments - general growing conditions, irrigation and drought.
In this comparison, genetic gain was 1.47 bu/acre per year, and overall gain (Iowa-based gain) was 1.97 bu/acre per year. Percent genetic gain* is calculated as 1.47/1.97, or 75%. The calculation demonstrates that a preponderance of total yield gain can be attributed to genetic gains, bearing in mind that genetic and management gains cannot be completely separated.
As the graph indicates, average yield gains under irrigation in Nebraska are about 1.96 bu/acre per year from 1965 to 2011. Pioneer studies documented a 1.4 bu/acre per year genetic gain under irrigation (average of 2 irrigated sites). Thus, genetic gain as a percent of overall gain under irrigated production is 1.4/1.96, or 71%.
The primary importance of era studies is in the insights they provide regarding possible corn yield increases in the future. Another way to gauge future corn yield potential is by evaluating yield trends in the highest yielding environments under top management. Yields achieved in the National Corn Growers Association (NCGA) National Corn Yield Contest are often considered the best estimation of current yield potential in corn. Researchers and growers, among others, are interested in whether those contest yields are increasing at a consistent rate, leveling off, or perhaps even increasing at a higher rate of gain. NCGA yield contest trends are shown in Figure 5.
NCGA irrigated yields are increasing at a rate of about 2.5 to 3.0 bu/acre per year, and non-irrigated yields by about 3.0 bu/acre per year. By comparison, Nebraska irrigated farm yields and Iowa farm yields are increasing by about 2.0 bu/acre per year. Thus, it is apparent that NCGA contest yields, which represent a small sample of growers using the very best genetics and management practices, are increasing faster than Nebraska and Iowa mean annual farm yields, which represent a very broad diversity of genetics and practices. However, it is not always cost-effective to attempt to bridge some or all of the yield gap between contest plots and farm yields, as this is dependent on grain prices and the cost of additional crop inputs needed to boost yields.
In 1997, a completely new genetic development was introduced to the marketplace - hybrids that contained an insect protection trait inserted into the corn genome using transgenic methods. Because of the potential impact of this technology on future genetic gains, it is valuable to examine the genetic gain attributable to transgenic insect resistance. However, this process is made more difficult because of:
European corn borer (ECB): The optimal method for measuring a trait effect is to evaluate pairs of hybrids that are genetically identical except for the trait in question (i.e., "isogenic" hybrids). Pioneer researchers tested 15 isogenic pairs of hybrids with adaptations spanning the predominant maize maturity zones in North America. Each hybrid pair had either no transgene or a +Cry1Ab transgene that confers protection against ECB. Hybrids were planted in plots 15 feet long by 2 rows wide arranged in a randomized complete block design. Tests were conducted at 3,739 environments distributed among 24 U.S. states and 2 Canadian provinces during 2000 to 2007 (Figure 6).
Standard agronomic practices were used at all sites. Neither artificial infestations nor pesticides were applied to manage the ECB or any other Lepidopteran populations. As expected, ECB levels ranged from low to high across the environments tested in the study. The yield advantage for the traited hybrid of each pair ranged from 2.5 to 8% (mean of 5.3% or 9 bu/acre). Because of the effectiveness of ECB resistance conferred by Bt traits, "it has been increasingly difficult to find ECB in most producers' fields and moth flights have been negligible" since 2006 (Steffey and Gray, 2008). Consequently, as long as protection remains resilient, future studies in the United States of genetic protection against the ECB, if reliant on natural infestations alone, will underestimate the positive effects provided by insect resistant hybrids.
Corn rootworm (CRW): Across maize growing regions of the U.S., Alston et al. (2002) estimated average yield increase factors resulting from the use of CRW resistant maize hybrids (in comparison with untreated conditions) ranging from:
There are significant interactions with soil type, water, and insect biotype. CRW infestation is more prevalent in wet clay soils, but if water and nutrient availability can be maintained, then yields might not suffer.
Two methods were used to compare the rates of genetic yield gain in U.S. corn contributed by simply inherited traits with the advances made by breeding with "base" germplasm. The results showed that native germplasm accounted for 93% of genetic gain in 1 method of analysis and 77% in another (Smith, et al., 2014). While these calculations are speculative, they suggest that multigenic mediated yield gain is overwhelming compared to that of monogenic sources.
Morphological data collected in Pioneer era studies include harvest index, tassel size, leaf angle, anther-silk interval ("silk delay"), stalk and root lodging, barrenness and staygreen. Previous studies reported that the mean harvest index (grain dry weight/total plant dry weight) increased from 0.46 for 1961 era hybrids to 0.49 for 2004 released hybrids (with maxima of 0.53) (Duvick, 1984, 2005a, 2005b; Duvick et al., 2004a, 2004b). In other words, grain weight has become a higher percentage of total plant weight over time. Of those traits categorized as contributing to increased harvest index, the current study found a continued trend toward reduced tassel size. In contrast, a trend toward more upright leaves plateaued in the 1990s.
Duvick (1984, 2005a, 2005b) also categorized several morphological changes associated with higher planting densities. The current study detected a continuing trend toward shorter anther-silk intervals (less "silk delay) and a lessening of stalk lodging, plus a slight but consistent improvement in root lodging resistance. In contrast, resistance to plant barrenness (failure to produce grain) and greater stay-green plateaued in the 1990s. Other studies using a computerized "crop modeling" approach suggest that root architecture improvements (deep, vertical roots rather than more shallow, horizontal roots) may contribute more to yield gains than leaf architecture improvements (more erect leaves) (Hammer et al.). Among all changes to the corn plant, however, the one deserving primary credit for corn yield gains over time is the ability to endure stresses imposed by higher plant populations and still produce an ear, resulting in more yield per unit area.
aSteve Butzen, Agronomy Information Consultant, Pioneer, Johnston, Iowa.
bStephen Smith, Research Fellow, Pioneer, Johnston, Iowa.
Alston, J.M., J. Hyde, M.C. Marra, and P.D. Mitchell. 2002. An ex ante analysis of the benefits from the adoption of corn rootworm resistant transgenic corn technology. AgBioForum 5(3):71-84.
Duvick, D.N. 1984. Genetic contributions to yield gains of U.S. hybrid maize, 1930 to 1980. In: W.R. Fehr, editor, Genetic contributions to yield gains of five major crop plants. CSSA Spec. Publ. 7. ASA and CSSA, Madison, WI. p. 1-47.
Duvick, D.N. 2001. Biotechnology in the 1930s: The development of hybrid maize. Nat. Rev. Genet. 2:69-74. doi:10.1038/35047587.
Duvick, D.N. 2005a. Genetic progress in yield of United States maize (Zea mays L.). Maydica 50:193-202.
Duvick, D.N. 2005b. The contribution of breeding to yield advances in maize (Zea mays L.). Adv. Agron. 86:83-145. doi:10.1016/S0065-2113(05)86002-X
Duvick, D.N., J.S.C. Smith, and M. Cooper. 2004a. Long-term selection in a commercial hybrid maize breeding program. In: J. Janick, editor, Plant breeding reviews: Long-term selection: Crops, animals, and bacteria. Vol. 24. Part 2. John Wiley & Sons, Hoboken, NJ. p. 109-151.
Duvick, D.N., J.S.C. Smith, and M. Cooper. 2004b. Changes in performance, parentage, and genetic diversity of successful corn hybrids, 1930-2000. In: C.W. Smith et al., editors, Corn: Origin, history, technology, and production. John Wiley & Sons, Hoboken, NJ. p. 65-97.
Hammer, G.L., Z. Dong, G. McLean, A. Doherty, C. Messina, J. Schussler, C. Zinselmeier, S. Paszkiewicz, and M. Cooper. 2009. Can changes in canopy and/or root system architecture explain historical maize yield trends in the U.S. Corn Belt? Crop Sci. 49:299-312 (2009).
Smith, S., M. Cooper, J. Gogerty, C. Löffler, D. Borcherding and K. Wright. 2014. Maize. In Yield Gains in Major U.S. Field Crops. CSSA Special Publication 33. ASA, CSSA, and SSSA, 5585 Guilford Rd., Madison, WI 53711-5801, USA. p. 125-171.
Steffey, K., and M. Gray. 2008. Survey for the second-generation European Corn Borer larvae, Illinois, 2008 (Revised). The Bulletin: Pest management and crop development information for Illinois. University of Illinois, Urbana-Champaign. (accessed 14 Aug. 2013).