Sunflower's Genetic Gains
Friday, March 28, 2014
filed under: Research and Development
Editor’s Note: Sunflower is among the crops being featured in Yield Gains in Major U.S. Field Crops – a monograph scheduled for publication this summer by the Crop Science Society of America. The book’s chapter on sunflower has been written by Brent Hulke and Larry Kleingartner. Hulke is sunflower research geneticist with the USDA Agricultural Research Service in Fargo, N.D. Kleingartner served as executive director of the National Sunflower Association for more than 30 years prior to his retirement.
Hulke and Kleingartner first provide brief overviews of sunflower’s U.S. history as well as the main cultural and environmental factors influencing production. Their focus, however, is on the genetic gains made in recent decades for defensive traits, quality traits — and yield. Here are some highlights from that discussion.
Genetic Gain for Defensive Trait
“Breeding for defensive traits has been the single most important advance in the genetic improvement of sunflower,” Hulke and Kleingartner write, “because many biotic stresses plague this crop, and ease of production through mitigation of risk is essential for keeping producers interested in the crop.”
For the eastern Dakotas and Minnesota, the most significant genetic advance in sunflower to date, they contend, is resistance to downy mildew. This resistance is termed “vertical” because it is conditioned by single dominant genes. After downy mildew overcame metalaxyl fungicides (e.g., Apron) in the mid-1990s, “there was a critical need for genetic resistance to fill in the void.” Resistance was found, providing universal protection against all then-known races of downy mildew. More recently, though, new races of downy mildew “have made it apparent . . . that several different resistance genes were needed” to counter the threat. Fortunately, the USDA-ARS Sunflower Research Unit in Fargo has identified and released a number of resistance genes to the breeding community, and there presently is effective resistance against all currently known races of downy mildew.
Another big advancement in sunflower genetics, Hulke and Kleingartner report, has been “the development of herbicide resistance genes that provided the sunflower plant with resistance to broad-spectrum herbicides.” Specifically, they’re referring to imidazolinone and sulfonylurea herbicides, i.e., the Clearfield® and ExpressSun® systems. The development of commercial sunflower hybrids with resistance to these herbicides — which effectively control numerous broadleaves and grasses — began with the discovery of wild sunflower plants in eastern Kansas that exhibited resistance to these herbicide classes. Hulke’s predecessor Jerry Miller, isolated the resistance via traditional breeding, leading to the eventual release of resistant lines to the private breeding community — which then developed the generations of new hybrids for commercial production.
While downy mildew resistance and resistance to the imidazolinone and sulfonyurea herbicides have been built around single dominant genes with nearly 100% heritability, “Sclerotinia resistance is quantitative (horizontal) with less than 100% heritability,” Hulke and Kleingartner point out. “There are no known vertical resistances to any of the three Sclerotinia diseases (basal stalk rot, mid-stalk rot and head rot) of sunflower.”
Given that very formidable obstacle, no sunflower hybrids have been brought to market thus far that can be termed “Sclerotinia resistant,” and there may never be a hybrid that is completely immune. However, progress definitely has been made in the identification of resistance and incorporating the resistance into commercial hybrids. “Sclerotinia resistance has been introgressed into modern breeding lines through traditional breeding, molecular breeding – and even cytogenetic techniques,” the authors confirm. “The public lines and associated molecular information are then used by the private sector to improve resistance in modern hybrids.”
One big issue has been the negative correlation between Sclerotinia resistance and yield (i.e., yield drag). But work with molecular breeding methods and tools such as SNP markers are helping address that problem.
Genetic Gain for Seed Quality Traits
NuSun® comprises the biggest part of this story. Hulke and Kleingartner explain that traditional sunflower oil — that produced during the ’70s, ’80s and well into the ’90s — is high in linoleic acid, an essential fatty acid also found in other vegetable oils, most notably soybean. Soybean oil has dominated the domestic veg oil market for many years, with a market share as high as 85%. Sunflower oil always has had a very difficult time competing, given soybean oil’s advantages: vast supply, broad functionality and typically less cost. So historically, most U.S.-produced sun oil ended up going into export.
An opportunity began to unfold in the early 1990s, however, when trans fatty acids started gaining attention. Human nutrition studies found that trans fatty acids, created during the oil hydrogenation process, were detrimental to heart health. “Members of the National Sunflower Association recognized this development as ˚an opportunity to gain domestic market share if naturally stable sunflower oil could be developed,” Hulke and Kleingartner write. “[The] market was interested in any oleic-based sunflower oil that could be produced in place of the traditional linoleic sunflower oil.”
As that time, high-oleic sunflower oil’s profile and seed were under patent. So the best option was a mid-oleic oil. The sunflower industry quickly made the transition to NuSun – and by 2009, about 90% of U.S.-produced sunflower oil was being consumed within North America.
“Redirecting an entire industry to NuSun production was a monumental undertaking that required a great deal of planning and coordination,” Hulke and Kleingartner emphasize. “The entire market chain – from the producer, seed handlers and the crushing plants – had to work together and develop an infrastructure to segregate NuSun production.” For breeders, the biggest challenge was to create NuSun hybrids “with the critical agronomic characteristics of yield, oil content, disease resistance and oleic acid content stability.” That was achieved, “and today traditional sunflower is rarely grown.”
Meanwhile, the confection (nonoil) sunflower sector was making a major transition as well. The world in-shell market for human consumption was demanding larger seed size, and confection breeders responded. Average seed size in the 1999-2002 crop period was 60% of seed over a 20/64 sieve size. The average seed size during the 2008-12 period, by comparison, was 83% over 20/64. There was a trade-off involved, however. The sources of increased seed size “lacked a number of important traits that are required in modern hybrids – including adaptability to modern production practices, disease resistance and self-compatibility,” Hulke and Kleingartner report. Crossing them with existing nonoil lines that had better agronomics has helped, but there’s still a lag in defensive traits and yield, compared to the current generation of elite oilseed hybrids.
Genetic Gain for Yield
A hybrid’s yield potential and its actual yield in a farmer’s field in a given year can be – and almost always are – quite different. Pests, adverse weather and cultural practices all impact the farmer’s yield. Each year, about 10% of the surveyed fields in the annual NSA Sunflower Crop Survey end up designated as having “no problem,” i.e., no yield-limiting production issues. For the three-year period of 2010-12, those “no problem” fields averaged just under 2,300 lbs/ac – compared to the average USDA-NASS U.S. sunflower yield for those combined years of about 1,460 lbs/ac.
“Considerable yield potential is clearly not being realized due to a combination of pests, poor cultural practices and weather,” Hulke and Kleingartner write. They estimate that the “strategies employed thus far by sunflower breeders have resulted in yield gains, in terms of the U.S. average seed mass harvested,” of 6.89 lbs/ac per year for nonoil sunflower and 8.90 lbs/ac per year for oilseed sunflower.
While moving in the right direction, those rate-of-increase numbers are not as great as those for maize (corn) and soybean. On average, maize yield increases in the U.S. are double those for both oilseed sunflower and nonoil sunflower.
But, add Hulke and Kleingartner, one weakness of using actual production data is that doing so does not allow for a differentiation between the relative contributions of genetic gain versus the gain from improved cultural practices. There currently is no independent analysis of these two contributors in the U.S., but there is in certain other countries, such as Argentina. There, best linear unbiased predictions (i.e., a statistical separation of genetic gain from improvements due to other factors) of commercial and experimental hybrids, over a 20-year period showed that oil yield, on a per-acre basis, increased by 10.6 lbs/ac per year. However, seed yield did not increase from the mid-1990s to mid-2000s. “Improvement of oil yield over seed yield was believed to be due to the philosophy of breeders in the region that oil yield was the main target of selection,” they report.
Another factor needs to be noted as well, they add: the shift of the bulk of sunflower acreage away from higher-rainfall areas into semi-arid regions – not only in the U.S. but in Argentina and South Africa as well. Hybrids developed specifically for conditions in such regions will perform better than hybrids targeted toward growing areas with a significantly different growing environment.
Future Prospects
“The U.S. sunflower crop has had a very rocky history,” Hulke and Kleingartner state. Predictions in the latter 1970s that acreage could climb to as high as 10 million contrasted sharply to “later assessments that sunflower would not continue to be viable into the 21st century.” Over the past four decades, “nearly every possible obstacle has impacted the sunflower crop,” they note – from federal farm programs that favor planting wheat, feed grains and cotton rather than oilseeds, to major disease outbreaks, to competition with GMO crops (e.g., corn and soybeans). Still, sunflower has maintained a consistent grower base in certain regions, such as the central and western Dakotas.
“The future of sunflower production in the USA is dependent on increasing yields and consistent annual levels of production (yield stability),” Hulke and Kleingartner emphasize. While they expect yield potential to continue to increase annually, “the critical question,” they observe, “is if yield enhancement can be maintained at the same or higher pace than competing crops such as soybean and maize.”
Sunflower is at a disadvantage with soybean and maize for three primary reasons: First, it is “not the crop of choice on the best agricultural land and is increasingly being cultivated in semi-arid regions.” Second, “it is not the recipient of the same level of research investment, public or private, as maize or soybean,” which in turn delays key breeding-enhancing technologies. Third, “sunflower will have some difficulty getting regulatory acceptance of a genetically modified (GM) event in the USA” due to the close proximity of commercial sunflower production areas to many populations of wild sunflower (i.e., the outcrossing issue) and other species of native sunflower that have some propensity for interfertilization.
Despite these obstacles, the sunflower breeding community has been — and definitely is — making progress. “High-throughput genotyping technologies, such as single nucleotide polymorphisms (SNPs), are currently being used to develop trait-marker associations and improve the efficiency of breeding,” Hulke and Kleingartner note. “Whole genome sequence, while not a reality today for sunflower, is in development, and a completed genome is expected soon – which would greatly simplify targeted genomic studies.” Also, doubled-haploid technology, if adaptable, would lead to more efficiencies in development of breeding lines.
The chapter authors also point out that some of sunflower’s “weaknesses” simultaneously can be viewed as “strengths.” For example, the large populations of wild sunflower and native species, while a potential barrier to GM commercialization, “are a valuable resource for gene mining and introgression.” The application of cytogenetics is helping scientists cross various sunflower species that otherwise would not be interfertile. That could, if successful, result in development of sunflower immune to Sclerotinia via the transfer of genes from Helianthus species that already have natural immunity.
“Fatty acid improvements, such as NuSun and high oleic, have already added value to sunflower oil and spurred demand,” Hulke and Kleingartner also point out. “Additional fatty acid profiles with greatly reduced saturated fat levels, in combination with high-oleic acid, will soon be introduced in the USA, further enhancing value and demand.” While other oilseeds (e.g., soybean) are trying to make similar advances, they have the potential market negative of being GM. With some states and some large retailers considering mandatory labeling of all GM ingredients, this could be a major trend working in sunflower’s favor.
Finally, there’s glyphosate resistance. With the glyphosate-resistant weed threat expanding across the U.S., “it could also be advantageous to have crops like sunflower, with alternative herbicide systems, available for the producer to use in a rotation.”
A central challenge for the U.S. sunflower industry in the future, Hulke and Kleingartner emphasize, is to be known by the food industry as a reliable supplier – one that can be counted upon year in and year out. Meeting this challenge can be aided with improved hybrids possessing disease and insect resistance, and by expanding the geographic base of sunflower production. “Sunflower production in the High Plains region, in particular, could be enhanced as irrigation water is further rationed and economically sound production of other common, less-drought-tolerant crops becomes difficult,” they state.
Sunflower also could gain acres in prime farmland to the east of the current production region, they contend, should droughts (such as those in the central U.S. in 2012) become more commonplace due to climate change.
In the end, “consistently improved yields via enhanced genetics hold the key to the future of U.S. sunflower production,” Hulke and Kleingartner conclude. “This will involve adaptation of new breeding technology to improve defensive, quality and yield traits simultaneously, in the context of the production environment.”