Corn History and Breeding
A. Forrest Troyer

A Retrospective View of Corn Genetic Resources. J Hered 81(1): 17-24. (1990)
Hybrid corn represents an extraordinary research success. The modem average increase in U.S. corn yield is two bushels per acre, or about $330 million, annually with virtually no additional Input. This return is fueled by research investments In corn breeding that must be recovered. Seed companies meet the farmer's demand for more profitable hybrids (higher yield, lower harvest moisture, and easier harvest). Tens of thousands of hybrids are tested at hundreds of locations annually to identify widely adapted hybrids. Corn genetic resources include wild relatives, races, open pollinated varieties, genetic and cytogenetic markers, synthetics, and elite inbreds. Popular modem hybrids contain one or more elite inbreds. Successful corn breeders use the best genetic resources available; elite inbreds are now the best genetic resource. The U.S. patent system requires public access in return for a grant of exclusivity; this ensures continued breeding gain from elite inbreds, reducing feed and food costs for the public good. I suggest that we broaden the U.S. corn germ plasm base by sending elite inbreds to foreign climates, crossing them with superior local races and varieties, backcrossing to the elite inbreds, and selecting for better-adapted progeny before returning them for use in the United States.

Selection for Early Flowering in Corn: Three Adapted Synthetics. Crop Sci. 30(4): 896-900 (1990)
Corn (Zea mays L.) breeders strive to broaden genetic diversity within shorter-season growing areas by selecting for early flowering in late-maturity germplasm. This creates earlier flowering material with most of the characteristics of the late germplasm, thus broadening the diversity of germplasm grown in the shorter-season area. The present study evaluates selection for early flowering in adapted germplasm; relative to flowering date of selection materials, not only place of selection but also place of intended use differs from the usual approach. This study measures the selection response for early flowering in three adapted synthetics. The synthetics were grown at high plant density (87 000 plants ha-1) near Mankato, MN; the earliest 5% to flower (55 of 1100 plants) were selected for four generations, then the original populations and the four cycles for each synthetic were compared in performance trials at 48 300,62 600 and 75 500 plants ha-1 for 5 yr. Selection response for early flowering for four cycles averaged 0.8 d less to flowering, 10 g kg-1 less grain moisture, 4.0 cm less plant height, 2.7 cm less ear height, 0.2 d less silk delay, 0.15 Mg ha-1 (3%) less yield, and 4% more broken stalks. The decrease in yield due to selection showed close association with decrease in plant size (r = 0.98**, significant at P = 0.01), which probably reduced photosynthetic capacity. Later-flowering synthetics respond more to selection for early flowering than do earlier flowering synthetics. Shorter-season growing areas with longer, cooler days at flowering favor selection for early flowering. The results indicate that selection for early flowering can help export new, diverse germplasm to shorter-season growing areas, to broaden genetic diversity in the shorter-season area.

The Location of Genes Governing Long First Internode of Corn. Genetics. 145(4): 1149–1154. (1997)
Knowing breeding behavior and cytological location of traits helps breeders. My objective was to locate dominant genes for long first internode of corn (Zea mays L.). I determined that Hopi Indian corn PI213733 (variety Komona) displayed the trait and grew well in the U.S. Corn Belt. I crossed PI213733 to 26 translocation tester stocks in Minnesota inbred A188 background, backcrossed semi-sterile plants carrying the translocation to A188 the next generation, and grew the segregating generation planted in trenches 15 cm deep with ridges of dirt 10 cm high one year, in trenches 25 cm deep the other year and also at normal (6 cm) depth. Emerged plants were classified for semi-sterility or for normal pollen. I concluded from multiple testers for each chromosome arm that dominant genes for long first internode are located (chromosome and region) on 3S, on 6 near the centromere, and on 9S; spurious associations occurred for two testers. Measurement of cell lengths indicated that PI213733 had more cells than A188 both in upper and in lower mesocotyl sections and that lower, older cells elongated sooner. I found a normal-sized kernel with twin embryos that developed two long first internode seedlings indicating that the amount of endosperm did not limit mesocotyl growth.

Background of U.S. Hybrid Corn. Crop Sci. 39(3): 601-626 (1999)
Understanding the history of a crop helps plant breeders select. This historical narrative discusses the background of hybrid corn (Zea mays L.) in the USA. It attempts to explain why certain open-pollinated cultivars persisted into today's hybrids. Domesticated corn originated by selecting larger, non-shattering ears in tropical, southern Mexico. Corn moved to temperate areas requiring adaptation to more variable, more stressful conditions. Flint corn arrived in the USA about 1000 BC and dent corn arrived after Columbus 2500 yr later. The first southern Corn Belt of the 1830s (Tennessee, Kentucky, and Virginia) moved northwest (Iowa, Illinois, and Missouri) by 1880; shorter-season, more drought tolerant cultivars were developed. About 1000 open-pollinated cultivars resulted from natural and artificial selection in flint x dent backgrounds. A few cultivars were widely adapted and became popular. Virtually all 1000 cultivars became parents of inbred lines. Yellow corn (containing vitamin A) proved better for feeding; white corn production diminished from 50% in 1920 to 1% of total in 1970. Earlier planting, higher plant densities, more nitrogen application, and wider adaptation affected hybrid selection. Pedigree background frequencies of 33 Pioneer Hi-Bred International Inc. proprietary, elite inbred lines directly estimate background of >40% of U.S. hybrid corn hectarage and indirectly estimate the rest. Forms of 'Reid Yellow Dent' ('Iodent Reid', 'Troyer Reid', 'Osterland Reid', 'Stiff Stalk Synthetic', Reid Yellow Dent per se, and 'Funk Yellow Dent Reid') are 50%. 'Minnesota 13' is 13%. 'Northwestern-Dent' is 5%. 'Lancaster Sure Crop' is 4%. 'Leaming Corn' is 4%. These cultivars were widely adapted and popular. The results are consistent with evolutionary (adaptation) expectations. Genes for adaptedness to the temperate, U.S. Corn Belt (longer days, cooler minimum temperatures, and shorter, drier, more stressful seasons) were naturally and artificially selected; first, in widely adapted, openpollinated cultivars then, in widely adapted hybrids. Old cultivar and inbred line background sources, few in number, indicate adaptedness is more important than diversity to increase yield.

More on the background of U.S. Hybrid Corn. (2003)
I present new or corrected background information to Crop Sci. 39:601-626. The phrase, PUR YEL DENT, should correctly be in the TROYER bar for Pioneer inbred B164 under REID YELLOW DENT in the bar graph. I have added LAN below the line and RL for Richey Lancaster to the LLE and LE bars formerly under LINDSTROM LONG EAR and now placed under LANCASTER SURECROP in the bar graph (J. Agron. 23:652-661). I have moved inbred CI540 which should correctly be called inbred TEA developed by Henry Wallace with Leaming in the bar graph. New information is provided for CORNBELT FEMALE COMPOSITE, DAWES CO. NB., LONGFELLOW, MIDLAND, PROLIFIC COMP, DEKALB 56, DDRF101, M3204, HT4, and HOWE'S ALBERTA FLINT.
    Reid Yellow Dent is 50% of U.S. hybrid corn background including Stiff Stalk Synthetic at 10%, Minnesota 13 is 13%, Lancaster Sure Crop is 12%, Northwestern Dent is 5%, and Leaming Corn is 5%. Five, century-old, open-pollinated varieties account for 85% of the background of today's U.S. hybrid corn. Think about it. Tens of thousands of open-pollinated corn varieties are maintained by germplasm banks, and virtually every one of them has been pollinated in an attempt to develop a useful inbred line. I have pollinated several hundred of them myself. Only five varieties made it, made it big, out of tens of thousands of varieties.

Background of U.S. Hybrid Corn II. Crop Sci. 44(2): 370-380 (2004)
Understanding the history of a crop helps plant breeders select. This paper supplements and corrects Background of U.S. Hybrid Corn (Zea mays L., Crop Sci. 39:601-626). I explain which open-pollinated cultivars and landmark inbreds persisted into today's hybrids and attempt to explain how and why. I briefly discuss human (artificial) selection, natural selection, and food production. Pedigree background frequencies from 33 elite, 1990s era inbreds directly estimate about 40% of U.S. hybrid corn hectarage and indirectly estimate the rest. I've searched historical records to trace the succession of cultivars and inbreds. Reid Yellow Dent is 51% of the documented U.S. hybrid corn background (Iodent Reid, 13%; Troyer Reid, 12%; Osterland Reid, 11%; Stiff Stalk Synthetic, 8%; Reid per se, 4%; and Funk Reid, 3%). Minnesota 13 is 13%, Lancaster Sure Crop is 13%, Northwestern Dent is 5%, and Leaming Corn is 5%. Five widely adapted, century-old, open-pollinated cultivars surpassed in use tens of thousands of genetically divergent cultivars to account for 87% of the known background of today's U.S. hybrid corn. Adaptedness mattered; U.S. corn breeders are adapting a tropical crop to a temperate climate. New, diverse background sources will probably be better adapted to longer daylengths, cooler minimum temperatures, drought, shorter seasons, and improved crop production practices. Selecting for adaptation to these conditions and to improved agronomic production practices will continue to increase yield. Traditional corn breeding methods will continue to accommodate climate change. Plant breeding and crop production research provides plentiful food in the USA.

Since 1950, the USA has never gone so long (8 yr) without an increase in record yield of corn. The average interval between record U.S. average corn yields for this period (1950-1994) is 2.1 yr. This recent delay in record yield increase is probably because of more emphasis on insect and herbicide resistance and less emphasis on yield per unit area. The regression of U.S. average corn yield on years for the decade (1994-2003) since transgenic corn is about two-thirds (b = 85.4 vs. 125.4 kg ha-1 or 1.4 vs. 2.0 bushels acre-1) of that for the period (1950-1994) when conventional breeders bred all hybrid corn.

Heterosis: Effects of Selection on Patterns and Adaptedness on Constancy. (2004)
Heterosis continues to be poorly understood. As corn germplasm becomes more complex, more recent corn breeders question the origin of Corn Belt corn and of heterotic patterns. I examine some of the first hybrids of original landmark inbreds and identify contrasting Northern Flint and Southern Dent traits (Troyer, 1999, 2004). This exercise supports Anderson and Brown (1950) who state: "the heterosis of Corn Belt maize seems to be due to the mingling of Northern Flints and Southern Dents." Shamel (1907) provides James L. Reid's verbatim affidavit on the origin of Reid Yellow Dent. Duvick (1999) compared successful hybrids and parent inbreds over 60 years and found hybrid yield increased about 50 bushels per acre, but % heterosis increased only when stresses affected inbreds more than hybrids. Why? We are adapting a tropical crop to a temperate climate. Increasing adaptedness to the Corn Belt climate affects both hybrids and their parent inbreds accounting for the constancy of % heterosis. I cite body size, which is female dominant in species hybrids of the genus Equus (horses and donkeys) as an easier-to-understand example of adaptedness being associated with heterosis. Horses are native to northern Europe and the mule (female horse / male donkey) is superior in northern Europe. Donkeys are native to Africa and the hinny (female donkey / male horse) is superior in Africa (Zeuner, 1963). In general, evolution favors larger animals that hold body heat nearer the colder poles and favors smaller animals that disperse body heat nearer the warmer equator. Heterotic patterns are used to predict size and weight of mules. Heterotic patterns are thousands of years old.

Adaptedness and Heterosis in Corn and Mule Hybrids. Crop Sci. 46(2): 528-543 (2006)
The origin of U.S. Corn Belt corn (Zea mays L.), heterotic groups, and heterotic patterns becomes less obvious with more cycles of breeding. Heterosis is poorly understood; simple curiosity cries out for more information. I endeavor to shed light on the effect of adaptedness and heterosis on U.S. Corn Belt corn. I relate pertinent happenings in the phenomenal increase in U.S. corn production. I briefly review the origins of Northern Flint and Southern Dent races of corn and two major, persistent open-pollinated cultivars; and how corn hybridization was preceded and eased by hybrid species of the horse (Equus spp.). I discuss heterotic groups and patterns. The objective of U.S. corn breeding has been to adapt a tropical crop to a temperate climate. Adaptedness is important. Open-pollinated cultivars emphasized local adaptation, but some cultivars were more popular, widely grown, and better adapted over a broad geographic region. Hybrid seed corn companies grew larger by selling more widely adapted hybrids that favored germplasm from the more popular, widely grown, better adapted open-pollinated cultivars containing more genes for adaptedness. I examine morphological differences between inbred parents of a widely adapted hybrid. Relatively constant percentage of heterosis of well-adapted hybrids over years is due to seasonal climate affecting hybrids and their parent inbreds in a like manner because of their selection for adaptedness. Adaptedness has been more important than heterosis in the U.S. corn yield and production increases. Adaptedness in analogous heterotic species hybrids of the genus Equus, where body size is female dominant, apparently discriminates for body size between mules and hinnies that have virtually identical genotypes—adaptedness determines superiority over and above heterosis.

and W. L. Brown: Selection for Early Flowering in Corn: Seven Late Synthetics. Crop Sci. 16(6): 767-772 (1976)
Seven corn (Zea mays L.) synthetics adapted to southern Iowa were grown in southern Minnesota at high plant density, and the earliest 5% to flower were sibmated for five generations. The original populations and the five selected cycles were then compared in performance trials at three plant densities over 2 years. Selection effect per cycle averaged: 1.1 q/ha yield increase, 2.2 ears/100 plants increase (1-year data), 1.6 percentage points grain moisture decrease, 2.8% stalk breakage increase, 6.1 cm plant height decrease, 4.1 cm ear height decrease, 1.7 days less to flower, and 0.4 days less silk delay.
    We observed a cycle-by-density interaction wherein advanced cycles exhibited prolificacy at a low density after selection against barrenness at high densities. The decrease in silk delay and increase in ears per plant indicate that selection for early flowering during stress is effective in adapting materials to high plant densities.

and R. W. Rosenbrook: Utility of Higher Plant Densities for Corn Performance Testing. Crop Science 23(5): 863-867 (1981)
We grew 84 different corn (Zea mays L.) performance tests in paired-density comparisons averaging 51,600 and 64,500 plants per hectare over a 9-year period to examine the utility of higher plant densities for corn performance tests. The higher density reduced yield test means from 76 to 73 q/ha, increased ranges among hybrids from 40 to 44 q/ha, and decreased hybrid F values for tests from 3.6 to 3.2. Broken stalk test means increased from 10 to 12%, ranges among hybrids increased from 35 to 39%, and hybrid F values for tests increased from 2.5 to 2.9 with the higher density. Dropped ears showed a large period-of-years effect due to effective selection. Testing at above optimum plant densities increased barrenness, stalk breakage, and ear droppage; it also increased the range among entries, thereby increasing the ease of selection against these traits. We analyzed 5 years' data from 250 strip tests comparing two widely grown hybrids at three plant densities and found that increased densities reduced number of tests needed to differentiate the hybrids. On the average twenty strip tests with alternate check strips grown at plant densities at or above 56,000 plants per hectare successfully differentiated (P = ca. 0.05) 7% yield at 75 q/ha average yield, 1.8% broken stalks at 5% average broken, and 2.4% root lodged at 2.5% average lodged; the two hybrids did not differ for dropped ears. Superior commercial hybrids resulted via higher plant density testing.

and Lorrene S. Palmer: Background and Importance of Troyer Reid Corn. Crop Sci. 46(6): 2460-2467 (2006)
Corn (Zea mays L.) history and American westward expansion were intertwined when several hundred newer, better adapted, open-pollinated corn varieties were developed by human and natural selection. Chester E. Troyer was a pioneer corn breeder who bred 'Troyer Reid', an improved 'Reid Yellow Dent' variety. We relate how Chester got to the particular Indiana farm whose pervious, river-bottom soil affected natural selection. Better, deeper rooting probably helped Troyer Reid obtain more nutrients and water. Chester's formative years were spent as a teacher, and his later achievements included being honored four times as Corn King of the World and also as a successful corn breeder of productive corn varieties and proprietary hybrids. He was first to produce and sell hybrid seed corn in Indiana in 1925, received the Purdue University Certificate of Distinction and was a successful seed corn businessman and employer, successful gladiolus (Gladiolus x gandavensis Van Houtte) breeder of award winning varieties, successful banker, and beloved civic philanthropist. Troyer Reid accounts for about 15% of the background of documented U.S. Corn Belt hybrid corn through inbreds developed by Purdue University, Pioneer Hi-Bred International, University of Minnesota, and Iowa State University corn breeders.

and Lois G. Hendrickson: Background and Importance of 'Minnesota 13' Corn. Crop Sci. 47(3): 905-914 (2007)
Background knowledge of germplasm helps corn (Zea mays L.) breeders develop inbreds and predict hybrids. The background of 'Minnesota 13' is still not generally understood. We provide an explanation for the mystery and provide conclusive evidence for Minnesota 13's origin from plant breeding records at the University of Minnesota, whose archives contain corn breeding nursery books. We also cite Minnesota Agriculture Experiment Station annual reports, bulletins, and other pertinent sources. A seed lot collected in 1888 designated as University No. 13 and reported as being improved by Prof. Willet Hays and Andrew Boss in an 1890 bulletin was later destroyed by a seed house fire. They collected a new seed lot in 1893 and the same designation, No. 13, was again used. We show that this second seed lot was common corn grown in the St. Paul, MN, area. We also review the improvement of Minnesota 13 and its influence on the history of corn growing in Minnesota, Wisconsin, and France. We diagram and discuss the background of Pioneer Hi-Bred Inbred PH207 and the importance of Minnesota 13 as 13% of the documented background of U.S. hybrid corn.