​As it became clear that corn hybrids could create large boosts in yield when two unrelated inbreds were crossed, it became clear that the biggest gain came when the inbreds were derived from distinct heterotic groups.  Lancaster Sure Crop was created in Lancaster county Pennsylvania.  It featured long, flinty ears with disease resistance consistent with the humid eastern USA environment.  Meanwhile, the Reid dent corn, developed from the accidental crosses of New England Flint with Southern dent in central Illinois spread throughout the corn belt with many sub-varieties selected for differing moisture and soil stresses.  These two heterotic groups became major sources of inbreeding in the 1920 and 30’s as hybrid development advantages were clear.
 
In 1935, George Sprague began intercrossing 16 inbreds and 4 inbred parents that were mostly from these many sub-varieties of Reid dent corn.  This became known as Iowa Stiff Stalk Synthetic population (SS).  New inbreds from this populations were test crossed with Lancaster inbreds, the successful SS inbreds were intercrossed, new selfs made, crossed again to Lancaster inbreds.  These cycles were continually resulting in more populations at Iowa State University.  Other public and private corn breeding efforts built on Stiff Stalk Synthetic, selecting from portions of it as well as constructing their own genetic populations.  Inbreds from these efforts continuously improved hybrid performance as national corn yields increased. A third heterotic group, Iodent, was developed by mostly private breeders in the USA.  Internationally, similar processes of identifying inbreds that combined with unrelated inbreds to give hybrid vigor.
 
The cause of heterosis lack some clarity but most of the evidence implies that negative, recessive genes in one inbred are overcome by the dominant version of the gene in the other inbred.  Given that a corn plant has at least 30,000 genes, and that any selfing, whether intentional to make inbreds or coincidental in open pollinated varieties, probability of homozygous recessive genes is high. Despite some randomness in matching dominant genes for the recessives of the other parent, inbreds derived from heterotic populations increase the probability of covering up the negative recessive genes.

.Changes in approaches to corn breeding in early 1900’s was concurrent with knowledge of genetics of all biological forms.  Corn breeding up to this time had been done predominantly by farmers and others, selecting for practical features of ear shape, kernel characters, adaptation of local environment and personal choice.  The result was multiples of varieties and sub-varieties.  Separation of male and female structures assured ease of mixing genetics within a variety and selecting preferred ears.
 
At the same time, researchers' understanding of genetics of any organism was moving towards trying to understand how did traits get transferred to the next generation.  The terms ‘gene’ and ‘genetics’ were not established until 1909!  Academically-trained researchers were becoming interested in maize to better understand inheritance.  G.H. Shull, purifying corn traits by selfing while at a New York Experimental station. E.M. East at Connecticut Experimental station and his student, H.K. Hayes at University of Minnesota, realized that the move to crossing the trait-pure inbreds from different varieties could result in huge yield gains.  They worked to convince seed producers that at least double cross hybrids could be economic.  The ideas, and the yields, caught on in the 1930’s and momentum expedited quickly.
 
The timing of this move, beginning with the interest of how genetics works by purifying trait genes in inbreds and then in hybrids of corn showed the yield benefits of producing hybrids.  Although research in trait genetics was incentive for more of the academically inclined, it should not be forgotten, especially in the commercial world, that much of the selection of premier inbreds was done by less academic, hands-on corn breeders, selecting for those that performed well in yield trials.  That combination continues today as a pursuit of knowledge of corn genetics concurs with practical breeding for increased performance in today’s environments. 

​From the base varieties and races of corn distributed in the USA, and as some were mixed, by individual farmers, many small seed companies emerged to produced popular varieties for an area. By the year 1900, it was estimated that 1000 distinct varieties of corn were grown by USA farmers but most were selections from 10 basic ‘races’. From these, individual public and private corn breeders began developing inbreds for hybrid production.  Selection pressure in varieties emphasized appearance of ears in farm shows, but also for disease resistance to local diseases, pests and drought.
 
Some of the same selection pressures were applied to inbred development but ultimately it was the performance in a hybrid that became most significant. These attempts in the early 1900’s had to be disappointing to many, as the ear size reduced greatly and relationship with inbred appearance and hybrid performance was not always strong.  It presented new dynamics over variety selection but the payoff for these early efforts was great.  Realizing that making 4-way hybrids, with both parents being hybrids, made seed production cost efficient especially in comparison to the boost in grain yields over variety yields. The lack of genetic relationship between the two hybrid parents covered up the genetic deficiencies of each other, giving the double cross hybrids a boost over either parent. Hybrids were introduced in the 1920’s and gained significantly in the 30’s and by 1950, nearly all USA corn grain production was made with hybrids. Separation of male and female flowering structures encouraged genetic variability and multiples of people selecting for varieties fitting their needs, established the basis for the huge hybrid vigor revealed in corn hybrids.  
 
Corn history has intrigued many and literature is rich with details.  One comprehensive source is ‘Corn and Corn Improvement’, 3rd edition published by American Society of Agronomy in 1988. Internet search engines lead to more references including more recent literature.
 

​Development of corn from a wild species of Teosinte was a remarkable human endeavor.  Along the way it depended upon mutations allowing people to select more desirable types. As it was moved to multiple environments over nearly 10000-year history, varieties were selected by locals to best fit their environmental and cultural preferences.  Open pollination, because of separation of male and female flower structures, encouraged continual mixing of genetics within a variety as well as occasional mixing of varieties. Mixing of varieties, sometimes by accident as in the case of the Northern flint with the Southern dent in the case of Reid farms resulted in new vigor to the next generation.
 
Open pollination had allowed for variability and, therefore, adaptation to many environments but it also more or less capped the yield, as there would remain individual plants that were genetically inferior sometimes from concentration of negative recessive genes. The challenge was how to take advantage of the variability and yet obtain the uniformly superior, adapted plants.
 
Concurrently, plant researchers studying the principle of heterosis, in which crosses between unrelated species or subspecies showed vigor were showing interest in this phenomenon in corn.  George Shull in 1908, at Cold Spring Harbor invented the term heterosis for the increased vigor that results when unrelated varieties are crossed.  This became especially clear when corn was selfed sufficiently to become inbreds, resulting in puny plants with small ears and then crossed with other inbreds to create hybrid plants.  Other university plant researchers in the early 1900’s were experimenting with inbreeding to obtain more genetic uniformity but struggling with the reduced plant size of inbreds.
 

​Separation of tassel and ear in the corn plant prevented severe selfing but individual farmers in the late 1800’s probably did not make large enough saves to avoid their versions of a widely-used variety to become too genetically narrow. It is probable that ‘negative’ recessive genes would become homozygous whereas new varieties developed by crossing unrelated varieties would show more yield.  It did become apparent by 1900 that crossing distinct genetic populations resulted in a vigor boost.
 
The strength of maintaining some genetic variability with open pollinated seed also included the weakness of the population genetically narrowing and at the same time genetically drifting with annual environmental pressures. Avoidance of the drift could only be done by selfing plants to make them homozygous and therefore more repeatable.  USDA researchers in the early 1900’s started developing inbreds from the various populations and discovering hybrid vigor.  Those early inbreds, however, were deemed as producing too little seed to be economic.  The solution was to cross related inbreds to make hybrids as parents.  These double crosses still showed advantages over open pollinated varieties and became popular in USA from 1930-1950.  These gave way to three-way crosses in which only the female parent was a hybrid. These and double crosses had less variability than open pollinated varieties but still varied in individual plant characters such as flowering dates and plant heights.  Not only did the move to more mechanized corn production favor uniformity but also, as plant densities increased, completion for light favored plants of the same height.  Studies done at University of Missouri in the 70’s reinforced the view of many corn specialists that short plants among taller ones yield less than when the short plants are planted separately. 
 
As inbred yields increased and seed production practices improved, the move to single cross hybrids became fixed in the US corn belt in the 70’s. 

​My dad told me of his dad keeping chosen corn ears in the attic of their house to use for seed next year.  This must have been in the 1920-1930 on their central Iowa farm.  This practice occurred throughout the USA for a few centuries, building on the several thousand years of selection done by others. 
 
Probably the most significant contribution, however, was made when the male and female parts of the corn plant were separated morphologically on the plant.  This assured continual cross pollination in these varieties, even with farmers saving seed.  Pollen from tassel does not fall perpendicularly but drifts slightly with the slightest of wind, assuring that open-pollinated plants maintain some variability.  Hybrid seed producers are very aware of the ease for maize pollen to contaminate seed production as well as anyone attempting to keep grain free of GMO traits.
 
We should not complain about this character. The ease of cross pollination in corn allowed genetic variability available to modern breeders that is distinct from any other crop.  When Robert Reid, in east central Illinois, got a poor stand with his southern dent variety, he planted an early new England flint variety to fill in the field.  Future generations from that field not only produced winners in corn show contests in the early 1900’s but also the foundation of Reid Cornbelt Dent, a variety commonly used in the Midwestern USA corn belt for many years. And later became a major source for development of parent inbreds of current hybrids.  It has been estimated that there were over 500 established varieties of corn in USA in 1900.  There must have been countless numbers of sub-varieties as well as individuals saved their preferred ears for the next season. Separation of tassels from ear shoots, the feature that occurred and was selected early in the move from Teosinte to corn, allowed continuous genetic variability within each of the varieties.  We continue to tap into that variation with today’s hybrids.

​The first European exposure to corn was about 500 years ago.  Europeans called it corn because that was the common name for other grains. Seed was moved to Europe where it spread from even further to Africa and Asia, again with locals selecting for adaptation to their conditions. This included a wide range of required time from planting to harvest, disease pressure and local food uses.
 
Within the USA, corn that moved through the Southwest and then north and east tended to be flint types whereas the Southeast corn was floury types perhaps with genetics influenced by Caribbean corn migration.  Flint corns in the Northeast USA tended to have fewer kernel rows than the semi-dent types of the southeast.  As people became more stationary, with each farmer having the opportunity to save seed that favored their environment, food and livestock needs, multiple uniquely genetic varieties developed across the continent. Although local selections were based upon gross performance, ultimately selections were having effects on root structures, photosynthesis parameters, moisture stress tolerance, germination in cool soils, and disease resistance.  Separation of the male and female flowers of corn became a major asset to avoid complete inbreeding as farmers kept desirable ears to save for the next season.  Most pollen from an individual corn plant does not fall on its own silk, almost guaranteeing cross pollination in open fields. Until 80 years ago, open pollinated varieties were the sources of corn, with yields in the USA usually less than 30 bushels per acre. 

​As the early corn-teosinte crosses were moved away from Teosinte locations in the valleys of southcentral Mexico, the intermixing with Teosinte genetics must have lessened. After hundreds of generations and selections by people from central highlands of Mexico, north into current southwest USA and south to Northern South America.  Aided by natural open pollination, and mutations, different types of corn evolved.  There were dented, semi-dented and flinted kernels with colored the non-colored aleurone layers and yellow and white flinty or floury endosperm. There were sweet corn and popcorn types 5000 years ago.
 
Whereas Teosinte needed to survive with seed naturally distributed by dehiscence from its ‘cob’, people selected types that would retain the seed on the cob.  Corn became dependent upon people to grow the next season. It also became easily transported by people as it spread to Southwestern US about 3-4000 years ago. From there it spread as far north as Alberta Canada. There is evidence of corn in central USA by 2000 years ago and that it reached the Northeast USA about 1500 years ago.  Movement away from the equator required changes in the factors influencing the time to flowering.  Heat accumulation became a more significant trigger to stimulating the change in the meristems to produce flowering parts instead of more stem and leaf tissue.  Development of modern corn varieties are the result of those multiple selections done by many people across multiple environments several thousand years ago.
 

Ancient corn breeders spent a few thousand years working with the teosinte mutations, further adapting them to human culture.  They changed an ‘ear’ from having two rows of kernels to a cob with at least 8 rows of seeds. The web site http://en.ancientmaize.com, traces the migration of corn from the southcentral Mexico origin to the south and then the north over the next thousands of years.  Maize does produce fertile seed when crossed with Teosinte and surely that occurred frequently as the breeders saved the best adapted to their environments and food needs. 
 
Corn kernels must have been easily transported and traded as neat little packets of food.  One seed could be multiplied to a couple hundred in only one season. It became the main ag product for the Astecs in Mexico, the Inca in the mountain chain from the Ecuador thru Peru and Chile, and the Mayan regime in the lowlands of central America. All three large regimes formed about 5000 years after the first domestication of maize and became major sources of further development of the crop. Within each geographical environment, selection of plants best fit to both the successful reproduction and the local food and culture needs, distinct populations, or races, of corn followed.  In addition to those doing selections in the large regimes there were multiples of smaller groups also selecting corn varieties that fit their needs. Although selection for food production was primary, selection resulted in a range of internal leaf, stem and root structures had to occur to meet the challenging and different environments.